recent formation of arroyos in the little missouri badlands of southwestern north dakota abstract in the little missouri badlands of southwestern north dakota the channels of ephemeral streams are incised 2 to 10 m or more into mid to late holocene alluvium the objectives of this study were to determine the timing and cause s of the most recent episodes of fluvial incision and to develop a process response model that illustrates the formation and evolution of arroyos in this region the purpose was to distinguish natural from anthropogenic changes to the landscape and to discriminate allogenic from autogenic causes of incision thereby gaining a greater sense of how steep relatively small ephemeral streams evolve dendrochronologic and dendrogeomorphic analyses of riparian cottonwoods provide an inexpensive high resolution dating method to constrain the time of incision thereby permitting determination of the cause s of incision by evaluating environmental conditions prior to and at the onset of fluvial incision an examination of seven small 10 to 100 km2 drainage basins indicated ephemeral streams have undergone a four stage cycle of change within the past 200 years comprising i an initial period of relative geomorphic stability with pedogenesis on the flood plain and low rates of lateral channel migration ii a period of channel incision with subsequent widening of the flood plain through lateral corrasion along middle and upstream reaches iii a concomitant period of aggradation along downstream reaches and finally iv a period of downstream incision dendrochronologic data and dendrogeomorphic relations indicate there have been three distinct periods of fluvial incision in the past 200 years the first period of incision began in the 1860s and 1870s prior to the onset of european settlement and intensive grazing by domesticated cattle in the area this period of incision occurred along the middle reaches of all seven of the streams examined and coincided with a severe protracted drought suggesting an allogenic cause the second period of incision occurred during the end of the 19th century and into the 20th century along the upstream reaches of streams this period of incision was mentioned in some written accounts of early settlers and coincided with the introduction of cattle and historical overgrazing of the region however overgrazing may not have caused the incision of this period but instead exacerbated or accelerated the upstream migration of knickpoints formed earlier along middle reaches a third period of incision has occurred in the past 20 years along the lower reaches of two of the seven streams examined this incision was caused by the process of meander cutoff this autogenic process decreases sinuosity and increases channel slope stream power and the potential to incise along adjacent stream reaches

the influence of diet on the δ13c of shell carbon in the pulmonate snail helix aspersa abstract the influence of diet and atmospheric co2 on the carbon isotope composition of shell aragonite and shell bound organic carbon in the pulmonate snail helix aspersa raised in the laboratory was investigated three separate groups of snails were raised on romaine lettuce c3 plant 13c 25 8 corn c4 plant 13c 10 5 and sour orange 12c enriched c3 plant 13c 39 1 the isotopic composition of body tissues closely tracked the isotopic composition of the snail diet as demonstrated previously however the isotopic composition of the acid insoluble organic matrix extracted from the aragonite shells does not track diet in all groups in snails that were fed corn the isotopic composition of the organic matrix was more negative than the body by as much as 5 whereas the matrix was approximately 1 heavier than the body tissues in snails fed a diet of c3 plant material these results indicate that isotopic composition of the organic matrix carbon cannot be used as an isotopic substrate for paleodietary reconstructions without first determining the source of the carbon and any associated fractionations the isotopic composition of the shell aragonite is offset from the body tissues by 12 3 in each of the culture groups this offset was not influenced by the consumption of carbonate and is not attributable to the diffusion of atmospheric co2 into the hemolymph the carbon isotopic composition of shell aragonite is best explained in terms of equilibrium fractionations associated with exchange between metabolic co2 and hco3 in the hemolymph and the fractionation associated with carbonate precipitation these results differ from previous studies based primarily on samples collected in the field that have suggested atmospheric carbon dioxide contributes significantly to the shell 13c the culture results indicate that the 13c of aragonite is a good recorder of the isotopic composition of the snail body tissue and therefore a better recorder of diet than is the insoluble shell organic carbon because the systematic fractionation of carbon isotopes within the snail is temperature dependent the 13c of the shell could provide an independent technique for estimating paleotemperature changes

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

rheology of the earth s mantle a historical review abstract historical development of our understanding of rheological properties of the earth s mantle is reviewed rheological properties of the earth s mantle control most of the important geological processes such as the style of mantle convection e g stagnant lid versus plate tectonics and the nature of thermal evolution however inferring the rheological properties of the earth s mantle is challenging because of the presence of multiple mechanisms of deformation that have different dependence on time scale strain rate stress levels and other parameters through the integration of a broad range of observations including the elastic stiffness from tidal deformation and the viscosity from the concept of isostasy a gross picture of rheological stratification of the earth s mantle a strong lithosphere a weak asthenosphere and a strong layer below was proposed in the mid 19th century however not only the physical basis for such a model was weak due to the lack of proper understanding of some materials science issues such as the interplay between elastic and viscous deformation but also the lack of understanding of temperature depth relation associated with convection prevented our understanding of the rheological structure of the earth s interior major progress occurred in the first half of the 20th century in our understanding of the atomistic mechanisms of plastic deformation in solids and much of the theoretical framework on the plastic deformation of solids was established by late 1960s those developments provided a basis for scaling analyses that are critical to the applications of laboratory results to the earth s interior major progress in laboratory studies on rheological properties occurred in the mid 1960s to the early 1970s in griggs lab using a new type of solid medium high pressure deformation apparatus to pressure 2 gpa and temperature 1600 k the basic concepts such as the water weakening non linear rheology and deformation induced lattice preferred orientation were identified by their studies however large uncertainties in the stress measurements with this type of apparatus were recognized in the late 1970s and high resolution experimental studies using synthetic samples were initiated in paterson s lab in the mid 1980s using a high resolution gas medium deformation apparatus however experimental studies with such a gas medium deformation apparatus can be conducted only at low pressures

the olivine spinel transformation and the rheology of subducting lithosphere during subduction relatively hot lithospheric slabs are expected to deform in a superplastic flow regime below a depth of 300 km if olivine reacts to fine grained spinel this process reduces the flow strength of such slabs below the phase boundary so that they become aseismic at about this depth in colder slabs temperatures are too low for superplastic deformation and a high flow strength and seismic activity are maintained to a depth of 600 700 km

si and o diffusion in mg fe 2sio4 wadsleyite and ringwoodite and its implications for the rheology of the mantle transition zone si and o diffusion rates on polycrystalline mg fe 2 sio 4 wadsleyite and ringwoodite have been determined at pressures between 16 and 22 gpa and temperatures between 1400 and 1600 c using a kawai type multi anvil high pressure apparatus pre synthesized polycrystalline wadsleyite or ringwoodite was used as starting materials diffusing sources of 29 si and 18 o enriched mg fe 2 sio 4 thin film were deposited on the surface of wadsleyite and ringwoodite by pulsed laser deposition the diffusion profiles were obtained using secondary ion mass spectrometry in the depth profiling mode all measured diffusion profiles were composed of volume and grain boundary diffusion regimes arrhenius relations for volume and grain boundary diffusion rates of si and o in wadsleyite and ringwoodite have been determined the results show that si is the slowest diffusing element among the major elements in both wadsleyite and ringwoodite under mantle transition zone as well as slab conditions therefore si should be the rate controlling species for high temperature creep in wadsleyite and ringwoodite comparisons with si and o diffusion rates obtained in this study and those in olivine and silicate perovskite suggest that si and o diffusion rates are enhanced at both 410 and 660 km seismic discontinuities the deformation mechanism maps constructed from si diffusion data in wadsleyite and ringwoodite suggest that the dominant deformation mechanism operating in the mantle transition zone is dislocation creep which may be the origin of the observed global seismic anisotropy under cold subduction zone conditions grain size sensitive diffusion creep becomes dominant when the grain size is reduced to less than 10 100 m below the metastable olivine wedge

reaction enhanced ductility the role of solid solid univariant reactions in deformation of the crust and mantle abstract in collision belts and subduction zones penetrative deformation of thrust sheets may be catalyzed by metamorphic reactions if the reactions have a significant effect on the mechanical properties of rocks reaction enhanced ductility may result if a reaction produces a significant decrease in grain size thus allowing the rock to deform superplastically the product grain size of a reaction is a function of the ratio of nucleation and growth rates this ratio varies along any pressure temperature time ptt path and only attains a significant value at a certain distance from an equilibrium boundary a comparison of rates of change of p and t along typical segments of ptt paths for crustal rocks leads to the conclusion that a high nucleation rate a small grain size and resulting superplastic deformation are more likely to be achieved when reaction boundaries are crossed during underthrusting rather than during the thermal readjustments of a perturbed geotherm following crustal thickening or during uplift an example of reaction enhanced ductility occurs in eclogite facies metagranitoids in the sesia lanzo zone western alps where medium grained plagioclase reacted to fine grained jadeite quartz during early alpine underthrusting microstructural evidence from shear zones shows that the jadeite quartz aggregates were highly ductile and it is inferred that the small grain size allowed superplastic deformation further evidence suggests that this process operated during regional deformation of metagranitoids rubie 1982 the reaction of olivine to spinel or the beta phase by metastable overstepping of the two phase stability fields in a subducting plate ringwood 1975 may result in fine grained products and thus may be analogous to the albite jadeite quartz reaction the consequent decrease in effective viscosity is likely to have important consequences for plate dynamics at and below the depths of major phase transitions

nomad rover field experiment atacama desert chile 1 science results overview nomad was deployed for a 45 day traverse in the atacama desert chile during the summer of 1997 during this traverse 1 week was devoted to science experiments the goal of the science experiments was to test different planetary surface exploration strategies that included 1 a mars mission simulation 2 a science on the fly experiment where the rover was kept moving 75 of the operation time the goal of this operation was to determine whether or not successful interpretation of the environment is related to the time spent on a target the role of mobility in helping the interpretation was also assessed 3 a meteorite search using visual and instrumental methods to remotely identify meteorites in extreme environments and 4 a time delay experiment with and without using the panospheric camera the results were as follow the remote science team positively identified the main characteristics of the test site geological environment the science on the fly experiment showed that the selection of appropriate targets might be even more critical than the time spent on a study area to reconstruct the history of a site during the same operation the science team members identified and sampled a rock from a jurassic outcrop that they proposed to be a fossil the presence of paleolife indicators in this rock was confirmed later by laboratory analysis both visual and instrumental modes demonstrated the feasibility in at least some conditions of carrying out a field search for meteorites by using remote controlled vehicles finally metrics collected from the observation of the science team operations and the use team members made of mission data provided critical information on what operation sequences could be automated on board rovers in future planetary surface explorations

fast rise times and the physical mechanism of deep earthquakes earthquakes at depths of 300 km are similar to shallower events in that they are dominantly of double couple character1 implying that shearing motion has taken place at depth but because increased friction at these high pressures inhibits brittle fracture2 3 various other mechanisms related to phase transformations have been invoked to explain the occurrence of deep earthquakes4 10 as yet however no consistent differences have been found between the source characteristics of deep 300 km and intermediate depth 300km earthquakes2 11 13 here we report a systematic global survey of the rise times and stress drops of deep and intermediate earthquakes the rise time is defined as the time from rupture initiation to peak moment release rate when the rise times are scaled to the seismic moment release of the events their average is nearly twice as fast for events deeper than 450 km as for shallower events this difference may ultimately provide an experimental means of testing proposed mechanisms for the generation of deep seismicity

stable carbon isotope record of middle to late holocene climate changes from land snail shells at hinds cave texas abstract stable carbon isotope ratios of shell carbonate and shell organic matter from land snails rabdotus alternatus from hinds cave texas are used to reconstruct the climate history of the drier western part of the southern great plains present mean annual rainfall of 400 mm over the last ca 5000 yr ams radiocarbon analyses of four individual shells from each of two strata indicate that there are no problems of reworking or age mixtures within strata however burning is evident in many shells amino acid composition and racemization were used to identify shells that had been significantly heated and these were rejected for isotopic analysis because of possible alteration of the isotopic signal fourteen samples of snails from hinds cave each consisting of 2 6 individual shells from the same provenience were radiocarbon dated by ams and analyzed for stable carbon isotopes in addition a sample collected alive ca ad 1890 from a nearby site was analyzed to represent modern conditions the shells show no decrease in organic content with age indicating good preservation of shell organic matter both the organic and carbonate isotope values show similar temporal trends the oldest samples 4100 4600 cal yr bp are relatively depleted in 13 c maximum 13 c values are reached at ca 3500 cal yr bp thereafter there is a regular depletion in 13 c with the recent sample showing the lowest 13 c value the change in the isotopic values over time is quite large range of 4 5 for organic matter and indicates substantial climatic changes the variations in isotopic values are interpreted as resulting primarily from changes in moisture conditions affecting the abundance of drought adapted cam crassulacean acid metabolism plant species enriched in 13 c thus relatively moist mid holocene conditions are indicated with increasingly dry conditions reaching a maximum at ca 3500 cal yr bp followed by progressively moister conditions through the late holocene

succession and stable isotope composition of gastropods in holocene semi arid alluvial sequences bardenas reales ebro basin ne spain palaeoenvironmental implications three alluvial units ranging from the middle to late holocene in age were distinguished in bardenas reales natural park ebro basin ne spain based on morphosedimentary analysis and radiocarbon data in this sensitive mediterranean alluvial system the succession and isotopic composition of gastropods greatly contribute to establishing the prevailing palaeoenvironmental conditions the oldest unit 4763 87 to 2848 55 cal yr bp represents fluvial channels and swamped areas includes the greatest proportion of aquatic gastropods ancylus fluviatilis and lymnaea truncatula and marsh species vallonia pulchella and presents the most negative values of 18o mean 2 21 pdb the intermediate unit 1403 60 to 836 65 cal yr bp shows a high rate of alluvial activity related to irregular torrential rainfall with sedimentary features of aridity and this unit includes only dry open ground gastropods xeroplexa sp chondrinidae indet and granopupa granum and presents the least negative values of 18o

climate change on the yucatan peninsula during the little ice age we studied a 5 1 m sediment core from aguada x caamal 20 36 6 n 89 42 9 w a small sinkhole lake in northwest yucatan mexico between 1400 and 1500 a d oxygen isotope ratios of ostracod and gastropod carbonate increased by an average of 2 2 and the benthic foraminifer ammonia beccarii parkinsoniana appeared in the sediment profile indicating a hydrologic change that included increased lake water salinity pollen from a core in nearby cenote san jose chulchaca showed a decrease in mesic forest taxa during the same period oxygen isotopes of shell carbonate in sediment cores from lakes chichancanab 19 53 0 n 88 46 0 w and salpeten 16 58 6 n 89 40 5 w to the south also increased in the mid 15th century but less so than in aguada x caamal climate change in the 15th century is also supported by historical accounts of cold and famine described in maya and aztec chronicles we conclude that climate became drier on the yucatan peninsula in the 15th century a d near the onset of the little ice age lia comparison of results from the yucatan peninsula with other circum caribbean paleoclimate records indicates a coherent climate response for this region at the beginning of the lia at that time sea surface temperatures cooled and aridity in the circum caribbean region increased

grain size evolution in subducted oceanic lithosphere associated with the olivine spinel transformation and its effects on rheology we investigate the role of grain size reduction during the olivine spinel transformation on rheological properties of subducting slabs on the basis of a scaling model for microstructural development during nucleation and growth in this model the size of spinel grains nucleating at olivine grain boundaries is controlled by the relative rates of nucleation and growth taking into account the impingement through the collision of grains due to growth when the volume fraction of spinel reaches a certain threshold value critical volume fraction 1 10 depending on the p t conditions in the slab the new phase will form a continuous film and will significantly reduce the strength of the two phase aggregate if spinel grain size is small the size of spinel grains d at this stage is calculated and is shown to be highly sensitive to 0 size and 2 a subducting slab has a complicated rheological structure containing a weak region below the tip of a metastable olivine bearing wedge in a cold slab possible implications of these anomalous rheological structures of slabs on the dynamics of subduction are discussed including the mechanisms of deep earthquakes

stable isotope δ18o δ13c and δd signatures of recent terrestrial communities from a low latitude oceanic setting endemic land snails plants rain and carbonate sediments from the eastern canary islands abstract stable isotopes of oxygen and carbon extracted from fossil land snail shells have been used increasingly to interpret past environments to evaluate the utility of this approach for low latitude oceanic islands populations of the modern helicid land snail theba geminata a species also abundant in the quaternary fossil record of the region were sampled at ten low altitude d and 2 v smow for 18 o ranging from 11 to 2 v smow and from 2 6 to 0 7 v smow respectively the local vegetation is heterogeneous including c 3 c 4 and cam plants 13 c values vary from 13 0 to 29 0 v pdb across plant species of the 24 species five are c 4 15 are c 3 and four are cam plants the 18 o values for shells represent a narrow range of values from 0 3 to 2 5 v pdb which is consistent with the low climate seasonality typifying low latitude oceanic settings hypothetical model of the expected 18 o value for shell aragonite precipitated in equilibrium suggests that the most negative 18 o shell represent the closest estimate for 18 o rain water the 13 c values of shells range from 9 4 to 1 7 v pdb the most positive 13 c values are attributed to a diet based on c 4 plants the comparison of 13 c values of soft tissues and shells suggests that snails ingested notable amounts from 20 up to 40 of foreign carbonates consequently fossil shells with the most negative 13 c values should be selected for radiocarbon dating in future geochronological studies of the region the 13 c values of body tissues vary from 12 0 to 27 2 v pdb indicating that land snails consumed c 3 and c 4 plants indiscriminately the mean carbon isotopic composition averaged across multiple fossil specimens may thus provide a useful tool for reconstructing paleoclimates and paleoenvironments throughout the quaternary history of the canary islands and other comparable low latitude oceanic settings

physical conditions producing slab stagnation constraints of the clapeyron slope mantle viscosity trench retreat and dip angles abstract recent seismic tomography has revealed various morphologies in the subducted lithosphere in particular significant flattening and stagnation of slabs around the 660 km boundary are seen in some areas beneath the northwestern pacific subduction zones we examined the cause of slab stagnation in terms of the clapeyron slope of the phase transformation from ringwoodite to perovskite magnesiowustite trench retreat velocity dip angles and high viscosity of the lower mantle based on two dimensional 2 d numerical simulations of thermal convection in particular we examined the conditions necessary for slab stagnation assuming a very small absolute value of the clapeyron slope which were proposed based on recent high pressure high temperature high p t experiments our calculations show that slabs tend to stagnate above the 660 km boundary with an increasing absolute value of the clapeyron slope viscosity jump at the boundary and trench retreat velocity and a decreasing initial dip angle stagnant slabs could be obtained numerically for a realistic range of parameters obtained from high p t experiments and other geophysical observations combining buoyancy high lower mantle viscosity and trench retreat we found that a low dip angle of a descending slab at the bottom of the upper mantle plays an important role in slab stagnation two main regimes underlie slab stagnation buoyancy dominated and viscosity dominated regimes in the viscosity dominated regime it is possible for slabs to stagnate above the 660 km boundary even when the value of the clapeyron slope is 0 mpa k

diffusion creep dislocation creep and mantle rheology abstract viscosity depth profiles in the earth s mantle are estimated on the basis of microrheological equations for ultrabasic silicates the preliminary reference earth model prem and two assumed geotherms one with and one without a lower mantle upper thermal boundary layer the estimation includes assessment of variations of creep parameters with depth both constant strain rate non linear viscosity and newtonian viscosity are in the range 1020 1021 pa s in the upper mantle in the lower mantle there is a moderate increase in viscosity with depth and upper limit estimates are of the order of 1022 and 1023 pa s respectively as uncertainties are one order of magnitude the newtonian viscosity mantle inferred from geophysical observations cannot be rejected on the strength of rheological considerations by the same token convection should be mantle wide as the viscosity layering is not sufficient to prevent it on the other hand if the mantle is chemically stratified and separate convective circulations exist in the upper and lower mantle the thermal boundary layer below the 670 km discontinuity should have a discernible viscosity signature

the influence of rheological weakening and yield stress on the interaction of slabs with the 670 km discontinuity results of high resolution seismic tomography showing subducting slabs deflected in the transition zone and thickened in the lower mantle seem to call for slab material weaker than inferred from mineral physics deformation mechanisms a possible mechanism suggested by several authors could be the weakening due to grain size reduction which should occur in the cold portion of fast slabs after an exothermic phase transition at a depth of 400 km since the amount of weakening as well as the rate of subsequent strengthening due to the grain growth are not precisely known we present here a parametric study of slab behavior in the transition zone and upper part of the lower mantle we simulate a subducting slab in a two dimensional 2 d cartesian box in the numerical model with composite rheology including diffusion creep dislocation creep and a general stress limiting rheology approximating peierl s creep we concentrate on two rheologic effects the dynamic effect of slab weakening due to grain size reduction at the phase boundary and the effect of yield stress of stress limiting rheology the effect of trench migration on slab deformation is also included in our study results show that the slab ability to penetrate into the lower mantle is not significantly affected by a trench retreat in the absence of grain size weakening however in case of a 4 cm yr trench retreat grain size weakening provides a viable mechanism to deflect the slab in the transition zone provided that stress limiting deformation mechanism would limit the effective viscosity outside the areas of grain size weakening to about 10 24 pa s 2002 elsevier science b v all rights reserved

the holocene fluvial chronology of spain evidence from a newly compiled radiocarbon database abstract a critical analysis of 74 radiocarbon dates selected from a total of 102 published and unpublished dates from holocene fluvial environments in spain has identified a number of periods of increased fluvial activity in spain namely 11 170 10 230 9630 8785 7980 6860 5800 4800 3880 3085 2895 1820 1300 0 cal bp the latter period reflecting sustained fluvial response to increased human impact the radiocarbon samples were classified according to type of depositional environment a alluvial overbank b flood basin c alluvial channel gravels d fluvio torrential deposits and e slackwater flood deposits palaeofloods dates from slackwater flood deposits indicated at least five phases of increased frequency of large magnitude floods during the holocene 10 855 10 230 9530 8780 2880 2430 975 790 and 520 265 cal yr bp flood basin type deposition occurred from 7980 to 4830 cal yr bp the main clusters of dates from alluvial floodplain deposits occurred at 2750 2150 and 930 520 cal yr bp the record is discussed in relation to other palaeoenvironmental archives including pollen records of holocene vegetation change and palaeoclimatic proxies such as the north atlantic drift ice record comparison with the latter indicates that major holocene flooding coincided with cold climate phases during 9530 9280 2880 2430 and 520 265 cal yr bp a cooling phase at 10 855 10 230 and warming phases at 9030 8780 and 975 790 cal yr bp

transformation shear instability and the seismogenic zone for deep earthquakes we use a numerical model for olivine spinel transformation to study deep earthquake nucleation and to delineate the seismogenic region within a subducting slab the model includes laboratory derived flow laws latent heat release and phase transformation kinetics we calculate deformation transformation state grain growth and rheology for several paths within a subducting slab strain rate perturbations are imposed to define the necessary conditions for instability strain rate perturbations decay for a critical value c and thus the coldest interior portion of the metastable wedge deforms stably for c strain rate perturbations grow shear strength decreases with strain and the system is potentially unstable the instability condition is mapped to delineate the seismogenic zone within a subducting slab the model seismogenic zone is bounded by c and at larger percent transformations by coarsening of spinel grains and saturation of the transformation weakening effect the model predicts a narrow seismogenic region along the outer edges of the metastable wedge and thus a double seismic zone consistent with some seismic observations several simplifying assumptions are required due to lack of thermo kinetic data and incomplete knowledge of constituent mineralogy and rheology however the model provides a quantitative definition of the instability condition and a framework for testing the hypothesis of transformation induced instability

the role of solid solid phase transitions in mantle convection abstract with changing pressure and temperature conditions downwelling and upwelling crustal and mantle rocks experience several solid solid phase transitions that affect the mineral physical properties owing to structural changes in the crystal lattice and to the absorption or release of latent heat variations in density together with phase boundary deflections related to the non null reaction slope generate important buoyancy forces that add to those induced by thermal perturbations these buoyancy forces are proportional to the density contrast between reactant and product phases their volume fraction the slope and the sharpness of the reaction and affect the style of mantle convection depending on the system composition in a homogeneous pyrolitic mantle there is little tendency for layered convection with slabs that may stagnate in the transition zone because of the positive buoyancy caused by post spinel and post ilmenite reactions and hot plumes that are accelerated by phase transformations in the 600 800 km depth range by adding chemical and mineralogical heterogeneities as on earth phase transitions introduce bulk rock and volatiles filtering effects that generate a compositional gradient throughout the entire mantle with levels that are enriched or depleted in one or more of these components phase transitions often lead to mechanical softening or hardening that can be related to a different intrinsic mechanical behaviour and volatile solubility of the product phases the heating or cooling associated with latent heat and the transient grain size reduction in downwelling cold material strong variations in viscosity would enhance layered mantle convection causing slab stagnation and plume ponding at low temperatures and relatively dry conditions reactions are delayed due to the sluggish kinetics so that non equilibrium phase aggregates can persist metastably beyond the equilibrium phase boundary survival of low density metastable olivine ringwoodite pyroxene and pyrope garnet in the transition zone and uppermost lower mantle produces positive buoyancy forces that decrease the subduction velocity and may lead to slab stagnation in the transition zone the presence of deep metastable portions is still debated and should not be associated a priori with a completely dry slab as field observations suggest that heterogeneously hydrated oceanic plates could contain metastable dry portions surrounded by transformed wet rocks

localized polymorphic phase transformations in high pressure faults and applications to the physical mechanism of deep earthquakes earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes such mechanisms in unmodified form are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300 700 km several alternative mechanisms have been proposed here i discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me these shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics 1 a single sharp shear zone forms in the maximum shear stress orientation 2 confining pressure does not suppress the instability and under some conditions raising confining pressure pc lowers the differential stress at which it occurs 3 stress drops can occur repeatedly after reloading at about the same differential stress 4 the mean stress vs temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice ii and diopside talc respectively although i have not as yet confirmed the existence of these high pressure phases in the shear zones transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non hydrostatically near transition boundaries moreover by analogy the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double couple first motion distribution finally if this is the earthquake source mechanism in deeply subducted lithosphere then earthquakes should shut off when all of the polymorphic phase changes have occurred this may explain the maximum depth of earthquakes at about 680 km

experimental deformation of a synthetic dunite at high temperature and pressure i mechanical behavior optical microstructure and deformation mechanism abstract we have performed a series of 27 deformation experiments on a very dry synthetic dunite using the griggs solid medium apparatus strain rates ranged from 1 10 4 to 1 10 7 sec 1 temperatures varied from 1100 to 1300 c and the confining pressure was maintained at 10 or more usually 15 kbar macor im a commercially available anhydrous machinable glass ceramic with a low melting point was used in many of the experiments we find that strength measurements performed using macor as a confining medium are similar to results obtained using sodium chloride as the confining solid tests performed in undried sample assemblies using air dried dunite specimens resulted in creep strengths greater than those found in any earlier studies except those of post 1973 1977 a single test on a sample for which both sample and assembly were dried at a temperature sufficiently high to drive off any adsorbed water resulted in a creep strength comparable to that determined by post 1973 1977 for very dry mt burnett dunite despite our experimental difficulties we are led to believe that our synthetic dunite exhibits mechanical behavior consistent with that determined in other experimental studies in which natural dunites and peridotites were used furthermore we conclude that dry dunite is very probably as strong as the oft disputed results of post earlier indicated finally we show that the optical textures of our highly recrystallized experimental specimens are essentially identical to naturally produced porphyroclastic textures and that the deformation mechanism in both the experimental and natural specimens is probably dislocation creep with recovery by dynamic recrystallization

kinetics of the olivine spinel transformation in subducting lithosphere experimental constraints and implications for deep slab processes abstract the persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep focus earthquakes the magnitude of buoyancy forces driving plate motion and the state of stress in the slab the depth to which metastable olivine can survive in a subduction zone and the depth interval over which transformation to or mg fe 2sio4 occurs have been evaluated from experimental kinetic data for the mg2geo4 and ni2sio4 transformations and the mg2sio4 transformation the data were extrapolated using a kinetic model for grain boundary nucleation and interface controlled growth under non isothermal and non isobaric conditions the results predict that metastable mg1 8fe0 2sio4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere the onset of transformation to mg fe 2sio4 spinel depends only on growth kinetics and coincides with the 550 50 c isotherm including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval at the onset of transformation high nucleation rates and low growth rates are consistent with the formation of very fine grained reaction products which are required for the transformational faulting mechanism of deep focus earthquakes when olivine crosses the equilibrium boundaries at higher temperatures e g higher than 700 c at 400 km depth transformation to or occurs much closer to equilibrium at a depth controlled by nucleation kinetics in this case the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting below the depth of olivine breakdown cold slabs are likely to have a complex rheological structure owing to temperature dependent microstructural evolution during the phase transformation

holocene landscape response to seasonality of storms in the mojave desert abstract new optically stimulated and radiocarbon ages for alluvial fan and lake deposits in the mojave desert are presented which greatly improves the temporal resolution of surface processes the new mojave desert climate landscape record is particularly detailed for the late holocene evidence from ephemeral lake deposits and landforms indicates times of sustained stream flow during a wet interval of the latter part of the medieval warm period at ca ad 1290 and during the little ice age at ca ad 1650 the former lakes postdate megadroughts of the medieval warm period whereas the latter match the maunder minimum of the little ice age periods of alluvial fan aggradation across the mojave desert are 14 9 cal ka and 6 3 cal ka this timing largely correlates to times of increased sea surface temperatures in the gulf of california and enhanced warm season monsoons this correlation suggests that sustained alluvial fan aggradation may be driven by intense summer season storms these data suggest that the close proximity of the mojave desert to the pacific ocean and the gulf of california promotes a partitioning of landscape process responses to climate forcings that vary with seasonality of the dominant storms cool season pacific frontal storms cause river flow ephemeral lakes and fan incision whereas periods of intense warm season storms cause hillslope erosion and alluvial fan aggradation the proposed landscape process partitioning has important implications for hazard mitigation given that climate change may increase sea surface temperatures in the gulf of california which indirectly could increase future alluvial fan aggradation

oxygen isotope composition of living land snail shells data from italy sixty seven shells of living land snails were selected from 31 different localities across italy to investigate the relationships between their oxygen isotopic composition 18os and the mean value of local rainfall 18op despite the difficulty for some localities to correctly estimate the oxygen isotopic composition of local rainfall a good relationship was found this indicates that the oxygen isotopic composition of environmental water used by the snails is in italy strictly related to that of the local meteoric precipitation however the relationship found is substantially different from that previous established for central europe indicating that regional climatic conditions are important in defining the final isotopic composition of land snail shells moreover these equations may not hold for different climatic conditions existing during for instance glacial and interglacial periods the data also show that isotopic differences among species are relatively narrow and that they may depend on ecological factors i e style of life rather than any clearly identifiable different vital offset

observations of a potential mars analog at the microscale using rover inspired methods a 10 sol observation of fort rock tuff ring 1 the terrestrial geologist s hand lens is a fundamental tool for identifying and correlating rocks and minerals we used rover inspired methods of remote hand lens scale data acquisition to conduct reconnaissance of a well characterized martian analog field site the objective was to determine if the current methodologies associated with the use of remote hand lens scale imagers maximizes science return field geologists provided with hand lens scale images of targets in geologic context could correctly identify many important characteristics of those targets however they could not fully confirm or rule out any formation hypothesis using the data provided solely through rover driven observational strategies this was due to 1 a lack of intermediate scale or millimeter to centimeter scale images providing important contextual information for the targets studied and 2 the limited number of hand lens scale images that were taken using rover driven methodology we conclude that the benefits of the hand lens as an effective triage tool and discriminator of microtexture are limited using current rover driven methodology because the hand lens type imager is not deployed frequently and resulting data cannot be used to fully support geochemical observations we recommend pursuing ways to increase the number of images that can reasonably be acquired at the hand lens scale in order for hand lens scale imaging to be fully effective textural characteristics diagnostic of the nature of a geologic site need to be identifiable at a number of different resolutions rover microscale observational strategies must include more contextual imaging

stable isotope composition of land snail body water and its relation to environmental waters and shell carbonate abstract day to day and within day diel variations in d and 18 o of the body water of the land snail theba pisana were studied at a site in the southern coastal plain of israel three phases of variation which relate to isotopic changes in atmospheric water vapor were distinguished 1 on rain days snail water becomes isotopically depleted approximately in the direction of the rain isotope values but always less depleted in d as is atmospheric water vapor 2 during the 1 3 days following a rain the snail water becomes isotopically enriched along a line with slope 18 o space this relates to an increasing influence of humidity derived from the mediterranean sea 3 a period of relative stability of the isotopic composition persists until the next rain event the isotopic variations can be explained by isotopic equilibration with atmospheric water vapor and or uptake of dew derived therefrom during the winter when the snails are active there is only very minor enrichment in 18 o relative to equilibrium with water vapor or dew apparently as a result of metabolic activity but this enrichment becomes pronounced after long periods of inactivity within day variation in body water isotopic composition is minor on non rain days shell carbonate is enriched in 18 o by ca 1 2 relative to equilibrium with body water in most regions the isotopic composition of atmospheric water vapor or dew is a direct function of that of rain because the isotopic composition of snail body water is related to that of atmospheric water vapor and the isotopic composition of shell carbonate in turn is related to that of body water land snail shell carbonate 18 o should provide a reliable indication of rainfall 18 o however local environmental conditions and the ecological properties of the snail species must be taken into account

sensitivity of glacial isostatic adjustment to a partially molten layer at 410 km depth we present a sensitivity analysis aimed at testing whether observables related to glacial isostatic adjustment can support or refute the occurrence of a low viscosity melt rich layer mrl above the mantle transition zone as required by the transition zone water filter model bercovici karato 2003 in total 1600 model runs were performed sampling a range of mrl thicknesses 1 10 and 20 km and viscosities 10 15 10 19 pa s plausible viscosity values in the upper and lower mantle regions and four distinct ice histories to determine decay time constraints we consider relative sea level rsl data from two sites angerman river ar sweden and richmond gulf rg canada and use a new method of observational sea level data correction comparing model output of postglacial decay times and j 2 to observational constraints we find numerous possible solutions largely as a result of parameter trade off the investigated observables are sensitive to the existence of an mrl and reasonable variations in its thickness and viscosity the magnitude and nature of this sensitivity varies between the two data types as well as the adopted background viscosity structure decay time results from either considered location do not strictly support or exclude mrl existence however both locations offer mrl viscosity requirements for given thicknesses with ar being more restrictive rg constraints allow mrl viscosities as low as 10 16 pa s 10 km and 10 17 pa s 20 km ar results narrow these permitted viscosity ranges to 10 18 pa s or greater for both 10 and 20 km mrl thicknesses in the case of a 1 km thick mrl ar constraints permit the viscosity to be as low as 10 17 pa s whereas those of rg permit any mrl viscosity the decay time observations are satisfied by only a small subset of background mantle viscosities regardless of the mrl properties none of which support a spherically symmetric solution of earth viscosity finally comparing model output to the observed j 2 value did not provide any constraints on mrl properties however our results show that this observable has a strong preference for viscosity values in the lower mantle that are equal to or greater than 10 22 pa s

magnetobiochronology of lower miocene ramblian continental sediments from the tudela formation western ebro basin spain abstract the calibration of european lower miocene continental stages is poorly constrained due to the lack of robust magnetostratigraphic data from continental sediments of this age in this paper we present the first magnetostratigraphic results from the mammal fossil bearing continental sediments of the lower middle miocene tudela formation western ebro basin spain the study is based on 331 paleomagnetic sites distributed along a 790 m thick composite section the resulting magnetostratigraphy provides an independent correlation to the geomagnetic polarity time scale and allows accurate dating of previously known and new ramblian upper lower miocene fossil localities of the tudela formation the new age constraints derived for the tudela formation fossil localities provide together with a revaluation of other iberian faunas the first magnetochronology for the ramblian continental stage the lower boundary of the ramblian is placed at the lower part of chron c6r ca 20 4 ma and the boundary between the lower and upper ramblian is located at the middle of chron c6n ca 19 6 ma the upper boundary of the ramblian and hence the base of the aragonian is placed at the upper middle part of chron c5cr 16 8 17 ma our results also provide the first magnetostratigraphically dated mn3 faunas in spain which together with a revaluation of other iberian and european faunas provide a revised calibration of european mammal zone mn3 the lower boundary of zone mn3 is placed at the boundary between chrons c6r and c6n ca 20 1 ma and its upper boundary is located at the upper middle part of chron c5cr 16 8 17 ma

intraplate volcanism due to convective instability of stagnant slabs in the mantle transition zone the study of volcanism can further our understanding of earth s mantle processes and composition continental intraplate volcanism commonly occurs above subducted slabs that stagnate in the mantle transition zone mtz such as in europe eastern china and western north america here we use two dimensional numerical models to explore the evolution of stagnant slabs in the mtz and their potential to sustain mantle upwellings that can support volcanism we find that weak slabs may go convectively unstable within tens of million years upwellings rise out of the relatively warm underbelly of the slab are entrained by ambient mantle flow and reach the base of the lithosphere the first and most vigorous upwellings rise adjacent to lateral heterogeneity within the slab ultimately convective instability also acts to separate the compositional components of the slab harzburgite and eclogite from each other with harzburgite rising into the upper mantle and eclogite sinking into the lower mantle such a physical filtering process may sustain a long term compositional gradient across the mtz

overview of the mars pathfinder mission launch through landing surface operations data sets and science results mars pathfinder successfully landed at ares vallis on july 4 1997 deployed and navigated a small rover about 100 m clockwise around the lander and collected data from three science instruments and ten technology experiments the mission operated for three months and returned 2 3 gbits of data including over 16 500 lander and 550 rover images 16 chemical analyses of rocks and soil and 8 5 million individual temperature pressure and wind measurements path finder is the best known location on mars having been clearly identified with respect to other features on the surface by correlating five prominent horizon features and two small craters in lander images with those in high resolution orbiter images and in inertial space from two way ranging and doppler tracking tracking of the lander has fixed the spin pole of mars determined the precession rate since viking 20 years ago and indicates a polar moment of inertia which constrains a central metallic core to be between 1300 and 2000 km in radius dark rocks appear to be high in silica and geochemically similar to anorogenic andesites lighter rocks are richer in sulfur and lower in silica consistent with being coated with various amounts of dust rover and lander images show rocks with a variety of morphologies fabrics and textures suggesting a variety of rock types are present rounded pebbles and cobbles on the surface as well as rounded bumps and pits on some rocks indicate these rocks may be conglomerates although other explanations are also possible which almost definitely require liquid water to form and a warmer and wetter past air borne dust is composed of composite silicate particles with a small fraction of a highly magnetic mineral interpreted to be most likely maghemite explanations suggest iron was dissolved from crustal materials during an active hydrologic cycle with maghemite freeze dried onto silicate dust grains remote sensing data at a scale of a kilometer or greater and an earth analog correctly predicted a rocky plain safe for landing and roving with a variety of rocks deposited by catstrophic floods which are relatively dust free the surface appears to have changed little since it formed billions of years ago with the exception that eolian activity may have deflated the surface by 3 7 cm sculpted wind tails collected sand into dunes and eroded ventifacts fluted and grooved rocks pathfinder found a dusty lower atmosphere early morning water ice clouds and morning near surface air temperatures that changed abruptly with time and height small scale vortices interpreted to be dust devils were observed repeatedly in the afternoon by the meteorology instruments and have been imaged

the 1999 marsokhod rover mission simulation at silver lake california mission overview data sets and summary of results we report on a field experiment held near silver lake playa in the mojave desert in february 1999 with the marsokhod rover the payload descent imager pancam mini tes and robotic arm camera data volumes and data transmission receipt windows simulated those planned for the mars surveyor mission selected for 2001 a central mast with a pan and tilt platform at 150 cm height carried a high resolution color stereo imager to simulate the pancam and a visible near infrared fiberoptic spectrometer operating range 0 35 2 5 m monochrome stereo navigation cameras were mounted on the mast and the front and rear of the rover near the wheels a field portable infrared spectroradiometer operating range 8 14 m simulated the mini tes a robotic arm camera capable of close up color imaging at 23 m pixel resolution was used in conjunction with the excavation of a trench into the subsurface the science team was also provided with simulated images from the mars descent imager and orbital panchromatic and multispectral imaging of the site obtained with the french spot airborne thermal infrared mapping spectrometer and landsat thematic mapper instruments commands sequences were programmed and sent daily to the rover and data returned were limited to 40 mbits per communication cycle during the simulated mission 12 commands were uplinked to the rover it traversed 90 m six sites were analyzed 11 samples were collected for laboratory analysis and over 5 gbits of data were collected twenty two scientists unfamiliar with the location of the field site participated in the science mission from a variety of locations accessing data via the world wide web remote science interpretations were compared with ground truth from the field and laboratory analysis of collected samples using this payload and mission approach the science team synergistically interpreted orbital imaging and infrared spectroscopy descent imaging rover based imaging infrared spectroscopy and microscopic imaging to deduce a consistent and largely correct interpretation of the geology mineralogy stratigraphy and exobiology of the site use of imaging combined with infrared spectroscopy allowed source outcrops to be identified for local rocks on an alluvial fan different lithologies were distinguished both near the rover and at distances of hundreds of meters or more subtle differences such as a contact between dolomite and calcite were identified at a distance of 0 5 km a biomarker for endolithic microbiota a plausible life form to be found on mars was successfully identified microscopic imaging of soils extracted from the surface and subsurface allowed the mineralogy and fluvial history of the trench site to be deduced the scientific productivity of this simulation shows that this payload and mission approach has high science value and would contribute substantially to achieving the goals of mars exploration

strength of mg fe 2sio4 wadsleyite determined by relaxation of transformation stress abstract the growth of a reaction rim around a crystal undergoing a polymorphic phase transformation generates elastic strain energy as a consequence of the reaction volume change the strain energy inhibits the transformation by counteracting the chemical free energy driving force for growth stress relaxation enables growth to continue at a rate controlled primarily by the mechanical properties of the rim we studied this process in high pressure experiments in order to constrain the rheology of wadsleyite one of the high pressure polymorphs of olivine single crystal spheres of san carlos olivine surrounded by a quasi hydrostatic pressure medium of nacl or au were partially transformed to wadsleyite at 1100 c and 16 17 gpa to determine the rate of growth as a function of time microstructural observations indicate that the relict olivine remains largely undeformed while the wadsleyite rims deform by dislocation creep application of an elastoplastic model to fit the observed decrease in growth rates with time constrains the yield strength of wadsleyite at these conditions to between 4 and 6 gpa a decrease in growth rates was also observed in another study in which varying amounts of water were added to the samples kubo t ohtani e kato t shinmei t fujino k 1998a experimental investigation of the transformation of san carlos olivine single crystal phys chem miner 26 1 6 kubo t ohtani e shinmei t fujino k 1998b effects of water on the transformation kinetics in san carlos olivine science 281 85 87 modeling these results gives values for the yield strength of wadsleyite between 2 and 5 gpa under various conditions a strength difference of 1 gpa is indicated by experiments at 1030 c conducted under nominally dry 200 ppm h2o and water added 500 ppm h2o conditions supporting the hypothesis that hydrolytic weakening is an important process in wadsleyite the calculated yield strengths for wadsleyite indicate that it is three to six times stronger than olivine at the same conditions based on an extrapolation of the low temperature plasticity flow law for olivine these results imply a strong increase in the strength of subducting slabs in the mantle transition zone provided that the deformation mechanism is the same as in the experiments furthermore inhibition of growth by transformation stress may be an important factor in extending the depth range over which metastable olivine transforms to its high pressure polymorphs in subducting slabs

acoustic emissions produced by anticrack faulting during the olivine spinel transformation anticrack faulting during the phase transformations of olivine to its denser polymorphs is a principal contender for the mechanism of deep focus earthquakes the original work on mg2geo4 established that initiation of faulting by this mechanism is not due to brittle fracture and subsequent experiments on olivine of mantle composition showed that it can operate at pressures of at least 14 gpa here we report acoustic emissions accompanying anticrack faulting in mg2geo4 thereby confirming that this mechanism involves a dynamic instability accompanied by elastic radiation

deformation of olivine spinel aggregates in the system mg ni 2geo4 deformed to high strain in torsion implications for upper mantle anisotropy 1 deformation of mantle phases to high strain is fundamental for understanding mantle dynamics however it is technically challenging to perform deformation experiments yielding accurate mechanical results at the high pressures required for certain mantle phases such as wadsleyite and ringwoodite deformation experiments on analog materials of such high pressure phases can provide insight into the deformation properties of the mantle the mg ni 2geo4 system undergoes the olivine spinel transformation at much lower pressures than the silicate counterpart hence the high strain deformation properties of an olivine spinel rock can be studied at experimentally tractable pressures and temperatures experiments were conducted in a gas medium deformation apparatus at constant angular velocity a temperature of 1473 k and 300 mpa confining pressure at these conditions with a mg mg ni ratio of 0 9 the resulting aggregate comprised 70 olivine 30 spinel and 1 orthopyroxene the applied strain rate ranged from 10 4 to 10 5 s 1 yielding shear stresses supported by the samples of between 100 and 250 mpa samples were deformed to a range of shear strains between 1 and 7 in order to study the microstructural development during high strain deformation in these experiments the spinel phase did not deform to produce a crystallographic preferred orientation cpo the olivine phase produced a cpo which is consistent with slip on the 001 hk0 slip system if this slip system is dominant in olivine below 250 300 km in the upper mantle the observed seismic anisotropy can be explained without resorting to changes in flow regime or deformation mechanism at this depth

paleoenvironment of the folsom archaeological site new mexico usa approximately 10 500 14c yr b p as inferred from the stable isotope composition of fossil land snail shells abstract well preserved aragonitic land snail shells vallonia from late pleistocene eolian sediment in the folsom archaeological site in new mexico exhibit an overall decrease of 18opdb from maximum values of 2 7 more positive than modern to younger samples with lower average values of about 3 6 within the modern range the age of the samples approximately 10 500 14c yr b p suggests that the decrease in 18o may manifest climatic changes associated with the younger dryas some combination of increased relative humidity and cooler temperatures with decreased 18o of precipitation during the times of snail activity can explain the decrease in shell 18o a well known paleoindian bison kill occurred at the folsom site during this inferred environmental transition average 13c values of the aragonite shells of the fossil vallonia range from 7 3 to 6 0 among different archaeological levels and are not as negative as modern values this suggests that the proportion of c4 vegetation at the folsom site approximately 10 500 14c yr b p was greater than at present a result which is consistent with other evidence for higher proportions of c4 plants in the region at that time

the rheology of faults triggered by the olivine spinel transformation in mg2geo4 and its implications for the mechanism of deep focus earthquakes the relevance of the anticrack faulting mechanism to the origin of deep focus earthquakes depends critically on the physical processes operating during fault generation and movement we show here that the resistance to sliding on fault zones produced by this mechanism in mg2geo4 depends only weakly on confining pressure but strongly on sliding rate both of these characteristics are inconsistent with friction the microstructures of the fault zones and the crystal plastic rheology of the constituent phases indicate that sliding must occur primarily by grain boundary sliding previously we reported that anticrack faulting is accompanied by elastic radiation of energy acoustic emissions and that it can operate during the transformation in mg fe 2sio4 at the pressures and temperatures at which deep earthquakes occur the present results and the recent demonstration that metastable olivine is present in the subducting slab beneath japan provide additional support for the anticrack theory of deep focus earthquakes

lithosphere rheology a note of caution abstract it has become common practice to use laboratory determined low temperature frictional sliding data together with power law equations for steady state creep of rocks at high temperatures to construct inferred profiles of rock strength and mode of failure with depth in the lithosphere in some cases this may involve unwarranted extrapolation of rock mechanics behaviour beyond the region of its validity the form of transition from frictional behaviour to intracrystalline plastic flow in the earth is much more complex than is suggested by the above model within the regime of plastic flow substantial changes in flow strength and mode of failure whether flow becomes localized into shear zones may accompany microstructural changes developed over large strains when deformation is accompanied by metamorphic changes existing flow laws for rocks are likely to be wholly inapplicable until a clearer understanding has emerged of the full range of expression of the rheological behaviour of rocks existing models of lithosphere rheology should be treated with caution

remotely sensed geology from lander based to orbital perspectives results of fido rover may 2000 field tests 1 blind field tests of the field integration design and operations fido prototype mars rover were carried out 7 16 may 2000 a core operations team cot sequestered at the jet propulsion laboratory without knowledge of test site location prepared command sequences and interpreted data acquired by the rover instrument sensors included a stereo panoramic camera navigational and hazard avoidance cameras a color microscopic imager an infrared point spectrometer and a rock coring drill the cot designed command sequences which were relayed by satellite uplink to the rover and evaluated instrument data using aerial photos and airborne visible and infrared imaging spectrometer aviris data and information from the rover sensors the cot inferred the geology of the landing site during the 18 sol mission including lithologic diversity stratigraphic relationships environments of deposition and weathering characteristics prominent lithologic units were interpreted to be dolomite bearing rocks kaolinite bearing altered felsic volcanic materials and basalt the color panoramic camera revealed sedimentary layering and rock textures and geologic relationships seen in rock exposures the infrared point spectrometer permitted identification of prominent carbonate and kaolinite spectral features and permitted correlations to outcrops that could not be reached by the rover the color microscopic imager revealed fine scale rock textures soil components and results of coring experiments test results show that close up interrogation of rocks is essential to investigations of geologic environments and that observations must include scales ranging from individual boulders and outcrops microscopic macroscopic to orbital remote sensing with sufficient intermediate steps descent images to connect in situ and remote observations

grain growth kinetics of ringwoodite and its implication for rheology of the subducting slab abstract grain growth rates of mg2sio4 and mg0 9fe0 1 2sio4 ringwoodites have been studied under the conditions of the mantle transition zone at a pressure of 21 gpa and temperatures ranging from 1473 to 2023 k using a kawai type high pressure apparatus the grain growth rates were expressed by gn g0n k0texp h rt where g m is grain size at time t s and g0 is the initial grain size at t 0 with n 4 5 0 8 and h 414 kj mol and log k0 20 4 1 6 m4 5 s for mg0 9fe0 1 2sio4 ringwoodite and h 456 kj mol and logk0 20 2 1 4 m4 5 s for mg2sio4 ringwoodite using these kinetic parameters for grain growth we estimated the grain size of ringwoodite in the cold subducting slabs to be less than 100 m this suggests that the dominant deformation mechanism in the subducting slab is diffusion creep it is also likely that some cold slabs are softer than the warmer surrounding mantle

rheology of mg2geo4 olivine and spinel harzburgite implications for earth s mantle transition zone geophysical modeling requires an upper mantle low viscosity zone lvz recent results suggest that the lvz could be within the mantle transition zone mtz or above it depending on boundary conditions the rheology of the olivine dominated uppermost mantle is well constrained experimentally but not the mtz here we report first time measurement of the rheology of mg2geo4 spinel analogue of ringwoodite the dominant mineral of the lower mtz in the dislocation creep regime and compare it with that of mg2geo4 olivine in both cases we incorporate 15 20 pyroxene the second most abundant mineral in the upper mantle to provide a realistic comparison of rock rheologies under similar conditions spinel dominated material is about twice as strong as olivine dominated material our results suggest that unless h2o weakening of ringwoodite or phase transformation induced grain size reduction occurs in the lower mtz it will be more viscous than above the 410 km discontinuity

a reevaluation of the flow data for olivine abstract the solid medium deformation experiments on polycrystalline olivine at pressures of 500 1500 mpa show steadily increasing creep strength with decreasing water content thus water content can significantly influence the strength of olivine room pressure experiments on olivine single crystals have been conducted under conditions of low p h 2 o and therefore should be compared with solid medium experiments on dried samples deformed under anhydrous conditions the single crystals flow more rapidly by 2 3 decades in strain rate for a given stress three hypotheses regarding the origin of this strength discrepancy are 1 1 solid medium sample strengths are higher due to the finite strength of the confining medium 2 2 temperature gradients in the solid medium samples render them stronger 3 3 the application of high confining pressure has significantly increased strength these hypotheses have been analyzed and none of them appears to alter strain rates by more than a decade another hypothesis that dislocations are pinned by iron oxide precipitates in the dry solid medium experiments also does not appear likely unless some unknown effect in the solid medium apparatus results in anomalously high strength for olivine the discrepancy must be attributed to fundamentally different behavior of the single crystal and the aggregate

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

patterns of human occupation during the early holocene in the central ebro basin ne spain in response to the 8 2 ka climatic event abstract the central ebro river basin ne spain is the most northern area of truly semi arid mediterranean climate in europe and prehistoric human occupation there has been strongly influenced by this extreme environmental condition modern climate conditions single out this region due to the harsh environment characterised by the highest absolute summer temperatures of the ebro river basin the bajo aragon region se ebro river basin was intensively populated during the early holocene 9400 8200 cal yr bp but the settlements were abandoned abruptly at around 8200 cal yr bp we propose that this archaeological silence was caused by the regional impact of the global abrupt 8 2 ka cold event available regional paleoclimate archives demonstrate the existence of an aridity crisis then that interrupted the humid early holocene that environmental crisis would have forced hunter gatherer groups from the bajo aragon to migrate to regions with more favourable conditions i e more humid mountainous areas and only return in the neolithic coherently archaeological sites persist during this crisis in the nearby iberian range maestrazgo and the north ebro river area pre pyrenean mountains and along the northwestern ebro basin

the taravilla lake and tufa deposits central iberian range spain as palaeohydrological and palaeoclimatic indicators 21 pages 8 figures 3 tables available online oct 13 2007 issue title lake systems sedimentary archives of climate change and tectonics egu general assembly

kinking in mg2geo4 olivine an ebsd study we have used electron backscatter diffraction ebsd to characterize the internal deformation and orientation of olivine grains within an experimentally deformed mg2geo4 olivine aggregate we observe that a significant fraction of the grains contain kink bands our observations of kink band geometry combined with the work of earlier authors indicate that 001 slip is the dominant slip system operating in mg2geo4 olivine at 1000 1473 k at 0 6 1 3 gpa since mg2geo4 is a high pressure analog for natural compositions of olivine this is consistent with the observations made by others that the dominant slip system for olivine in the mantle switches from 100 to 001 as pressure increases we also observed that kink bands formed in all measured olivine grains with 001 axes oriented within 35 of the compression direction we thus conclude that in a randomly oriented polycrystal undergoing uniaxial deformation at low to moderate temperatures 18 of all grains will deform by kinking and that kink band formation is an important accommodation mechanism for olivine at these conditions

anisotropic growth in the olivine spinel transformation of mg2geo4 under nonhydrostatic stress abstract the olivine spinel phase transformation in mg 2 geo 4 does not occur by a martensitic mechanism the evidence from samples transformed in a griggs type solid medium deformation apparatus are 1 1 lack of microstructural features in the olivine phase which can be specifically associated with a martensitic mechanism 2 2 the orientation relationship between the two phases that is predicted by the martensitic mechanism does not occur nor is there any apparent consistency of relative orientations 3 3 application of a differential stress to the transforming sample resulted in an anisotropic growth rate for the spinel phase indicating that growth was externally controlled rather than crystallographically controlled anisotropic growth of the spinel phase results in elongation of the residual olivine phase grains in the plane normal to the direction of maximum principal compressive stress a velocity ratio of 1 7 0 7 5 4 has been determined for the growth rate of the spinel from measurements on residual olivine grains the interphase grain boundary in samples transformed under stress has cusp shaped fingers of spinel with a blunt end separated by thin spikes of olivine samples transformed isostatically do not exhibit this feature providing further confirmation of anisotropic growth of the spinel the preferred growth of the spinel is consistent with a theory of phase transformation under nonhydrostatic stress the predicted spinel finger shape based on this theory is generally consistent with observed shapes except for the blunt end the discrepancy may be due to surface energy which has not been considered here or to local deviations of the applied macroscopic stress

middle miocene climatic change in the atacama desert northern chile evidence from supergene mineralization at la escondida geochronology and paleotopographic reconstruction of the porphyry copper deposit at la escondida chile are used to calculate long term erosion rates and to deduce the timing of tertiary climatic change for a portion of the atacama desert region hypogene hydrothermal alteration and protore mineralization at la escondida took place between 33 7 1 4 and 31 0 1 4 ma based on k ar dating of hydrothermal biotite and sericite supergene weathering and copper sulfide en richment processes were active from 18 0 0 7 to 14 7 0 6 ma based on k ar dating of supergene alunite distinguished from hypogene alunite by grain size color and sulfur isotopic composition reworked lenses of volcanic ash in the vicinity of la escondida provide useful time stratigraphic markers at 8 7 0 4 6 5 0 2 and 4 2 0 2 ma within present soil profiles long term average rates of erosion are determined by these age dates and quantitative calculation of eroded leached capping thickness at la escondida using mass balance analysis of geochemical profiles coupled with an estimate of unmineralized lithocap thickness based on alteration petrology and fluid inclusion geobarometry at similar deposits the observed trend of decreasing long term average erosion rates with time is consistent with arid to semiarid conditions in the early miocene changing to hyperarid conditions during the middle miocene this climatic desiccation caused termination of significant supergene copper suffide enrichment at la escondida and elsewhere in the atacama region and preservation of surficial features including the ash horizons and the leached capping middle miocene climatic desiccation in northern chile and southern peru was probably related to a pronounced decrease in temperature of coastal waters supplied by an ancestral humboldt current and an increase in upwelling intensity as the antarctic ice cap became established at approximately 15 to 13 ma the central andes cordillera which now provides a rain shadow protecting the atacama region from precipitation from the east must have attained at least half its present elevation prior to about 15 ma to have played a similar role in the middle miocene

geological analysis of the silver lake marsokhod field test from ground truth sampling and mapping we compare the results of the observations and conclusions obtained from a remote science team and field geologists during the 1999 marsokhod silver lake field experiment this comparison is used to determine methods that enhance the ability of a remote team using a rover to determine the geology and paleoclimate of a landing site analogous to sites that may be encountered during the mars surveyor program the mobility of the field geologists and their ability to rapidly gain visual and physical data from the field site give an immediate confidence to their interpretation of the geology the finite time of teleoperation and the initial decisions and mechanical problems that caused the rover to linger and collect data in a small area of the test site for the first 5 days of the test limited the ability of the remote science team to gain a similar confidence nevertheless by appropriate use of orbital and descent images as well as the rover s science payload the remote science team interpreted the present and former climates the geology and geological history and present day biology relatively accurately the few erroneous interpretations by the remote team resulted in part from biases brought to the test about the nature of the geology that was likely to be encountered data quality and variety also impacted the quality of the geological interpretations significant misinterpretations in the remotely determined geology resulted from the lack of detail in the optical imagery a less than optimal understanding of the site s topography and misinterpretation of some infrared data all in all this field test showed that the geology paleoclimate and biology of a site can be assessed to a reasonable level of confidence using a remotely controlled vehicle it also showed that it is both feasible and desirable for a distributed science team to participate in long science missions from home institutions as long as data flows efficiently to and from these institutions

creep mechanism in mg2geo4 effects of a phase transition we deformed polycrystalline mg2geo4 olivine and spinel in a griggs type solid medium apparatus flow of mg2geo4 olivine can be represented by 6 5 l07 3 5 exp 105 kcal mol rt where kbar is the differential stress and s 1 is the natural strain rate for mg2geo4 spinel the flow law is 2 6 104 2exp 73 kcal mol rt the low stress exponent and activation enthalpy coupled with fine grain size 3 m suggest that mg2geo4 spinel deformed by a superplastic mechanism flow parameters for the olivine phase suggest a dislocation creep mechanism comparison of theoretical superplastic flow laws for mg2geo4 olivine with the spinel phase data suggests that strain rates in mg2geo4 spinel are only about a factor of 3 lower than for mg2geo4 olivine of the same grain size a similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion transformation from mg2geo4 olivine to spinel reduced grain size to approximately 3 m thus we might expect a similar reduction in grain size in the earth s transition zone which could result in superplastic deformation of the transformed phase and cause a weak decoupling zone at the transition boundary in the mantle superplasticity brought about by transformation induced reduction in grain size may also provide a mechanism for deep focus earthquakes and an explanation for the correlation observed between their distribution with depth and the depths of phase transitions within the mantle

faulting associated with the olivine to spinel transformation in mg2geo4 and its implications for deep focus earthquakes samples of metastable mg2geo4 olivine fail by faulting when loaded at temperatures where the incoherent nucleation and growth of spinel the stable phase just becomes possible at lower temperatures where the transformation is inhibited metastable olivine is ductile and strong while at higher temperatures where the transformation happens very quickly samples are ductile and weak microfaults that have just been created and have slipped very little contain only very fine grained spinel those that have slipped more contain a mixture of fine grained spinel and larger broken olivine fragments a new type of transformation microstructure that develops only under nonhydrostatic stress occurs in all specimens that failed the spinel nucleates on grain boundaries and on subgrain boundaries within olivine and grows as fine grained lens shaped bodies with a strong preferred orientation normal to maximum compression the correlation between the onset of transformation the new microstructure and the mechanical instability suggests a cause and effect relationship the grain size of the spinel within the microfaults suggests that these zones have superplastic properties in this case the instability should not be suppressed by increasing pressure as normal frictional processes are suppressed analogous conditions exist in down going slabs of lithosphere in the mantle and we postulate that the same mechanism that causes faulting in our samples operates in the mantle causing deep focus earthquakes

stable carbon and oxygen isotopic variations in modern rabdotus land snail shells in the southern great plains usa and their relation to environment abstract variations of stable isotopic ratios of carbon 13c 12c and oxygen 18o 16o were investigated in modern shells of two species of rabdotus land snails r dealbatus and r alternatus in the southern great plains geographic variation in relation to climate and vegetation microgeographic variation variability among individuals and detailed records of seasonal variations within individual shells were studied stable carbon isotopic ratios in shell carbonate are primarily a function of the isotopic composition of the diet of the snails as represented by the isotopic composition of shell organic matter this in turn reflects the presence or absence of cam crassulacean acid metabolism or c4 plants vegetation density may have a small effect on the carbon isotope ratios microgeographic variation samples within 25 to 300 m is greater than that seen across different climatic regions and points to very local control of isotopic variations predominantly related to vegetation seasonal variations as assessed through serial analysis of individual shells up to 35 samples per shell may provide a means for distinguishing between isotopic influences of perennial cam vs annual c4 plants carbon isotopic variations in time series of shells from a site provide a means of reconstructing temporal changes in environment and climate oxygen isotopic values of shell carbonate are uniform across the region and also show no significant microgeographic variation the oxygen isotopic composition appears to be mainly a function of the rainwater isotopic composition with no direct influence of rainfall amount or evaporative effects the 18o values are only 2 enriched relative to estimated equilibrium with rainwater variability is low sd of 0 8 among sites so the isotopic composition of fossil rabdotus shells can provide a precise record of changes in the isotopic composition of rain over time

analytical electron microscopy of a synthetic peridotite experimentally deformed in the β olivine stability field a synthetic harzburgite prepared by isostatic hot pressing of finely ground natural olivine admixed with 3 enstatite was deformed at high temperature and high pressure such that the olivine phase boundary was traversed both residual and new phases their grain sizes their deformation induced lattice defects and their compositions were then characterized by analytical electron microscopy owing to large heterogeneities in temperature pressure and deformation conditions contrasting microstructures were observed some areas consist essentially of untransformed olivine while the polymorph is the main constituent of other areas crystals of the polymorph are generally free of dislocations except at the grain boundaries of the larger grains where slip bands were nucleated probably induced by stress concentrations the activated glide systems are 021 100 the orthopyroxene grains have been transformed to very fine grained clinopyroxene and have reacted with chromite to form garnet with a majoritic component compared to olivine crystals of the phase contain significantly higher concentrations of fe and trivalent cations all crystals are free of inclusions confirming the conclusion from previous infrared spectroscopic analysis that the high oh concentration in this specimen is dissolved in the phase coupled with these previously measured oh concentrations our new measurements suggest that incorporation of hydrogen into the phase and substitution of trivalent cations into the octahedral sites may be coupled such increased complexity of mineral chemistry points to a need for detailed investigation of the spectrum of compositions stable in the structure and coexisting phases in order to better estimate the modal mineralogy of the transition zone

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

the origin and distribution of salts on alluvial fans in the atacama desert northern chile the atacama desert has been predominantly hyperarid since the middle miocene combined ionic and x ray diffraction analysis shows that calcium sulphate is prevalent on three similar alluvial fans in salar basins across a transect of different environments in antofagasta province northern chile differences between the fans were largely due to the effect of local factors on salt input secondary redistribution and deposit preservation thus carbonate was notable in the high andes fan c and pre cordillera fan b whereas in the central valley fan a greater qualities of the more soluble salts chlorides and nitrates probably reflect the higher level of aridity and an oceanic source the camanchaca calcium sulphate distribution on the alluvial fans and on adjacent hilltops indicates an aeolian origin evidence from the pre cordillera fan b however suggests that salt input may have been episodic related to changes in conditions within the general aridity and possibly to stone pavement and soil development calcium sulphate sources range from an input in andean precipitation and the camanchaca to abundant regional evaporite deposits it is probable however that products of andean volcanism constitute the dominant primary source of calcium sulphate and that the salt has subsequently been distributed widely within the endoreic basins of the region by a combination of groundwater surface flow and the wind 1997 john wiley sons ltd

a constitutive model for olivine spinel aggregates and its application to deep earthquake nucleation a constitutive model is developed to describe the creep behavior of olivine spinel aggregates and conditions for transformation shear instability and deep earthquake nucleation are predicted using the eshelby mori tanaka theory eshelby 1957 mori and tanaka 1973 a piecewise incremental method is employed to calculate the stress strain relation for the olivine spinel aggregate the creep strain and strain rate depend on differential stress the aspect ratio of spinel inclusions latent heat of the phase transformation the relaxation of the volumetric phase transformation strain e ph kk and the creep law parameters of spinel a constant strain rate calculation suggests that above a critical percentage transformation c an earthquake may nucleate owing to the system s potential for a shear stress drop provided e kk ph of 8 is not relaxed and spinel grain size d xp remains small a normalized critical stiffness calculated from the potential shear stress drop and the critical shear strain c needed to change from one steady state creep condition to another suggests that a plate like partially transformed region is required for deep earthquake nucleation such a region may be formed by localized deformation or preexisting heterogeneities in a subducting rock

evolution of grain sizes and orientations during phase transitions in hydrous mg2sio4 transformation microstructures in mantle minerals such as mg fe 2sio4 are critical for predicting the rheological properties of rocks and the interpretation of seismic observations we present in situ multi grain x ray diffraction experiments on hydrous mg2sio4 at the p t conditions relevant for deep cold subducting slabs up to 40 gpa and 850 c at a low experimental strain rate 4 10 6s 1 we monitor the orientations of hundreds of grains and grain size variations during the series of forsterite wadsleyite ringwoodite phase transformations microtextural results indicate that the and an intermediate phase grow incoherently relatively to the host phase consistent with a nucleation and growth model the and phase exhibit orientation relationships which are in agreement with previous ex situ observations the and intermediate show texturing due to moderate differential stress in the sample both the and transformation induce significant reductions of the mean sample grain size of up to 90 that starts prior to the appearance of the daughter phase apart from the in the newly formed and phases the nucleation rate is faster than the growth rate inhibiting the formation of large grains these results on grain orientations and grain size reductions in relation to transformation kinetics should allow refining existing slab strength models