growth by extension and reworking of a south western kalahari linear dune optically stimulated luminescence osl dating studies of linear longitudinal dunes have been used extensively to elucidate late quaternary environments and climates in arid or formerly arid regions yet understanding of the development of such dunes is incomplete in particular conflicting opinions have been presented regarding the propensity of linear dunes to migrate laterally the degree to which they rework their own sediment during accumulation and whether they form primarily by extension as opposed to lateral sand movement from adjacent interdunes this study focuses on this last point although the importance of the other controversies is discussed in context a simple linear dune in the south western kalahari which has a prominent termination on a pan playa surface provides an opportunity to directly test hypotheses of dune extension chronostratigraphy along a 600 m transect along the crest of the dune constrained by 42 osl ages reveals that the dune grew by extension on occasions in the late pleistocene and early holocene but has also been subject to reworking along its length which has continued until recent times dune development by extensional growth is suggested to operate under environmental conditions more conducive to net accumulation whereas reworking is largely independent of conditions throughout the last 18 ka and may represent seasonal fluctuations in the position of the dune crest the relative significance of these two modes of development is suggested to be a key control on the efficacy of linear dunes as archives of environmental and climatic change copyright 2011 john wiley sons ltd

experimental grain size sensitive flow of hot pressed brazilian quartz aggregates abstract grain size sensitive flow may be of particular importance in the flow of rocks in natural high strain zones hot pressed aggregates of synthetic ultrafine grained brazilian quartz of nominal grain sizes 0 4 1 3 and 4 5 m were deformed at 300 mpa confining pressure and temperatures of 1273 1473 k fully ductile flow at differential stresses below 200 mpa followed 0 4 exp 220000 r t d 2 where is strain rate s 1 is stress mpa d is grain size microns and r is the gas constant grains remained equant and mostly dislocation free without significant crystallographic fabric water pressure 25 50 mpa was estimated from water adsorbed onto grain boundaries during specimen fabrication flow was inferred to be controlled by silicon volume diffusion and to be sensitive to water fugacity at differential stresses 200 mpa non linear flow involved dislocation motion extrapolation to natural strain rates and temperatures that predict grain size sensitive flow in quartz should only be important at 800 k and very small grain sizes 1 m flow law parameters may change over the extrapolation interval e g through a transition to control by silicon grain boundary diffusion but geological evidence suggests that flow of even very fine grained quartzites commonly occurs by recrystallization accommodated dislocation creep rather than by diffusional accommodation

the interaction of fluid inclusions and migrating grain boundaries in a rock analogue deformation and annealing of polycrystalline camphor ethanol mixtures the results of deformation and annealing experiments using a rock analogue containing a liquid phase as observed in transmitted light microscopy are presented samples were made of camphor ethanol mixtures with the polycrystalline solid being a rhombohedral phase and the liquid a saturated solution of camphor in ethanol results show the in situ pore fluid morphology during grain boundary migration recrystallization samples were deformed at high homologous temperatures th 0 7 and strain rates of 1 10 6 to 9 10 4 s 1 for grain boundary migration rates ranging from 10 10 to 10 6 ms 1 zener pinning not only the drag and drop of fluid inclusions by migrating grain boundaries but also the passage of grain boundaries over fluid inclusions without noticeable interaction was observed the drag limiting migration rate is three times lower for post kinematic annealing than for dynamic recrystallization and shows a weak dependence on fluid inclusion size a detailed description is presented of fluid inclusion grain boundary interaction with respect to fluid inclusion sizes and grain boundary migration rates fluid inclusions show up to 39 reduction in diameter after passage of a grain boundary indicating fluid flow along the mobile grain boundary

quartz microstructures developed during non steady state plastic flow at rapidly decaying stress and strain rate abstract synseismic loading to very high stresses 0 5 gpa and subsequent creep during stress relaxation in the uppermost plastosphere at temperatures of ca 300 350 c near the lower tip of an inferred once seismically active crustal scale fault was proposed based on peculiar microstructures identified in rocks exposed over 100 km 2 in the sesia zone european western alps here we discuss the conspicuous and highly heterogeneous microstructural record of quartz in disseminated small scale shear zones sub basal deformation lamellae and arrays of elongate subgrains on the tem scale indicate an early stage of glide controlled deformation at high stresses distributed brittle failure is indicated by healed microcracks very fine grained recrystallised aggregates with a pronounced crystallographic preferred orientation reflect intense plastic flow by dislocation creep locally a fine grained foam microstructure indicates a final stage of static grain growth at low differential stress for the previously inferred peak stresses of about 0 5 gpa and given temperatures initial strain rates on the order of 10 10 s 1 are predicted by available flow laws for dislocation creep of quartz we emphasise the importance of short term non steady state deformation in the uppermost plastosphere underlying seismically active upper crust the related heterogeneous record of quartz is governed by the local stress history at constant temperature

evolution of pore microstructures during healing of grain boundaries in synthetic calcite rocks the morphologies of calcite grain boundaries were analyzed to provide insight into the evolution of pore networks in unfractured rock two synthetic calcite rocks were fabricated by hot isostatically pressing hip ing dried analytical grade powders of pure caco3 and caco3 plus 5 al2o3 at 600 c and 200 mpa confining pressure for 3 hours hip 1 some samples were hiped a second time at different temperatures and pressures to investigate the stability of the structures hip 2a c sem and tem were used to image both grain faces and grain boundary cross sections structures on grain faces vary from open shallow basins with peripheral rims to labyrinths of irregular ridges and channels to isolated circular depressions all of these structures are mirrored across the plane between grain faces the grain size in both the single and two phase samples increased markedly during hip 1 migrating boundaries either dragged pores along or broke away leaving grain interiors dotted with small voids the structures present after hip 1 were not stable but evolved considerably in a way dependent on the conditions of the hip 2 confining pressure had the most pronounced effect with low confining pressure the grain boundary porosity evolved into isolated circular depressions but the total pore volume did not noticeably decrease with high confining pressure the pore volume virtually disappeared the structures present after hip 1 are strikingly similar to those that develop in intragranular cracks during healing we infer that grain boundaries and intragranular cracks heal by similar processes decomposition localized melting impurities and anisotropic surface energies played no evident role in forming the grain boundary structures the timing of the formation of the porosity and of the subsequent healing processes is more difficult to ascertain some structures appear to have evolved gradually throughout the constant high temperature stage of hiping the most obvious structures however appear to have evolved on grain boundary cracks that opened during cooling

the sedimentary structure of linear sand dunes linear sand dunes dunes that extend parallel to each other rather than in star like or crescentic forms are the most abundant type of desert sand dune1 but because their development and their internal structure are poorly understood they are rarely recognized in the rock record2 models of linear dune development2 3 4 5 6 have not been able to take into account the sub surface structure of existing dunes but have relied instead either on the extrapolation of short term measurements of winds and sediment transport or on observations of near surface internal sedimentary structures from such studies it has not been clear if linear dunes can migrate laterally2 7 8 here we present images produced by ground penetrating radar showing the three dimensional sedimentary structure of a linear dune in the namib sand sea where some of the world s largest linear dunes are situated these profiles show clear evidence for lateral migration in a linear dune moreover the migration of a sinuous crest line along the dune produces divergent sets of cross stratification which can become stacked as the dune height increases and large linear dunes can support superimposed dunes that produce stacked sets of trough cross stratification these clear structural signatures of linear dunes should facilitate their recognition in geological records

dune forming winds on titan and the influence of topography abstract numerous extended dunes on saturn s moon titan detected by the cassini radar constrain the long term pattern of surface winds we analyse the statistics of surface wind speed and direction and their spatial and temporal variability predicted by a general circulation model gcm in order to constrain surface wind predictions of this gcm by dune observations the model shows that modern winds are sufficient for saltation and dune formation at low latitudes in agreement with the presence of dunes there the best condition for the dune forming wind occurs with a threshold friction speed of 0 02 m s 1 or slightly less the equatorial region is conducive to longitudinal dunes because of a combined effect of a high sand drift potential and obtuse bimodal wind pattern oblique to the equator caused by the seasonal reversal of the hadley circulation the cross equatorial wind is steady and is characterised by a high weibull shape parameter k 4 the wind pattern at higher latitudes is more complex and gusty and neither longitudinal nor transversal dunes would be able to form putative large scale topography is found to have a profound influence on the near surface wind pattern generally mountains cause a convergence and speeding up on the flank while basins weaken the wind and cause a divergent flow longitudinal dunes can be deflected on the foot of mountains by up to 90 if xanadu is a hypothetical large mountain a wind pattern converging in xanadu that entirely disagrees with the dune observations is predicted if instead xanadu is a large basin the wind arcs clockwise north of xanadu and anti clockwise west and southwest of xanadu in agreement with the dune orientations in the vicinity of xanadu the albedo pattern has comparatively little influence on the wind field isolated mountain chains cause only local scale change in the wind pattern however the persistent surface easterlies in belet which are in conflict with the dune orientations do not disappear by any combination of large scale topography

strain path partitioning within thrust sheets microstructural and petrofabric evidence from the moine thrust zone at loch eriboll northwest scotland abstract quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the loch eriboll area of the moine thrust zone and are used to interpret the detailed processes involved in fabric evolution the intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude a correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the moine thrust within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate mutually interfering shear bands globular low strain detrital quartz grains whose c axes are aligned sub parallel to the principal finite shortening direction z and quartz c axis fabrics which are symmetric both in terms of skeletal outline and intensity distribution with respect to mylonitic foliation and lineation non coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation tectonic models offering possible explanations for the presence of kinematic strain path domains within thrust sheets are considered

factors determining desert dune type while most observers recognize four elemental types of desert dunes longitudinal transverse barchan and star1 3 there is little agreement about which factors determine these types the angular relationships between the resultant of sand shifting winds and both the crest and principal slipfaces of the elemental types have been discussed qualitatively for many decades these relationships have been quantified but the wide range of dune types that combine the elemental forms and the juxtaposition of different dune types show that wind regime is not the only determinant for example sand availability has been considered important we have now quantified the importance of both wind regime and sand availability as determinants of dune type we show here that the four elemental types occur in areas uniquely defined by both the equivalent sand thickness of sediments held in the dunes and a measure of directional variability of sand moving winds wind strength does not appear to be important vegetation has an ambiguous role and the particle size of dune sediments is unimportant

australian desert dune fields initiated with pliocene pleistocene global climatic shift development of continental aridity has been linked to late cenozoic global cooling but the evidence is indirect based on terrestrial loess deposits and eolian silt in marine sediments whereas direct dating of the inception of arid landforms has been frustrated by a lack of suitable methods here we report the first age determination of a major arid zone dune field based on cosmogenic 10 be and 26 al measurements of drill cores from dunes in the simpson desert central australia results show that the dune field began to form ca 1 ma whereas dating using quartz optically stimulated luminescence indicates episodic dune building during late quaternary ice ages less intense desertification began earlier the previous cosmogenic exposure dating showed that neighboring stony deserts began to form at the onset of quaternary ice ages 2 4 ma aridity deepened and the dune field formed when ice age cycles increased their amplitude and switched their periods from 40 k y to 100 k y ca 1 ma

the mesoproterozoic beaverhead impact structure and its tectonic setting montana idaho 40ar 39ar and u pb isotopic constraints abstract new 40ar 39ar and uranium lead u pb zircon data from the beaverhead impact structure first identified by extensive shatter coning of proterozoic quartzite and gneiss from the beaverhead mountains near the montana idaho border indicate that the structure formed at or after 900 ma the 40ar 39ar age spectra from fine grained muscovite and biotite from a breccia zone in high grade gneiss show significant argon loss but yield dates for highest temperature steps that cluster between 899 and 908 ma the dated minerals probably formed by recrystallization of impact glass so on both geologic and isotopic grounds the dates probably represent the minimum age of impact u pb data for zircons from the same breccia are strongly discordant and yield an upper intercept apparent age of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepac

infrared microspectroscopy analysis of water distribution in deformed and metamorphosed rocks infrared microspectroscopy has been applied to thin sections of various deformed and metamorphosed rocks in order to investigate water content of quartz in these rocks the broad ir band absorbance around 3400 cm 1 probably due to fluid inclusion molecular water h2o was used to calculate water contents deformed granitic rocks from the yanazawa kamimura area near the median tectonic line mtl showed an increase of water content in quartz from about 300 ppm to 2500 ppm toward the mtl with increasing degree of deformation metacherts from sambagawa metamorphic rocks asemigawa route showed a systematic decrease of water content in quartz from about 1000 ppm to 200 ppm with increasing metamorphic degree from chlorite garnet albite biotite to oligoclase biotite zones an archaean metachert from the napier complex granulite facies has only 40 ppm water comparing this with an inuyama unmetamorphosed chert sample having water contents of 3500 to 7000 ppm a starting point of metamorphism the systematic decrease of water in quartz with increasing metamorphic grade may extend from unmetamorphosed cherts to the highest grade metacherts water contents in high pressure metamorphic rocks in japan are mostly of the order of 500 ppm 300 700 ppm except for those from kurosegawa with a water content as high as 1700 ppm these results represent an exploratory analysis of intragranular water contents in quartz in various deformed and metamorphosed rocks further micro ft ir studies of the distribution of water in crustal rocks will provide a quantitative basis for examining the geochemical cycle of water in the earth s crust

modification of fluid inclusions in quartz by deviatoric stress i experimentally induced changes in inclusion shapes and microstructures fluid inclusions in quartz are known to modify their shapes and microstructures textures during weak plastic deformation however such changes have not been experimentally demonstrated and criteria are not available to relate them to paleostress conditions to address these issues quartz crystals containing natural co2 h2o nacl fluid inclusions have been experimentally subjected to compressive deviatoric stresses of 90 250 mpa at 700 c and 600 mpa confining pressure strains of up to 1 cause the inclusions to develop irregular shapes and to generate microcracks in crystallographic planes oriented subperpendicular to the major compression axis 1 the uniform alignment of the microcracks imparts a planar fabric to the samples the microcracks heal and form swarms of tiny satellite inclusions these new inclusions lose h2o by diffusion thereby triggering plastic deformation of the surrounding quartz via h2o weakening consequently the quartz samples deform plastically only in domains originally rich in inclusions this study shows that fluid inclusions deformed by deviatoric stresses may indeed record information on paleostress orientations and that they play a key role in facilitating crystal plastic deformation of quartz

deformation processes at the down dip limit of the seismogenic zone the example of shimanto accretionary complex in order to constrain deformation processes close to the brittle ductile transition in seismogenic zone we have carried out a microstructural study in the shimanto accretionary complex japan the fossil equivalent of modern nankai accretionary prisms the hyuga tectonic melange was sheared along the plate interface at mean temperatures of 245 c 30 c as estimated by raman spectroscopy of carbonaceous material rscm it contains strongly elongated quartz ribbons characterized by very high fluid inclusions density as well as micro veins of quartz both fluid inclusion planes and micro veins are preferentially developed orthogonal to the stretching direction furthermore crystallographic preferred orientation cpo of quartz c axes in the ribbons has maxima parallel to the stretching direction recrystallization to a small grain size is restricted to rare deformation bands cutting across the ribbons in such recrystallized quartz domains cpo of quartz c axes are orthogonal to foliation plane the evolution of deformation micro processes with increasing temperature can be further analyzed using the foliated morotsuka a slightly higher grade metamorphic unit 342 30 c by rscm from the shimanto accretionary complex in this unit in contrast to hyuga tectonic melange recrystallization of quartz veins is penetrative cpo of quartz c axes is concentrated perpendicularly to foliation plane these variations in microstructures and quartz crystallographic fabric reflect a change in the dominant deformation mechanism with increasing temperatures above 300 c dislocation creep is dominant and results in intense quartz dynamic recrystallization in contrast below 300 c quartz plasticity is not totally activated and pressure solution is the major deformation process responsible for quartz ribbons growth in addition the geometry of the quartz ribbons with respect to the phyllosilicate rich shear zones shows that bulk rheology is controlled by quartz behavior consequently below 300 c the application of quartz pressure solution laws based on realistic geometry derived from hyuga microstructures results in strongly lowering the overall strength of the plate interface with respect to the classical brittle envelop

modification of fluid inclusions in quartz by deviatoric stress iii influence of principal stresses on inclusion density and orientation extraction of useful geochemical petrologic and structural information from deformed fluid inclusions is still a challenge in rocks displaying moderate plastic strain in order to better understand the inclusion modifications induced by deviatoric stresses six deformation experiments were performed with a griggs piston cylinder apparatus natural nacl h2o inclusions in an oriented quartz crystal were subjected to differential stresses of 250 470 mpa at 700 900 c and at 700 1 000 mpa confining pressure independently of the strain rate and of the crystallographic orientation of the quartz the inclusions became dismembered and flattened within a crystallographic cleavage plane subperpendicular to 1 the neonate newly formed inclusions that result from dismemberment have densities that tend towards equilibrium with pfluid 1 at tshearing these results permit ambiguities in earlier deformation experiments on co2 h2o nacl to be resolved the results of the two studies converge indicating that density changes in neonate inclusions are promoted by high differential stresses long periods at high p and high t and fluid compositions that maximize quartz solubility neonates spawned from large precursor inclusions show greater changes in density that those spawned from small precursors these findings support the proposal that deformed fluid inclusions can serve as monitors of both the orientation and magnitude of deviatoric stresses during low strain ductile deformation of quartz bearing rocks

longitudinal dunes on mars relation to current wind regimes longitudinal dunes are extremely rare on mars but constitute a substantial fraction of terrestrial desert dunes we report finding isolated examples of longitudinal dunes on mars and relate their occurrence to expected sand transport regimes terrestrial longitudinal dunes form in bimodal and multimodal transport regimes general circulation models and streak data indicate that bimodal and multimodal transport of sand should be very rare on mars thus the dearth of longitudinal dunes on mars is consistent with their apparent formation conditions on earth

water distribution in low grade siliceous metamorphic rocks by micro ftir and its relation to grain size a case from the kanto mountain region japan abstract infrared microspectroscopy reveals that liquid like water is always present in polycrystalline quartz grains from low grade metamorphic cherts and shale in the chichibu group and the mikabu greenrock complex of the kanto mountains area the water distribution in these rocks is heterogeneous and is related to rock textures in the microcrystalline quartz o h species due possibly to si oh are generally present the liquid like water is also present at grain boundaries and or in sub micron fluid inclusions in the microcrystalline parts on the other hand in quartz grains larger than 20 m in diameter within veins only the liquid like water is present possibly as fluid inclusions trapped during quartz vein formation in the microcrystalline cherty matrix of the metamorphic rocks a general decrease in liquid like water content is observed with increasing metamorphic grade associated with an increase in grain size two representative grain shapes a cube and a regular tetradecahedron having 14 planes are used to estimate the surface area between grains per unit volume with the grain size d m 3 d for cubic and 2 37 d for tetradecahedron grain boundary volumes were then calculated assuming grain boundary widths from 0 5 to 20 nm and normalized by unit volume of the rock the measured ir data on low grade metamorphic cherty rocks fall closely on these curves with grain boundary width of around 10 nm for the both models assuming saturation of h2o at grain boundary the observed decrease in water content with increase in grain size can be rigorously explained by the decrease in grain boundary volume per unit volume these results suggest that liquid like water occurs mainly between grain boundaries in the microcrystalline quartz of low grade metamorphic rocks although this grain boundary water model is the first approximation and requires further details of quantitative grain boundary textures the present model can provide a new approach to understand water distribution in polycrystalline rocks

dynamic processes acting on a longitudinal seif sand dune abstract field measurements were made on a longitudinal dune in the sinai desert in order to understand its morphology and dynamics the field measurements contradicted the wind structure indicated by the helicoidal flow theory rather it was found that winds coming from two basically different directions at different times and striking the dune obliquely were responsible for sand transport and erosion or deposition along the lee flank the essence of this mechanism is the deflection of the wind airflow on the lee flank of the dune to a direction parallel to the crest line the occurrence of erosion or deposition depends upon the angle of incidence between the wind and the crest line when this angle is 40 the velocity of the deflected wind drops and deposition takes place on the lee flank the angle of incidence in each wind storm is changed intermittently between 30 and 100 along the dune because the dune meanders and because of the sinuous outline of the crest line in this manner sand transport and erosion or deposition occurs along the lee flank depending on the angle of incidence between the wind and the crest line as a result of the deflection of the wind the dune elongates at an average rate of more than 1 m per month peaks and saddles along the crest line advance at an average rate of 0 7 m per month the lack of uniformity in the effects of the wind on both sides of the dune creates a lack of uniformity in the rate of erosion and deposition this can explain the formation of peaks along the crest line of the dune

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

long term evolution of an accretionary prism the case study of the shimanto belt kyushu japan the shimanto belt in sw japan is commonly described as a paleo accretionary prism whose structure is explained by continuous accretion like in modern accretionary prisms such as nankai we carried out a structural study of the cretaceous to miocene part of the shimanto belt on kyushu to test this hypothesis of continuous accretion most deformation structures observed on the field are top to the se thrusts fitting well the scheme of accretionary wedge growth by frontal accretion or underplating in particular the tectonic melange at the top of the hyuga group records a penetrative deformation reflecting burial within the subduction channel in contrast we documented two stages of extension that require modifying the traditional model of the belt as a simple giant accretionary wedge the first one in the early middle eocene is mostly ductile and localized in the foliated bases of the morotsuka and kitagawa groups the second one postdating the middle miocene is a brittle deformation spread over the whole belt on kyushu integrating these new tectonic features to existing data we propose 2 d reconstructions of the belt evolution leading to the following conclusions 1 erosion and extension of the margin in the early middle eocene resulted from the subduction of a trench parallel ridge 2 the late eocene to early miocene evolution is characterized by rapid growth of the prism followed by a middle miocene stage where large displacements occurred along low angle out of sequence thrusts such as the nobeoka tectonic line 3 from middle miocene the strain regime was extensional

testing models of linear dune formation by provenance analysis with composite sediment fingerprints abstract the formative mechanisms of linear longitudinal dunes and dunefields remain uncertain and multiple hypotheses have been proposed a central debate is the degree to which dunes act as along dune sediment transport corridors implying that dunes grow primarily by extension or whether they are comprised of locally derived sands moved from adjacent interdunes the wind rift model sediment fingerprinting studies with origins in fluvial science have been shown to offer the possibility to trace the provenance of aeolian sands and thus elucidate transport pathways two models a monte carlo framework and a generalized likelihood uncertainty estimate framework are used here to provide quantitative estimates of the sediment sources that have supplied a linear dune in the central simpson desert of central australia four possible sources are identified that may have supplied the dune two adjacent interdunes one upwind low ridge of sand and a merging upwind dune two sites near the dune s crest are used as the target and provided twenty surface samples for analysis following geochemical assay stepwise discriminant function analysis identified optimum elemental sediment fingerprints for a variety of possible sediment pathway configurations results suggest that the sands of the dune are sourced predominantly from upwind dunes and sand sources and that likely contributions from neighbouring dune swales are typically the findings suggest recent sediment accretion of a linear dune dominated by along dune sand flux and thus support an extensional component for the development of such dunes whilst it is noted that at a point by point basis this might not exclude accretion by vertical growth as some have observed there is no clear support for a substantive contribution to the dune sands from adjacent interdunes moreover the use of contemporary sediment fingerprinting methods to question hypotheses of aeolian geomorphology suggests that such methods have great potential for addressing other terrestrial geomorphological questions where identifying sediment pathways can provide vital insight

star and linear dunes on mars abstract a field containing 11 star and incipient star dunes occurs on mars at 8 8 s 270 9 w examples of linear dunes are found in a crater at 59 4 s 343 w while rare dune varieties that form in bi and multidirectional wind regimes are not absent from the surface of mars the occurrence of both of these dune fields offers new insight into the nature of martian wind conditions and sand supply the linear dunes appear to have formed through modification of a formerly transverse aeolian deposit suggesting a relatively recent change in local wind direction the 11 dunes in the star dune locality show a progressive change from barchan to star form as each successive dune has traveled up into a valley into a more complex wind regime the star dunes corroborate the model of n lancaster 1989 progr physical geogr 13 67 91 1989 sedimentology 36 273 289 for the formation of star dunes by projection of transverse dunes into a complex topographically influenced wind regime the star dunes have dark streaks emanating from them providing evidence that the dunes were active at or near the time the relevant image was obtained by the viking 1 orbiter in 1978 the star and linear dunes described here are located in different regions on the martian surface unlike most star and linear dunes on earth both martian examples are isolated occurrences neither is part of a major sand sea previously published mars general circulation model results suggest that the region in which the linear done field occurs should be a bimodal wind regime while the region in which the star dunes occur should be unimodal the star dunes are probably the result of localized complication of the wind regime owing to topographic confinement of the dunes local topographic influence on wind regime is also evident in the linear dune field as there are transverse dunes in close proximity to the linear dunes and their occurrence is best explained by funneling of wind through a topographic gap in the upwind crater wall

lateral migration thermoluminescence chronology and colour variation of longitudinal dunes near birdsville in the simpson desert central australia the longitudinal dunes of the simpson desert in the vicinity of birdsville have been reworked largely during the holocene from dunes deposited up to 80000 years ago or earlier the widespread asymmetry of these roughly northward trending dunes with steeper eastern faces and more gentle western faces supports wind rose data showing sand transporting winds from the southwest obliquely intersecting the dunes while this suggests a change in the wind pattern since the dune field was oriented it does not indicate that the dunes are necessarily shifting leeward eastward as a consequence it is hypothesized that the direction of migration is controlled by the extent to which the dunes are vegetated relatively well vegetated dunes can accrete sand on their gentle stoss slopes and erode on their lee slopes causing them to shift westward and hence obliquely into the wind a condition that probably prevails in wetter regions and during episodes of relatively humid climate in contrast in very dry areas or during arid phases sand can move unimpeded up a sparsely vegetated stoss face and over the crest to form an avalanche or slip face on the lee side thereby causing the dunes to shift eastward despite evidence that longitudinal dune crests can shift laterally to some extent the dunes in the western part of the simpson desert have not migrated either westward or eastward more than 100m or so from their pleistocene cores aeolian transport and partial or complete removal of iron cutans from around quartz grains results in dunes of widely varying colour yet of similar age

stresses and pressures at the quartz to coesite phase transformation in shear deformation experiments coesite was found in quartz aggregates experimentally deformed at confining pressures of 1 0 1 5 gpa and temperatures between 600 c and 900 c the confining pressure pc and in most cases the mean stress m of the experiments were below those of the quartz to coesite phase transformation yet coesite formed when the maximum principal stress 1 was within the p t range of the coesite stability field in one sample the euhedral coesite grains were corroded indicating that coesite started to transform back to quartz it is inferred that this sample started to deform with 1 above the quartz to coesite phase transformation and with on going deformation 1 decreased to values in the quartz stability field due to strain weakening in all cases 1 triggered the quartz to coesite reaction as well as the reverse reaction suggesting that 1 is the critical parameter for the quartz to coesite transformation not pc or m with progressive deformation the coesite laths rotated towards the shear plane as more rigid particles with the sense of shear in case of back reaction new quartz grains exhibit no systematic crystallographic relationship with respect to old coesite the experiments cover different degrees of pressure overstepping different temperatures and different experimental durations at p and t and deformation always enhances the reaction kinetics the observation that 1 is critical for a pressure dependent phase transformation also for reversals poses questions for the thermodynamic treatment of such phase transformations

the rate of water penetration in experimentally deformed quartzite implications for hydrolytic weakening abstract in order to determine the rate at which water penetrates the grains of a quartzite we performed several types of experiments including water added deformation and hydrostatic experiments on vacuum dried samples and hydrostatic annealing experiments on previously deformed quartzites samples of heavitree quartzite with an average grain size of 200 m and intragranular water concentration of 1750 420 h 10 6 si as measured by ftir were vacuum dried at 800 c 6 pa for 12 h after vacuum drying the grains showed thermal cracks with an average spacing of 50 m and an intragranular water concentration of 240 60 h 10 6 si one constraint on the rate of water penetration into quartz was obtained by deforming vacuum dried samples with 0 3 wt 20 000 h 10 6 si water added at 800 c 1500 mpa and a strain rate of 10 6 s conditions at which as is quartzite undergoes climb accommodated dislocation creep the time between reaching experimental conditions and yielding of the sample was between 2 and 9 h the samples had yield stresses of 100 250 mpa values well below the yield stress of 1500 mpa for a vacuum dried sample deformation of the grains was homogeneous on both optical and tem scales indicating that the grains had fully equilibrated with the water related defect that is responsible for water weakening ftir measurements are consistent with this interpretation one of the deformed samples has an intragranular water concentration of 820 160 h 10 6 si and vacuum dried samples hydrostatically annealed for 3 or 10 h at the same conditions as the deformed samples have an average intragranular water concentration of 730 180 h 10 6 si to obtain a second constraint on the rate of water penetration hydrothermal annealing experiments were performed on portions of a sample of heavitree quartzite which were previously deformed without water added at 700 c 1200 mpa and 10 5 s and have a high 10 16 m 2 and homogeneous dislocation density two samples were annealed at 700 c 1500 mpa for 24 h one with 0 2 wt water and one with no water added the grains in the water added sample have a rim approximately 15 m wide with dislocation recovery microstructures indicating equilibration with the water related defect the sample with no water added has a homogeneous dislocation density throughout the grains which is similar to that of the original deformed sample the 15 m wide recovered rim suggests a penetration rate approximately an order of magnitude faster than the rate of oxygen diffusion under hydrothermal conditions the observed rate of penetration of the water related species is consistent with theoretical considerations and calculations which predict a diffusion rate for h 2 o that is an order of magnitude faster than the rate of hydrothermal oxygen diffusion we infer from our experiments that the rate of water penetration into quartz is consistent with diffusion of h 2 o

linear dunes on titan extensive dunes produced by wind transport of sand sized particles are common on earth and are also seen on mars and venus saturn9s moon titan is now shown to have similar dunes

dynamic recrystallization of quartz under greenschist conditions abstract the sem electron channelling technique has been used to investigate the dynamic recrystallization of three quartz grains representative of optical textures observed in quartzite deformed under greenschist facies conditions the degree of recrystallization has been determined at 10 20 50 and 100 respectively we demonstrate that dynamic recrystallization is a sequential process involving subgrain grain rotation and grainboundary migration it initiates via subgrain polygonization on a single crystal slip system and continues via subgrain and neoblast rotation on several slip systems in the early stages of recrystallization

the initiation of particle movement by wind when air blows across the surface of dry loose sand a critical shear velocity fluid threshold ut must be achieved to initiate motion however since most natural sediments consist of a range of grain sizes fluid threshold for any sediment cannot be defined by a finite value but should be viewed as a threshold range which is a function of the size shape sorting and packing of the surface sediment in order to investigate the initiation of particle movement by wind a series of wind tunnel tests was carried out on a range of pre screened fluvial sands and commercially available glass beads with differing mean sizes and sorting characteristics a sensitive laser monitoring system was used in conjunction with a high speed counter to detect initial grain motion and to count individual grain movements test results indicate that when velocity is slowly increased over the sediment surface the smaller or more exposed grains are first entrained by the fluid drag and lift forces either in surface creep rolling or in saltation bouncing or hopping downwind as velocity continues to rise larger or less exposed grains may also be moved by fluid drag on striking the surface saltating grains impart momentum to stationary grains this impact may result in the rebound of the original grain as well as the ejection of one or more stationary grains into the air stream at shear velocities lower than that required to entrain a stationary particle by direct fluid pressure as a result there is a cascade effect with a few grains of varying size initially moving over a range of shear velocities the fluid threshold range and setting in motion a rapidly increasing number of grains results of the tests showed that the progression from fluid to dynamic threshold based on grain movement can be characterized by a power function the coefficients of which are directly related to the mean size and sorting characteristics of the sediment

temporal observations of a linear sand dune in the simpson desert central australia testing models for dune formation on planetary surfaces linear dunes are the most common dune form found on planetary surfaces yet questions remain about their formation temporal observations of a linear dune located in the simpson desert of central australia were made to monitor dune movement and to test competing hypotheses regarding linear dune formation our observations were collected on three separate occasions from 2006 to 2014 rebar stakes were placed in a gridded pattern so that multiple measurements of sand thickness gps surveys and photographs could be taken at the same locations over time we observed widespread reworking of sand on and around the dune crest with sand accumulation locally exceeding a meter between surveys overall the height of the dune crest increased by several centimeters we also observed fluctuations in the sand cover in the adjacent swales that often exceeded 2 3 cm between surveys yet we did not observe any appreciable changes in the position of the dune s downwind terminus weather data indicate that the effective sand transporting winds in the simpson are widely unimodal net sediment flux resultant drift direction is toward the north northwest locally at an oblique angle to dune orientation collectively our results suggest that the linear dune is actively maintained by vertical accretion the implications from our observations are that linear dunes on other planetary surfaces could form in wind regimes that are widely unimodal even where the resultant drift direction is locally oblique to dune orientation in particular such findings may provide support for global circulation models of titan

chapter 7 middle and late proterozoic rocks and late proterozoic tectonics in the southern beaverhead mountains idaho and montana a preliminary report

lateral migration of linear dunes in the strzelecki desert australia linear dunes in the strzelecki desert trend roughly south north sand transport which is toward the nne has caused the dunes to migrate eastward while they extend or migrate northward eastward lateral migration is evidenced by 1 asymmetrical shape of the dunes east facing slopes are several times as steep as west facing slopes 2 asymmetrical accumulation of loose recently transported sand relatively abundant on east facing slopes 3 asymmetrical outcropping of older semiconsolidated aeolian sand on the dune surface more abundant on west facing slopes and 4 east dipping foreset beds that underly the west facing flanks of some dunes dunes in the strzelecki desert are still active in the sense that sand is transported along and across many dune crests however the dunes are composed primarily of pleistocene strata indicating that the trend of the dunes was established before the holocene the obliquity of the dunes to the transport direction is not merely an aberration of the wind regime of the last few decades preferential accumulation of sand on east facing flanks indicates that the dunes migrated eastward several metres during the holocene moreover the west facing flanks of some dunes have experienced a minimum of tens of metres of erosion this asymmetric erosion and deposition were caused by dune obliquity and lateral migration that may have begun as early as the pleistocene dunes in the strzelecki desert and in the adjacent simpson desert display a variety of grossly different internal structures computer graphics experiments demonstrate that many of these differences in structure can be explained by different angles of climb of the dunes

longitudinal dunes can move sideways longitudinal dunes occur in all major sand areas in the world their dominant mode of migration or extension is considered to be either prevailing or resultant wind parallel the dunes extending downwind via accretion of the terminal nose in the qaidam pendi in northwest china a series of active longitudinal dunes extend downwind at 5 10 m yr 1 internal sedimentary structures examined in the dunes however display beds dipping in one direction rather than two opposed directions as is expected analysis of aerial photographs confirms that these dunes migrate laterally up to three metres per year whilst maintaining a symmetrical longitudinal dune morphology

semi brittle flow of granitoid fault rocks in experiments field studies and seismic data show that semi brittle flow of fault rocks probably is the dominant deformation mechanism at the base of the seismogenic zone at the so called frictional viscous transition to understand the physical and chemical processes accommodating semi brittle flow we have performed an experimental study on synthetic granitoid fault rocks exploring a broad parameter space temperature t 300 400 500 and 600 c confining pressure pc 300 500 1000 and 1500 mpa shear strain rate 10 3 10 4 10 5 and 10 6 s 1 to finite shear strains 0 5 the experiments have been carried out using a granular material with grain size smaller than 200 m with a little h2o added 0 2wt only two experiments performed at the fastest strain rates and lowest temperatures have failed abruptly right after reaching peak strength 1400 mpa all other samples reach high shear stresses 570 1600 mpa then weaken slightly by 10 190 mpa and continue to deform at a more or less steady state stress level clear temperature dependence and a weak strain rate dependence of the peak as well as steady state stress levels is observed in order to express this relationship the strain rate stress sensitivity has been fit with a stress exponent assuming n and yields high stress exponents n 10 140 which decrease with increasing temperature the microstructures show widespread comminution strain partitioning and localization into slip zones containing at first nanocrystalline and partly amorphous material later during continued deformation fully amorphous material in slip zones despite the mechanical steady state conditions the fabrics in the slip zones and outside continue to evolve and do not reach a steady state microstructure below 5 within the slip zones the fault rock material progressively transforms from a crystalline solid to an amorphous material we present and interpret the experimental results both in terms of sliding friction and viscous flow and we discuss the possible effect that the formation of nanocrystalline and amorphous layers may have on earthquake nucleation

a second look at western sinai seif dunes and their lateral migration abstract tsoar et al tsoar h blumberg d g stoler y 2004 elongation and migration of sand dunes geomorphology 57 293 302 reported that seif dunes in the western sinai desert did not migrate laterally between 1973 and 1999 if the planform sinuosities of the dunes are removed by filtering spatial averaging or linear regression however it is evident that the dunes did in fact migrate laterally roughly 13 m during this 26 year period the measured migration distance is 1 2 orders of magnitude greater than the rms co registration error tsoar et al determined for the first and last air photos that were used to map the dunes the western sinai dunes provide another example demonstrating that linear dunes can migrate laterally and they illustrate some of the difficulties in documenting systematic lateral motion lateral migration of a dune can be important geologically or geomorphologically even where migration is too slow to detect from repeated topographic surveys this article explains the wind conditions for the lateral migration of seif dunes in western sinai and the possible wind occurrences that would not lead to such a migration

dunes on titan observed by cassini radar thousands of longitudinal dunes have recently been discovered by the titan radar mapper on the surface of titan these are found mainly within 30 of the equator in optically near infrared and radar dark regions indicating a strong proportion of organics and cover well over 5 of titan s surface their longitudinal duneform interactions with topography and correlation with other aeolian forms indicate a single dominant wind direction aligned with the dune axis plus lesser off axis or seasonally alternating winds global compilations of dune orientations reveal the mean wind direction is dominantly eastwards with regional and local variations where winds are diverted around topographically high features such as mountain blocks or broad landforms global winds may carry sediments from high latitude regions to equatorial regions where relatively drier conditions prevail and the particles are reworked into dunes perhaps on timescales of thousands to tens of thousands of years on titan adequate sediment supply sufficient wind and the absence of sediment carriage and trapping by fluids are the dominant factors in the presence of dunes

dune formation under bimodal winds the study of dune morphology represents a valuable tool in the investigation of planetary wind systems the primary factor controlling the dune shape is the wind directionality however our understanding of dune formation is still limited to the simplest situation of unidirectional winds there is no model that solves the equations of sand transport under the most common situation of seasonally varying wind directions here we present the calculation of sand transport under bimodal winds using a dune model that is extended to account for more than one wind direction our calculations show that dunes align longitudinally to the resultant wind trend if the angle w between the wind directions is larger than 90 under high sand availability linear seif dunes are obtained the intriguing meandering shape of which is found to be controlled by the dune height and by the time the wind lasts at each one of the two wind directions unusual dune shapes including the wedge dunes observed on mars appear within a wide spectrum of bimodal dune morphologies under low sand availability

two types of longitudinal dune fields and possible mechanisms for their development longitudinal dune fields characterized by nearly uniform interdune spacing are distinguished from longitudinal dune fields characterized by fairly variable interdune spacing and high frequencies of dune coalescence the empirical and theoretical evidence indicating that the former may be due to helical air currents aligned with the dunes is reviewed hypotheses arguing that the latter may arise indirectly from horizontal pressure gradients or bidirectional wind regimes are discussed evenly spaced linear sand banks aligned with tidal currents may be shown mathematically to result from energy optimalization within two dimensional sand transporting flow regimes and a similar simple or non rotational flow model is considered for the problem of desert longitudinal dunes an initial complex or rotational flow analysis is undertaken to discern the likely significance of roll vortices in desert sediment transport an evolutionary timescale is estimated for the formation of desert longitudinal dune fields a simple analysis is performed for the effect of regional sand mass change on longitudinal dune field ordering recommendations are made for future empirical and theoretical research

wolfe creek crater a continuous sediment fill in the australian arid zone records changes in monsoon strength through the late quaternary abstract a bolide that impacted nw australia during the late quaternary left a circular depression more than 100 m deep and nearly a kilometer in diameter with a crater rim 30 m above the regional terrain the resultant crater is a window into the regional water table the surface of the contemporary central pan is 25 m below the adjacent terrain coincident with the late holocene regional water table modified by local evaporative processes shielded from aeolian deflation by the crater rim the central depression has slowly filled with dust sand and chemical precipitates estimated to be 20 100 m thick based on geophysical surveys one of the few continuous depocenters in the australian arid zone the nature of the crater s sediment fill is controlled by interactions between the water table primarily in response to changes in summer monsoon rain changes in the delivery of sand and dust to the crater by the prevailing easterly winds and the level of the sedimentary fill surface optically stimulated luminescence osl and 14c dates constrain an age model indicating the upper 10 m of sediment fill recovered from the central pan span the past 60 ka the lowest 3 m consist of clayey sand deposited in perennial water during marine isotope stage mis 3 the water table subsequently dropped rapidly 35 ka and remained more than 7 m below the late holocene level through most of mis 2 during which 2 m of sandy clay was deposited on a dry crater floor confirming a dry and dusty last glacial maximum lgm climate by 14 ka a rising water table intersected the crater surface modifying the upper 50 cm of lgm sediment and syndepositionally modifying another 60 cm of subsequent sandy clay deposition aeolian sediment delivery effectively ceased 13 ka and the upper 4 8 m is a gypsum dominated precipitate which initially accumulated rapidly before equilibrating with the late holocene water table shortly after 6 ka lacustrine carbonate encrustations on rocks at the base of the crater wall and 4 m above the central pan with 14c ages 40 ka document a time when regional groundwater maintained a water body in the crater 3 5 4 5 m above the modern groundwater level the crater wall deflected the prevailing easterly winds creating a horseshoe dune extending westerly on both sides of the crater with an extension rate of 35 m ka 1 an augered hole through the northern dune revealed 10 m of sediment overlying ferricrete the lowest meter is a mixture of broken ferricrete and sand that we interpret to be debris from the bolide impact three osl dates through the dune project an age for the debris dune contact of 120 10 ka changes in physical properties and bulk sediment 13c through the 9 m of aeolian sediment indicate the lowest 1 8 m was deposited during mis 5 120 85 ka under a uniformly wetter climate than present the overlying 4 3 m of sediment was deposited between 85 and 14 ka mis 4 3 2 and exhibits transitional characteristics between the lower unit and the upper 3 8 of sand which was deposited primarily during the holocene large changes in the regional water table occurred over the past 60 ka including an lgm water table persistently 7 m lower than late holocene levels and 3 5 4 5 m higher prior to 40 ka plausibly in mis 5 indicative of a stronger australian summer monsoon than at any time subsequently age models and sediment properties from the two sedimentary records indicate the crater was formed 60 ka and most likely 120 ka more recently than previous estimates

hydrogeology and mechanics of subduction zone forearcs fluid flow and pore pressure at subduction zones fluid flow pore pressure and tectonic processes are tightly interconnected excess pore pressure is driven by tectonic loading and fluids released by mineral dehydration and it has profound effects on fault and earthquake mechanics through its control on effective stress the egress of these overpressured fluids which is in part governed by the presence of permeable fault zones is a primary mechanism of volatile and solute transport to the oceans recent field measurements new constraints gained from laboratory studies and numerical modeling efforts have led to a greatly improved understanding of these coupled processes here we summarize the current state of knowledge of fluid flow and pore pressure in subduction forearcs and focus on recent advances that have quantified permeability architecture fluxes the nature and timing of transience and pressure distribution thus providing new insights into the connections between fluid metamorphic mechanical and fault slip proc

the development of large aeolian bedforms abstract large aeolian bedforms complex and compound varieties of dunes or draas dominate over wide areas of many sand seas and are well represented in the rock record analyses of allometric relationships between dune height and spacing in several sand seas suggest that draas develop by vertical growth of existing dunes in the namib and other sand seas draa tend to occur in areas of low total and net sand transport rates the distribution of such bedforms in a sand sea reflects long term patterns of deposition which are related to regional scale changes in wind regimes and sand transport rates superimposed bedforms are almost ubiquitous on draa and develop only after a minimum size of draa has been reached formation of superimposed dunes reflects spatial and temporal variations in secondary flow patterns developed on the draa measurements of winds and depositional patterns on complex linear dunes in the namib sand sea suggest that an important mechanism for their growth is the migration of superimposed dunes from areas of high to low wind velocity on draa flanks leading to the deposition of sediment by bedform climbing

effect of microcracking on pressure solution strain rate the gratz grain boundary model different but reasonable and well accepted assumptions made about grain boundary structure during pressure solution ps creep may easily have an effect of more than 10 orders of magnitude on the calculated ps deformation rate understanding of grain boundary structure during ps creep is therefore extremely important experimental evidence is presented in support of a grain boundary model previously proposed by a j gratz on the basis of observations on naturally deformed rocks in this model boundaries are assumed to have a static island channel network structure channels are located where microcracks intersect the boundary the rate of material transport is governed by thin film diffusion at the islands the model predicts that ps deformation rates are sensitive to the microcracking process ps rate is therefore essentially dependent on whether microcracking can occur and how intense the microcracking is these factors in turn depend on among others stress temperature effective pressure and crystallographic orientation the model qualitatively explains the observation made in experimentally and naturally deformed quartz rocks that the ps rate in single grains depends on crystallographic orientation the model predicts that sudden increases in stress or fluid pressure may lead to microcracking and subsequently many orders of magnitude increase in ps deformation rate

global pattern of titan s dunes radar survey from the cassini prime mission 1 we present a map of the orientation and extent of titan s sand dunes derived from the complete radar imaging dataset from the cassini prime mission the 16 000 dune segments we have mapped cover 8 of the titan s surface suggesting a total coverage of 20 are confined within 30 of the equator and show local and regional deviations of dune orientation of up to about 40 from due eastwards there is no obvious global longitudinal pattern although some divergence with latitude is apparent the most striking pattern is one of collimation by and divergence around bright and or high terrain obstacles 100 300 m high obstruct dunes when the local slope is 1 50 or steeper while slopes 1 200 or shallower cause dunes to thin out as they climb or to deviate around the obstacles

linear dunes on titan and earth initial remote sensing comparisons thousands of dunes found in cassini radar images of the equatorial regions of titan a moon around saturn are similar in size and morphology to linear dunes on earth we present remote sensing images of terrestrial analogues to the dunes on titan obtained by landsat and radar both at considerably higher resolution than are available at titan that provide information about dune landforms and processes dunes are generally dark to radar indicating smooth surfaces and signal absorbing materials but at certain incidence angles dune surfaces can reflect the radar signal and lead to a bright return linear dunes on titan and earth diverge around topographic obstacles creating teardrop patterns that indicate mean direction of wind flow and sand transport when sand supply or wind conditions change in linear dune fields such as behind an obstacle or near the margin of the dune field dunes disappear change size and spacing or change dune type these comparisons of features on titan and earth provide a better understanding of the global sand transporting wind directions sand properties and supply and the nature of the underlying substrate on titan

extra basinal fluid infiltration mass transfer and volume strain during folding insights from the idaho montana thrust belt finite strain and geochemical variations along strain gradients were used to study cleavage development in carbonates in the lost river range idaho deformation accommodating layer parallel shear was partitioned into thin deforma tion zones during folding of willow creek anticline cleavage intensity is strong to very strong in deformation zones and weak in surrounding rocks strain magnitudes range from e s0 32 1 15 0 93 0 73 x y z principal axes of strain outside to e s0 64 1 29 0 80 0 52 inside deformation zones we found a positive linear correlation between strain cleavage development and negative dilation volume loss at a cubic centimeter scale ranges from 12 percent to 49 percent cleavage selvages are depleted in ca and 18 o and enriched in other elements relative to microlithons and less deformed protolith carbonates mass balance considerations indicate that cleavage was formed by incongruent pressure solution leading to a passive concentration of less soluble components during ca loss and metasomatic additions of si al and k to produce authigenic clay minerals in selvages data for the willow creek locality and elsewhere in the lost river range davidson and others 1998 pioneer mountains and the tendoy range bebout and others 2001 show that across the sevier orogen fluid infiltration was heterogeneous at centimeter kilometer scales and resulted from positive feedback between deforma tion and far traveled surficial fluids metasomatic strain softening enhanced deforma tion zone development which generated increased permeability as the thrust belt evolved from a porosity based closed system to a discontinuity based open system increased fluid infiltration and isotopic exchange was associated with volume loss increasingly prolate strains and crystallization of clays to produce the cleavage selvages mass transfer was accommodated by diffusion early and advection later in the deformation history mesoscopic structures deformation zones faults veins focused fluid flow and were kinematically related to larger scale structures faults fault related folds the inferred addition of surficial fluids to depths of 7 kilometers in the thrust belt implies a fluid regime involving significant topographically driven recharge deformed whole rocks and microsamples are lower in 18 o than unde formed samples which have o and c isotope compositions similar to those of marine carbonates veins are even lower in 18 ov smow with minimum values of 5 permil reflecting penetration of the crust by ocean derived precipitation with 18 o near to somewhat lower than 0 permil range of 7 5 to 2 5 calculated for h2 oi n equilibrium with these veins the inferred penetration into the thrust belt of nearshore meteoric waters is consistent with proximity to the reconstructed western interior seaway later fluid infiltration locally lowered the 18 o of carbonates even further based on the o isotope compositions of veins related to younger compres sional deformation and to the onset of crustal scale extension in the eocene this progression toward lowered 18 o of the surficial waters is compatible with the retreat of the seaway during emergence of the thrust wedge and paleogene extension uplift and subaerial volcanism

reaction induced weakening of plagioclase olivine composites abstract the localisation of strain into natural ductile shear zones is often associated with the occurrence of metamorphic reactions in order to study the effects of solid solid mineral reactions on plastic deformation of rocks we have investigated the shear deformation of plagioclase olivine composites during the reaction plagioclase olivine orthopyroxene clinopyroxene spinel garnet microstructures of plagioclase olivine composites were studied after shear deformation experiments in a griggs apparatus experiments were performed on anorthite forsterite an fo and labradorite forsterite lab fo composites at 900 c confining pressures between 1000 1600 mpa and with constant shear strain rates of 5 10 5 s 1 in absence of reaction lab fo composites are stronger than pure olivine and labradorite end members that deform with a high temperature plasticity mechanism lab fo composites strain harden due to the inhibition of extensive recrystallisation by interphase boundaries in an fo composites the reaction induces strain weakening by a switch from dislocation creep to grain size sensitive deformation mechanisms through the development of fine grained size

a natural example of crystal plastic deformation enhancing the incorporation of water into quartz abstract water content of quartz in and around a greenschist facies mylonitic shear zone located in the western adirondacks was analyzed by micro ftir spectroscopy the shear zone is within a pegmatitic dike which cuts across a granitic gneiss the thickness of the shear zone varies along strike from 15 cm wide and encompassing all of the pegmatite dike at its northern most exposure to 5 cm wide approximately 10 m south along strike microstructures including quartz ribbons and recrystallized grains indicate quartz and feldspar within the mylonite underwent dislocation creep infrared spectral analysis was carried out using a nicolet micro ftir on mylonitic quartz ribbons pegmatitic quartz and gneissic quartz a small aperture size 56 m by 50 m for the ir beam allowed optically clear regions of the quartz grains to be analyzed without any contribution from grain boundaries the smallest dimension of the quartz ribbons is 0 3 mm whereas the pegmatitic quartz has a grain size of 3 to 5 cm results show mylonitic quartz ribbons contain the most water 320 h 10 6 si average range of 50 to 1120 h 10 6 si pegmatite quartz contains much less water 30 h 10 6 si average range of 20 40 h 10 6 si and the gneissic quartz contained an intermediate amount 200 h 10 6 si average range of 20 to 870 h 10 6 si these data indicate that water was preferentially incorporated into the deformed quartz ribbons

the simpson strzelecki and tirari deserts development and sand provenance abstract sands of the simpson strzelecki and tirari deserts central australia can be divided into two main groups on the basis of their physical and chemical characteristics colour grainsize heavy minerals quartz oxygen isotope composition zircon u pb ages the first group encompasses the strzelecki tirari and southeastern simpson deserts while the second occupies the northern and the western simpson desert the boundary between the two groups corresponds approximately to the northern most extent of the kallakoopah lakes several lines of evidence suggest derivation of the sands mainly from local bedrock with very little subsequent aeolian transport ultimate protosources for the sands in order of importance are for the southeastern simpson tirari and strzelecki deserts the tasman orogenic system new england and lachlan fold belts georgetown inlier musgrave and arunta blocks gawler and curnamona cratons and for the north and western simpson desert arunta musgrave and mount isa blocks and tennant creek inlier sediment from the tasman orogenic system includes an additional exotic component from palaeozoic sediments probably derived mainly from antarctica sediment transport from these protosources across the several hundred kilometres to the surficial sedimentary basins was dominantly by fluvial not aeolian means quaternary aeolian transport or reworking has been minimal serving only to form the dunes by vertical corrasion of underlying sedimentary rocks or residual products of local basement weathering the deserts have received some recent localised sediment input from modern fluvial systems

the sand seas of titan cassini radar observations of longitudinal dunes the most recent cassini radar images of titan show widespread regions up to 1500 kilometers by 200 kilometers of near parallel radar dark linear features that appear to be seas of longitudinal dunes similar to those seen in the namib desert on earth the ku band 2 17 centimeter wavelength images show 100 meter ridges consistent with duneforms and reveal flow interactions with underlying hills the distribution and orientation of the dunes support a model of fluctuating surface winds of 0 5 meter per second resulting from the combination of an eastward flow with a variable tidal wind the existence of dunes also requires geological processes that create sand sized 100 to 300 micrometer particulates and a lack of persistent equatorial surface liquids to act as sand traps

how do longitudinal dunes respond to climate forcing insights from 25 years of luminescence dating of the australian desert dunefields abstract this paper presents a meta analysis of 689 luminescence age estimates of australian desert sand dunes this analysis had two aims to examine the hypothesis that quaternary climate changes have forced dune accumulation and to understand longitudinal dune behaviour from the age architecture of sand dunes for which stratigraphic information is often absent or poorly known a novel approach to the analysis of probability density curves of dune age frequency shows that in the mallee dunefield of southeastern australia only the period leading up to and including the last glacial maximum lgm 18 34 ka experienced marked dune growth which cannot be explained by random stochastic activity during the lgm most but not all mallee longitudinal dunes were accumulating more rapidly than before and after in the strzelecki dunefield source bordering sbd longitudinal dune tl n 61 and osl n 40 ages are more frequent at the pleistocene holocene transition but nearby non sbd longitudinal dunes n 21 ages were only active above background levels during the lgm however the strzelecki dune age data set suffers from being drawn from a small number of individual dunes 10 sbd dated using tl 12 sbd dated by osl 7 non sbd dated by osl with a marked bias away from surficial young and basal old depths even during the lgm many dunes do not record active growth and the dunefield was most likely a mosaic of stable dune surfaces and bare mobile patches supported by the scant pollen data from the arid zone hyper aridity has not been experienced in these dunefields these vegetated dunes exhibit conservative behaviour with limited response to the range of climate forcing experienced in the late pleistocene a more complete understanding of dunefield response to quaternary climates would require an intensive and structured dating program to overcome the noise from stochastic processes leading to small scale activity in these dunes and the simultaneous removal of parts of the sedimentary record

evolution in h 2 o contents during deformation of polycrystalline quartz an experimental study shear experiments were performed in a griggs type apparatus at 800 c and 1 5 gpa at a strain rate of 2 1 10 5s 1 using different starting materials i powder grain size 6 10 m of dry brazil quartz with 0 15 wt added h2o ii dry brazil quartz porphyroclasts grain size 100 200 m devoid of fluid inclusions embedded in the same fine grained powder and iii wet porphyroclasts grain size 100 200 m containing initially a high density of m scale fluid inclusions embedded in the same powder after hot pressing samples were deformed to large shear strains 3 to 4 5 in order for the microstructures and h2o distribution to approach some state of equilibrium the h2o content and speciation in quartz were analyzed by fourier transform infra red ftir spectroscopy before and after the experiments mechanical peak strength is generally lower in experiments with 100 hydrated matrix intermediate in experiments incorporating wet porphyroclasts with a proportion of 30 or 70 and highest in those with dry porphyroclasts all experiments with porphyroclasts show pronounced strain weakening and the strengths of most samples converge to similar values at large strain wet porphyroclasts are pervasively recrystallized during deformation while dry porphyroclasts recrystallize only at their rims and remain weakly deformed recrystallization of the initially fluid inclusion rich porphyroclasts results in a decrease in inclusion abundance and total h2o content while h2o content of initially dry clasts increases during deformation h2o contents of all high strain samples converge to similar values for matrix and recrystallized grains in samples with wet porphyroclasts shear bands with high porosity and fluid contents develop and they host the precipitation of euhedral quartz crystals surrounded by a free fluid phase these high porosity sites are sinks for collecting h2o in excess of the storage capacity of the grain boundary network of the recrystallized aggregate the h2o storage capacity of the grain boundary network is determined as a h2o boundary film of 0 7 nm thickness

age and dynamics of linear dunes in the namib desert bristow c s duller g a t lancaster n 2007 age and dynamics of linear dunes in the namib desert geology 35 6 555 558 sponsorship american chemical society

evidence for indurated sand dunes in the martian north polar region 1 we investigate the relevance of induration as an explanation for the occurrence of dunes of unusual morphology in the martian north polar region the evidence for induration of aeolian deposits on mars is reviewed an explanation for rounded elongated barchans in chasma boreale is presented the role played by induration in the formation of linear dunes is discussed our arguments are supported by the results of a computer simulation model

evolution of c axis pole figures and grain size during dynamic recrystallization results from experimentally sheared quartzite 1 quartzite samples experimentally sheared at conditions where subgrain rotation and grain boundary migration recrystallization are active show a striking change in the c axis crystallographic preferred orientation cpo with increasing shear strain up to 8 and degree of recrystallization up to 100 we used optical methods to determine the c axis cpo for the bulk samples and for porphyroclasts and recrystallized grains separately and to track the strengths of the cpos in five different orientation domains with increasing the c axis pole figure evolves from a broad peripheral maximum indicative of basal a slip to an inclined single girdle with two maxima indicative of rhomb a slip and finally an elongate single maximum at the girdle center indicative of prism a slip throughout this sequence the fabric skeleton rotates with the sense of shear the bulk preferred c axis orientation results from growth and shrinkage of differently oriented crystallographic domains grain boundary density analysis indicates that the size of prism a recrystallized grains is 1 5 times that of average and 2 times that of rhomb grains indicating that prism a slip is significantly easier at our experimental conditions this inference is supported by autocorrelation function analysis of the various orientation domains and how they evolve with strain similar progressive changes in c axis cpo are observed in quartzites naturally sheared at conditions where grain boundary migration recrystallization operates these results indicate that achievement of microstructural and mechanical steady state requires high strain and complete recrystallization

fluid circulation in the depths of accretionary prisms an example of the shimanto belt kyushu japan accretionary prisms constitute ideal targets to study fluid circulation and fluid rock interactions at depths beyond the reach of active margin deep drilling the highest grade rocks from the shimanto belt on kyushu were buried under 3 5 kbars at 300 c toriumi and teruya 1988 they contain abundant quartz veins formed throughout burial and exhumation and variably affected by brittle and ductile deformation cathodoluminescence cl reveals the existence of two distinct types of quartz characterized by a blue and brown color respectively cl blue quartz fills macro veins width 10 m while cl brown quartz is present in micro veins width 1 10 m and ductilely recrystallized domains on the basis of microstructures the fluids associated with the cl blue and cl brown quartz are interpreted as external and local respectively quartz growth rims of alternating cl colors as well as mutually cross cutting veins show that the two fluids cyclically wetted the host rock from fluid inclusions analysis the fluid associated with cl blue quartz has a salinity similar to seawater while the fluid associated with cl brown quartz is less saline in addition cl blue quartz is richer in aluminum than the cl brown one in contrast to the salinity aluminum signature the 18o isotopic signature of both quartz types is similar and buffered by host rock the difference between the preservation of the salinity signature of the fluid and the loss of its 18o signature is explained by quicker exchange kinetics and larger host rock buffering capacity for isotopic reequilibration the local fluid associated with cl brown quartz reflects the dilution of pore water by the pure water produced by prograde dehydration reactions of clay minerals the external fluid associated with cl blue quartz is interpreted as seawater or pore water from shallow depth 1 2 km below seafloor sediments we propose that downward percolation of shallow water to depths 10km is a transient process associated with mega earthquakes

aeolian fluvial interaction evidence for late quaternary channel change and wind rift linear dune formation in the northwestern simpson desert australia in central australia the most easterly extent of the macdonnell ranges border the simpson desert dunefield where widely spaced strike ridges intercept and isolate pockets of broad crested linear dunes that reflect regional changes in late quaternary climate flow regime and channel avulsion an energetic todd river reworked the eastern part of camel flat basin from 75 65 ka until the last glacial maximum lgm when it shifted eastwards but with some flows persisting through the basin until about 10 ka resulting desert surfaces of different age facilitate temporal comparisons of linear dune formation fine grained red dunes 75 65 ka in age occur on the western floor of the basin and are ramped against the foot slopes of the range after the lgm and especially during the holocene the river s departure enabled small pale coloured closely spaced coarser textured linear dunes to form on the abandoned floodplain in the eastern basin their orientation 20 farther west than the larger and older red dunes this realignment indicates that the australian wind whorl shifted southwards some 160 km or 1 5 after the lgm linear dunes in the northwestern simpson desert were formed by wind rifting involving vertical accretion of sand from a proximal source not by long distance sand transport with linear extension the blocking ranges have caused negligible downwind sediment accumulation over the past 75 ka

dislocation creep of dry quartz small scale shear zones within the permian truzzo meta granite developed during the alpine orogeny at amphibolite facies conditions in these shear zones magmatic quartz deformed by dislocation creep and recrystallized dynamically by grain boundary migration with minor subgrain rotation recrystallization to a grain size of around 250 750 m consistent with flow at low differential stresses fourier transform infrared ftir spectroscopy reveals very low water contents in the interior of recrystallized grains in the form of discrete oh peaks 20 h 10 6 si and very little broad band absorption 100 h 10 6 si the spectral characteristics are comparable to those of dry brazil quartz in ftir spectra magmatic quartz grains show a broad absorption band related with high water concentrations only in those areas where fluid inclusions are present while other areas are dry drainage of fluid inclusions and synkinematic growth of hydrous minerals indicates that a hydrous fluid has been available during deformation loss of intragranular water during grain boundary migration recrystallization did not result in a microstructure indicative of hardening these ftir measurements provide the first evidence that quartz with extremely low intragranular water contents can deform in nature by dislocation creep at low differential stresses low intragranular water contents in naturally deformed quartz may not be necessarily indicative of a high strength and the results are contrary to implications taken from deformation experiments where very high water contents are required to allow dislocation creep in quartz it is suggested that dislocation creep of quartz in the truzzo meta granite is possible to occur at low differential stresses because sufficient amounts of intergranular water ensure a high recovery rate by grain boundary migration while the absence of significant amounts of intragranular water is not crucial at natural conditions

extraterrestrial dunes an introduction to the special issue on planetary dune systems abstract aeolian dune fields have been described on earth mars venus and titan the plethora of data returned from recent planetary missions has enabled a new era in planetary geomorphic studies much of our understanding of planetary dune systems comes from the application of earth analogs wind tunnel experiments and modeling studies despite the range of atmospheric pressures composition and gravity many of the dune forms on extraterrestrial surfaces are similar to those on earth although some have notable differences in bedform scale and composition as an introduction to the special issue on planetary dune systems this paper summarizes the current state of knowledge of planetary dune studies and highlights outstanding questions that require further investigation

a mechanism for preferential h 2 o leakage from fluid inclusions in quartz based on tem observations preferential leakage of h2o from fluid inclusions containing multiple gas components has been suspected in natural metamorphic rocks and has been demonstrated experimentally for synthetic h2o co2 rich inclusions in natural quartz knowledge of the physical and chemical characteristics of the leakage mechanism which may be very complex increases the value of natural fluid inclusions to metamorphic geology it is proposed that crystal defects play a major role in nondecrepitative preferential h2o leakage through quartz and remain effective during metamorphism inclusions with either an internal overpressure or underpressure produce strain in the adjacent quartz crystal via the nucleation of many dislocations and planar defects like dauphine twin boundaries these defects allow preferential loss of h2o from h2o co2 rich inclusions at supercritical conditions the transport capacity of this leakage mechanism is enhanced by nucleation of small bubbles on defect structures the nucleation of these bubbles seems to be a recovery process in strained crystals solubility gradients of quartz in water in a crystal with internally underpressurized inclusions may result in optical visible implosion halos in a three dimensional spatial arrangement caused by the growth of small bubbles at the expense of the larger original fluid inclusion natural fluid inclusions from naxos greece are always associated with numerous interlinked dislocations these dislocations may have been produced by plastic derormation or by crystal growth related processes e g crack healing the presence of small bubbles on these dislocations indicates that a similar leakage mechanism for h2o must have occurred in these rocks

solubility of hydroxyl in natural quartz annealed in water at 900 c and 1 5 gpa we have measured the infrared spectra of a natural quartz after annealing it for up to 37 hours in water at 900 1000 c and 1 0 1 5 gpa under chemical conditions buffered through a hydrogen membrane with an assemblage of water nickel metal and nickel oxide from integral absorbances in the 3400 and 2500 cm 1 regions we calculated the concentration of hydrogen and deuterium present in the samples as hydroxyl concentrations reported in this work are calibrated to the deuterium content of a sample annealed 37 hours in d2o18 as determined by nuclear reaction analysis hydrothermal annealing increased the hydroxyl content of natural quartz a 1 from 26 to 75 11 h 106si absorption increased by growth of o h bands it 3318 3483 and 3432 cm 1 associated with a13 samples annealed in d2o18 showed an increase in bands near 2500 cm 1 associated with o d and a13 an o h band in the starting material at 3485 cm 1 associated with li was absent from the spectra of annealed samples fractured samples were characterized by the appearance of a sharp band at 3580 cm 1 and absorption several times larger than unfractured samples microscopic examination of fractured samples showed that they contained water inclusions and healed fractures previous hydroxyl solubility measurements made on fractured samples may be too large due to hydroxyl contamination along fractures at 900 c experiments equilibrated in times consistent with a diffusivity for hydrogen impurity in quartz of 10 7 cm s

multiple origins of linear dunes on earth and titan the formation of dunes is controlled by the direction of the prevailing winds and the characteristics of the sediments linear dunes in the qaidam basin china are shown to form from cohesive sediments a model that could be applicable to dunes on titan

morphodynamics boundary conditions and pattern evolution within a vegetated linear dunefield abstract the controls on the evolution of linear dunefields are poorly understood despite the potential for reactivation of dunefields which are currently stabilized by vegetation under the influence of 21st century climate change the relative roles of local influences i e boundary conditions and morphodynamic influences i e emergent properties remain unclear chronostratigraphic and sedimentological analyses were conducted on two pairs of linear dunes exhibiting different spatial patterning in the strzelecki desert of central australia it was hypothesized that morphodynamic influences via pattern coarsening would mean that dunes from the simpler pattern defined in terms of the frequency of defects i e junctions and terminations would be more mature older landforms optically stimulated luminescence osl dating of full depth regularly sampled profiles was used to establish accumulation histories for the four dunes and supported by sedimentological analysis to investigate possible compositional differences and similarities between the dunes whilst three of the dunes the two more simply patterned dunes and one of the more complex dunes have accumulation histories beginning between 100 ka and 150 ka and document sporadic net accumulation throughout the last interglacial glacial cycle to the late holocene one of the dunes with relatively complex patterning reveals that the majority of the dune accumulation 7 m at that site occurred during a relatively short window at 50 ka there is no clear sedimentological reason for the different behaviour of the younger dune the data suggest that small scale and essentially stochastic nature of the aeolian depositional erosional system can overprint any large scale morphodynamic controls the concept of dating landscape change by pattern analysis is thus not supported by this study and would require very careful interpretation of the scales being considered this further suggests caution when interpreting dune chronostratigraphies palaeoenvironmentally as different dunes are able to respond very differently to the same external stimulus e g climate in the case studied here a mechanism is proposed to account for the rapid accumulation of the anomalous dune by avulsion of the local aeolian accumulation from one dune ridge to another

degradation of linear dunes in northwest ngamiland botswana and the implications for luminescence dating of periods of aridity the long morphodynamic history of the linear dune system of northwest ngamiland comprises both construction and degradation processes here we describe features resulting from the latter crest lowering during wet periods is associated with lateral transfer of sand into the straats weathering and leaching have mobilised iron to the base of the dune profiles and have corroded the quartz grains satellite image interpretation indicates that dune sand has been diluted by bioturbation which transferred damaran saprolite from below the dunes to the surface flooding and waterlogging have modified and in places completely destroyed the original crisply linear morphology small linear collapse features across the linear dunes are associated with open fractures or faults where water infiltration is facilitated the implications for luminescence dating of dune sands to identify periods of aridity are discussed the dunes cannot simply be regarded as providing a stacked record of such periods they are very old and much of the record is missing as a result of crest lowering the chronostratigraphy includes degradational components which must be distinguished from those of construction bioturbation cannot be disregarded as a complication which overprints the luminescence signals

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

textural evolution of synthetic fluid inclusions in quartz during reequilibration with applications to tectonic reconstruction experimental reequilibration of synthetic 10 wt nacl h2o inclusions in natural quartz reveals that reequilibration textures show distinct differences depending upon the p t path followed by the inclusion after formation these differences combined with other geological information may be used to determine whether the sample rock followed a dominantly isothermal or isobaricp t path following entrapment the intensity and style of inclusion reequilibration features is related to the direction and magnitude of the departure of thep t path from the original isochore for the inclusion thus fluid inclusion reequilibration textures not only permit inclusionists to determine whether the rocks followed an isothermal or isobaric retrogradep t path but also the magnitude of departure of this path from one that is isochoric

the influence of stress history on the grain size and microstructure of experimentally deformed quartzite deformation of middle crustal shear zones likely varies with time as a result of the stress build up and release associated with earthquakes and post seismic deformation but the processes involved and their microstructural signature in the rock record are poorly understood we conducted a series of experiments on quartzite at 900 c to characterize microstructures associated with changes in stress and strain rate and to investigate the feasibility of carrying out grain size piezometry in natural rocks that experienced analogous changes differential stress referred to simply as stress was varied in two stage experiments by changing strain rate and by stopping the motor and allowing stress to relax the two stage samples preserve a microstructural record that can be interpreted quantitatively in terms of stress history the microstructure associated with a stress increase is a bimodal distribution of recrystallized grain sizes the smaller grains associated with the second deformation stage accurately record the stress of the second stage and the surviving coarse grains remain similar in size to those formed during the earlier stage the transient microstructure associated with stress decrease is a partial foam texture containing a larger concentration of stable 120 triple junctions than occur in samples deformed at a relatively constant strain rate our results indicate that microstructures preserved in rocks that experienced relatively simple two stage deformation histories can be used to quantitatively assess stress histories grain growth rates during deformation are similar to rates observed in previous isostatic growth experiments supporting theoretical approaches to recrystallized grain size such as the wattmeter theory austin and evans 2007 that incorporate static growth rates from an analysis of the experimental data for quartz recrystallized grain size we find 1 recrystallized grain size quickly reaches a value consistent with ambient deformation conditions we argue that this explains a good match between average grain sizes predicted by the wattmeter after complete recrystallization and the recrystallized grain sizes of the experiments 2 the present formulation of the wattmeter overestimates the rates at which porphyroclasts recrystallize by as much as an order of magnitude and 3 owing to problems with extrapolation of grain growth data for quartz the wattmeter is not presently applicable to natural samples deformed at low temperatures we present a simplified flow law for quartz and suggest that the change in slope of the quartz piezometer at high stress regime 1 is related to a switch to a linear viscous rheology

tidal winds on titan caused by saturn abstract the influence of saturn s gravitational tide on the atmosphere of titan is investigated by means of a three dimensional general circulation model titan s orbital eccentricity of 0 0292 gives rise to time dependent radial and librational tide whose potential circles eastward on titan unlike atmospheric tides on terrestrial planets saturn s tide on titan has a large impact on the dynamic meteorology down to the surface the surface pressure oscillates by up to 1 5 hpa through the orbit near the surface the tidal wind dominates the atmospheric flow and exhibits strong temporal and spatial variation the superposition of the annually present thermally forced latitudinal pressure gradient and tidally caused pressure variation produces a unique wind pattern near the surface characterized by equatorward flow and high latitude whirls at higher levels the tidal wind manifests itself as eastward traveling planetary scale wave of wavenumber 2 superposed on the background wind in general tidal winds are more significant in the troposphere where other forcing mechanisms are weak meridional tidal winds become as fast as 5 m s 1 in the troposphere and change direction periodically through the orbit and along the parallel of latitude except in the lower troposphere zonal winds always remain prograde because the tidal wind amplitude is usually smaller than the mean zonal wind the tide also has a large impact on the mean zonal circulation in the stratosphere a meridional drift of the descending huygens probe in the troposphere would be the easiest way to verify the tidal wind on titan but more complete observations of tropospheric wind and surface pressure by a future mission would be required to unveil the complete details of the tidal wind