stable isotope composition of middle miocene carbonates of the frontal cordillera and sierras pampeanas did the paranaense seaway flood western and central argentina abstract the geographic extent interconnectedness and chronology of the miocene paranaense epeiric seaway have been the subject of considerable debate understanding the timing and location of this marine incursion is significant for documenting sea level changes paleoclimatic changes and surface uplift or subsidence stable isotope analyses of carbonate strata in the flat slab segment of the sierras pampeanas and high andean cordillera previously purported to be of marine origin provide evidence that the parana seaway did not inundate this portion of west and central argentina no unambiguously marine facies or isotopic signatures were recognized for five representative stratigraphic units the saguion formation ese of salinas grande in cordoba province anta formation in the quebrada de la yesera and lerma valley of salta province del buey and del abra formations within the famatina ranges la rioja province and chinches formation in the manantiales foreland basin in the frontal cordillera san juan province paleontologic and lithologic features in support of a marine origin are reconsidered herein instead a lacustrine origin is inferred for these formations their contemporaneity perhaps related to concurrent global climate conditions and broad tectonic setting until more substantive evidence of marine deposition is found we reject correlation of these units with the parana seaway preserved in the chaco pampean plain subsurface and discourage mapping of the seaway as extending into the central sierras pampeanas or andean foreland our findings suggest the miocene andean foreland was elevated above sea level although it remained as a largely flat lying area west of the pampas and chaco plains which were inundated by the parana seaway

deformation of forsterite polycrystals at mantle pressure comparison with fe bearing olivine and the effect of iron on its plasticity abstract the rheology of polycrystalline forsterite was investigated in the deformation dia d dia using in situ x ray diffraction at pressure between 3 1 and 8 1 gpa temperature in the 1373 1673 k range and at steady state strain rate ranging from 0 5 10 5 to 5 5 10 5 s 1 microscopic observations of the run products show characteristic microstructures of the so called dislocation creep regime in wet conditions based on the present data at 1473 k the pressure effect on forsterite plasticity is quantified using an activation volume v fo 12 1 3 0 cm3 mol 1 a comparison between the strain rates of san carlos olivine and forsterite specimens deformed together indicates that at the experimental conditions they compare with each other within less than half an order of magnitude this comparison also allows for the determination of the stress exponent of forsterite of nfo 2 3 0 6 our results combined with data from the literature indicate a clear trend of increasing stress exponent with fe content in olivine

importance of pressure solution and coble creep in the deformation of polymineralic rocks the theoretical framework for modeling pressure solution in polyphase rocks is presented it is often assumed that pressure solution is a deformation mechanism of secondary importance deep in the lithosphere it is shown here that the rheology of rock during pressure solution may be drastically reduced in a bimineralic and polymineralic rock relative to its monomineralic counterparts and therefore the importance of this deformation mechanism importance is perhaps underestimated this behavior contrasts with that of a polymineralic rock undergoing dislocation creep where existing models show that the rheology will be some average of the rheologies of individual phases for instance the rheology of a dunite in pressure solution is governed by the diffusion of both mgo and sio2 whichever is the slowest will control the viscosity of the rock conversely in a harzburgite with a specific texture flow will be governed by the fastest diffusing species thus its strength may be as low as that of a quartz rock of comparable grain size given the comparative strength of many phases undergoing dislocation creep relative to quartz this prediction implies a larger range of conditions under which pressure solution forms an important part of bulk rock deformation than inferred previously predictions on textural evolution of polymineralic aggregates are also made and it is shown that some minerals grains may become elongate parallel to the maximum principal stress during deformation even though the rock as a a whole shortens in this direction

effect of quartz inclusions on plastic flow in marble to investigate the effect of variations in volume fraction of a second phase on the strength and microstructure of rocks we produced synthetic marbles with a grainsize of 5 25 m containing quartz of similar size with volume fractions of 0 5 1 and 20 4 vol and between 3 to 9 porosity these rocks were deformed in triaxial compression at a strain rate of 3 x 10 5 s 1 confining pressure of 200 mpa and 600 c marbles containing 20 4 vol quartz were almost 5 times stronger than those containing no quartz the work hardening rate at given strain increased with quartz content marbles containing 20 4 vol quartz work hardened up to 10 strain whereas in single phase samples work hardening was negligible beyond 3 strain the dislocation density in the calcite increased with increasing quartz content the data are consistent with models for the strength of ceramic matrix composites or with the hypothesis that strength is increased by decreasing porosity

petrographic and trace element analysis of uranium rich tufa calcite middle miocene barstow formation california usa an integrated petrographic and spectroscopic x ray diffraction phosphor imaging and synchrotron x ray absorption spectroscopy study of tufas from the miocene barstow formation california relates sample morphology mineralogy and geochemical composition the tufas composed mainly of calcite that formed at the interface between an ancient alkaline lake and ground or spring waters have textures similar to those of microbially mediated terrestrial stromatolites and travertines the tufas have elevated concentrations of a number of trace elements including mn fe sr and u synchrotron x ray fluorescence analyses show that u concentrations can exceed 500 p p m x ray absorption spectroscopy indicates that the u in these samples is incorporated as u iv it is suggested here that alkaline lake waters had a high u ca ratio and tufa calcite formed where groundwater or possibly epithermal springs brought in ca and trace elements such as sr the rapid and possibly microbially mediated precipitation of calcite allowed for incorporation of high concentrations of trace metals as either structural substitutes or extremely fine scale inclusions

climatic and tectonic controls on sedimentation and erosion during the pliocene quaternary in the qaidam basin china the qaidam basin is an internally drained basin located in the northeastern tibetan plateau presently over 50 of the basin floor exposes thick 1 km sections of pliocene quaternary strata that are deformed by folding and faulting we investigated this nearly continuous pliocene quaternary sedimentary record for the effects of global climate change and deformation on basin sedimentation new detailed stratigraphic magnetostratigraphic and stable isotope 18 o 13 c data from the pliocene shizigou and pleistocene qigequan formations along the southwestern flank of an intrabasin fold within the north central qaidam basin are presented here strata reveal climatically controlled meter scale parasequences within shallow lacustrine marginal lacustrine and deltaic lithofacies paleocurrents shift from eastward at the base to southwestward for the majority of the section but they abruptly shift toward the south southeast in the upper 400 m twenty two magnetozones constrain deposition between 5 2 ma and ca 0 8 ma and reveal that sedimentation rates were fairly constant 474 34 m m y from 5 2 to 3 0 ma after which time rates abruptly decreased to 154 40 mm yr before increasing again to 750 m m y since 1 2 ma the 18 o values shift from relatively constant values avg 6 8 range 9 6 to 4 5 relative to vienna peedee belemnite vpdb to less negative values avg 1 2 range 1 2 to 2 7 vpdb between 3 1 and 2 6 ma and to extremely variable values avg 2 9 range 8 3 to 4 0 after ca 2 6 ma the post 2 6 ma extreme variability in stable isotopes reflects the same timeframe of enhanced climatic cyclicity associated with northern hemisphere glaciation the 13 c values remain relatively constant average 4 0 range 5 7 to 1 0 until ca 0 9 ma when the values increase to 0 3 range 1 0 to 1 5 vpdb the appearance of growth strata at 3 0 ma shallow water evaporite deposition after 2 6 ma and the observation of paleoyardangs buried wind sculpted landforms within lake marginal strata at 2 4 ma imply that emergence of the adjacent anticline was followed by the shallowing and partitioning of the lake basin and subaerial exposure and erosion of marginal lake sediments these data reflect a significant change to a more arid climate in the qaidam basin between ca 3 1 ma and 2 6 ma overlapping with the onset of significant northern hemisphere glaciations and basin floor deformation lake level cycles were on 230 000 yr frequency from 2 6 ma to 1 2 ma before increasing to 12 000 yr frequency suggesting increased aridity and broad subaerial exposure of lake sediments after the pliocene quaternary transition deformation of intrabasin sediments by at least 3 0 ma caused uplift of the basin floor which provided an environment rich in friable material for wind deflation of the qaidam basin a likely source for sediments on the chinese loess plateau and nutrients for the pacific ocean carried by westerly winds by 2 6 ma deformation of the qaidam basin created closed depressions that facilitated evaporite sedimentation that continues today coeval intrabasin deformation combined with increasing aridity after 3 1 ma thus controlled both deposition and erosion within the region

miocene sedimentary environment and climate change in the northwestern qaidam basin northeastern tibetan plateau facies biomarker and stable isotopic evidences abstract facies biomarker and stable isotopic records from the miocene lacustrine sediments in the northwestern qaidam basin were investigated to reconstruct the miocene sedimentary environment and climatic history three distinct facies can be recognized these include the following 1 gray black laminated mudstone and marlstone which represent a semi deep fresh to semi brackish lake environment 2 gray yellowish massive mudstone marlstone and siltstone and 3 yellowish massive sandstone which imply a shallow brackish lake environment the decreasing c 27 c 31 and c 27 c 29 c 31 c 33 values the increasing acl mean chain length values of n alkanes and the vertical evolution of sedimentary environments indicate the overall intensified aridity which is considered to be an integrated result of high elevation of the himalaya tibetan system retreat of the paratethys and global cooling high fluctuations of the 18 o values and primary dolomite contents reveal the hydrologically closed paleolake with intermittently open conditions in the study area during middle late miocene the qaidam basin is suggested to be hydrologically segmented based on the stable isotopic data comparison between the study area and the northeastern area the most negative end of the oscillations of the 18 o values indicating the minimal evaporation which likely represents the isotopic ratio of the meteoric water surprisingly conveys stability in the shang youshashan and shizigou formations and displays a positive 2 5 shift this significant shift was probably due to the climatic aridification and air mass changes around 10 8 ma rather than the global cooling

controlling factors and dynamical formation models of lacustrine organic matter accumulation for the jurassic da anzhai member in the central sichuan basin southwestern china abstract the lacustrine shale in the jurassic da anzhai member is considered as an excellent source rock in the central sichuan basin however geochemical studies of lacustrine organic matter om accumulation factors and mechanisms are scarce this study investigated the variations in total organic carbon toc major elements trace elements tes and 13c and 18o values to reconstruct the paleoenvironmental conditions biotic productivity terrestrial inputs and other factors to improve the understanding of the controlling factors and dynamical formation models for lacustrine om the target shale was deposited in a fluctuating and complicated terrestrial open lake that exhibited oxic suboxic oxygen levels weak to moderate hydrodynamic conditions a dry to humid climate moderate weathering and fresh to saline water conditions the lake experienced moderate high biotic productivity under a warm humid climate and featured abundant flora and fauna compared to the da13 shale the da1 shale featured higher salinity levels a hotter and drier climate greater weathering and more reduced conditions the da13 shale experienced stronger hydrodynamic forces and a more humid climate moreover the toc values of the da13 correlate better with the indicators of redox conditions and exhibit little or no correlation with indicators of climate weathering salinity and biotic productivity indicating that the lacustrine om was mainly controlled by the redox conditions additionally factors such as hydrodynamic conditions and terrestrial inputs exerted some degree of influence based on these factors and their relationships two dynamical formation models are proposed a high salinity low oxygen dynamical formation model model i and a deep water low oxygen dynamical formation model model ii model i stresses that the low oxygen levels were mainly caused by salinity whereas model ii stresses that the low oxygen levels were mainly caused by deep depth both models can be classified as preservation models

rheology of two phase systems a microphysical and observational approach abstract ductile shear zones commonly contain distinctive bands of high strain rock characterized by intimately mixed fine grained two phase or polyphase material these ultramylonite bands are weaker than the surrounding material and may play a critical role in strain localization how such zones develop how the phases become evenly dispersed the bulk rheology and the controls on grain size are all unclear the following generic scenario may resolve some of these questions 1 dislocation creep and dynamic recrystallization cause grain size reduction commonly the recrystallized grain sizes of the two phases differ 2 grain size reduction causes a switch to grain boundary diffusion creep which requires grain boundary sliding diffusion allows one phase to fill spaces that open between grains of the other this will happen most rapidly in the finer grained phase the grain size of the resulting mixture is therefore controlled by that of the finer grained phase this leads to mixing and dispersion of the two phases producing a fine grained evenly dispersed two phase aggregate 3 the bulk rheology will be controlled by grain boundary diffusion creep of the two phases with the grain size controlled by the finer grained phase bulk flow laws can be developed for quartz feldspar and olivine orthopyroxene ultramylonites based on these concepts using appropriate mixing laws

comparison of lacustrine successions and their palaeohydrological implications in two sub basins of the triassic cuyana rift argentina fil benavente cecilia andrea consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet mendoza instituto argentino de nivologia glaciologia y ciencias ambientales provincia de mendoza instituto argentino de nivologia glaciologia y ciencias ambientales universidad nacional de cuyo instituto argentino de nivologia glaciologia y ciencias ambientales argentina

87sr 86sr ca sr and ge si ratios as tracers of solute sources and biogeochemical cycling at a temperate forested shale catchment central pennsylvania usa plant uptake and biological cycling processes are commonly the largest flux of nutrients in terrestrial ecosystems hydrologic and other losses are offset by inputs from atmospheric deposition and weathering this multi tracer study investigates these effects using 87sr 86sr ca sr and ge si ratios from solid soil profiles and bedrock biological leaves and sap waters and water pore ground stream waters samples to study ca sr ge and si sources and cycling in a forest catchment underlying gray shale in the temperate climate of central pennsylvania leaves and sap waters were found to have similar ge si ratios 1 mol mol which is consistent with biological fractionation occurring at the root soil water interface ge si ratios in soil porewaters were higher near the surface and increased over the growing season suggesting plant preferential uptake of si over ge ca sr ratios in oak leaves 1360 40 mol mol were significantly higher than those in maple leaves 650 20 mol mol ca sr ratios were also generally higher in leaves than in sap waters which are consistent with preferential sr adsorption or sr uptake during transpiration 87sr 86sr ratios in both leaves and sap waters were similar for a given site implying that trees access similar pools of sr and ca although there are site to site differences additionally 87sr 86sr ratios in plant tissues were most similar to shallow pore waters 0 50 cm suggesting that mineral nutrients are obtained by trees from the near sub surface 87sr 86sr ratios in soil solutions from ridgetop and swale sites are best explained by mixing sr derived from shale and atmospheric deposition valley bottom soil reservoirs and stream and groundwater samples include sr and ca derived from dissolution of two isotopically distinct generations of carbonates a preliminary estimate of the sr and ca stream fluxes and isotopic mass balances imply propagation of a carbonate weathering front of ca 300 m myr 1 a value larger than previously reported for regolith weathering advance rates calculated based on cosmogenic nuclides and u series isotopes the data for ca sr si and ge in soil soil solutions and stream waters reflect the interaction of slower weathering processes with rapid biologically driven cycling between soils and biomass

effect of water on the rheology of experimentally deformed quartzite empirical flow laws have been determined for simpson quartzite samples deformed to mechanical steady state in the quartz stability field using griggs blacic solid medium deformation apparatus experiments were conducted on samples both as received dry and with water added via the dehydration of a talc confining medium wet best fits of the power law type yield a stress exponent of 2 72 0 19 an activation enthalpy of 134 32 kj mol 1 and a preexponential constant of 1 16 1 15 0 58 10 7 mpa 2 72 s 1 for the dry quartzite law and are 2 61 0 15 145 17 kj mol 1 and 5 05 5 00 10 6 mpa 2 61 s 1 for the wet quartzite the enhanced hydrolytic weakening of the wet experiments appears to affect the flow laws mostly in the preexponential constants possibly as a defect concentration term that is higher in the wet than in the dry law over the range of experimental conditions 750 900 c 10 7 to 10 4 s 1 and 1 0 1 25 gpa confining pressure dry specimens are 1 5 2 5 times stronger than wet samples deformed at the same conditions the microstructures produced are analogous to those observed in many natural quartz tectonites recrystallization is well developed in the wet but not the dry specimens at flow stresses below 500 mpa microstructures indicative of recovery are well developed a comparison of our creep activation enthalpies with those determined from diffusion experiments are consistent with deformation of wet simpson quartzite that is limited by oxygen self diffusion inconsistencies between the present study and previous studies are probably the result of differences in sample material sample treatment and sample assemblies chemical environment

freshwater molluscan fauna from the florissant formation colorado paleohydrologic reconstruction of a latest eocene lake1 the freshwater molluscan assemblage from the uppermost eocene florissant formation 34 07 0 10 ma colorado usa provides a reliable proxy in reconstructing past ecology and environmental chara

high temperature deformation of enstatite aggregates synthesized polycrystalline enstatite samples were deformed in a paterson gas medium apparatus at 1200 1300 c oxygen fugacity buffered at ni nio and confining pressures of 300 mpa protoenstatite field or 450 mpa orthoenstatite field at both confining pressures the mechanical data display a progressive increase of the stress exponent from n 1 to n 3 with increasing differential stress suggesting a transition from diffusional to dislocation creep nonlinear least squares fits to the high stress data yielded dislocation creep flow laws with a stress exponent of 3 and activation energies of 600 and 720 kj mol for orthoenstatite and protoenstatite respectively deformed samples were analyzed using optical microscopy and scanning and transmission electron microscopy microstructures show undulatory extinction and kink bands evidence of dislocation processes crystallographic preferred orientations measured by electron backscatter diffraction are axisymmetric and indicate preferential slip on 100 001 most deformed grains comprise an interlayering of orthoenstatite and clinoenstatite lamellae while many lamellae may have formed during quenching from run conditions those in samples deformed in the orthoenstatite field are often bordered by partial 001 dislocations suggesting transformation due to glide of partial 001 dislocations in 100 planes comparison of our orthoenstatite creep law with those for dislocation creep of olivine indicates that orthoenstatite deforms about a factor of 2 slower than olivine at our experimental conditions however as orthoenstatite has a higher activation energy and smaller stress exponent than olivine this strength difference is likely smaller at the higher temperatures and lower stresses expected in much of the upper mantle

flow behavior and microstructures of hydrous olivine aggregates at upper mantle pressures and temperatures deformation experiments on olivine aggregates were performed under hydrous conditions using a deformation dia apparatus combined with synchrotron in situ x ray observations at pressures of 1 5 9 8 gpa temperatures of 1223 1800 k and strain rates ranging from 0 8 10 5 to 7 5 10 5 s 1 the pressure and strain rate dependencies of the plasticity of hydrous olivine may be described by an activation volume of 17 6 cm3 mol 1 and a stress exponent of 3 2 0 6 at temperatures of 1323 1423 k a comparison between previous data sets and our results at a normalized temperature and a strain rate showed that the creep strength of hydrous olivine deformed at 1323 1423 k is much weaker than that for the dislocation creep of water saturated olivine and is similar to that for diffusional creep and dislocation accommodated grain boundary sliding while dislocation microstructures showing the 001 slip or the 001 100 slip system were developed at temperatures of 1633 1800 k a much stronger pressure effect on creep strength was observed for olivine with an activation volume of 27 7 cm3 mol 1 assuming a stress exponent of 3 5 water fugacity exponent of 1 2 and activation energy of 520 kj mol 1 i e power law dislocation creep of hydrous olivine because of the weak pressure dependence of the rheology of hydrous olivine at lower temperatures water weakening of olivine could be effective in the deeper and colder part of earth s upper mantle

strength evolution and the development of crystallographic preferred orientation during deformation of two phase marbles abstract we conducted conventional triaxial compression and confined torsion tests on calcite rocks at 1023 k and equivalent strain rates between 10 6 and 10 3 s 1 to investigate the influence of variations in volume fraction and geometry of a second phase on the evolution of strength and cpo nominally dense two phase marbles were made by hot isostatic pressing of mixtures of calcite and amorphous carbon the carbon grains were either spheres or splinters with known size distributions were chemically inert and are nearly rigid adding 1 vol spheres caused only small changes in strength or cpo strengths were consistent with the activation of slip on f 10 1 1 the hardest system in calcite adding 10 vol splinters increased the strength at low strains and strain rates but had smaller effects at high strains or rates the cpo of rocks containing splinters were similar to those in pure samples but with reduced intensity when 10 vol spheres were added the aggregate strength under confined torsion was less than that of the pure material distinct and intense cpos developed the reduced stresses were consistent with those necessary for slip on systems weaker than f viscoplastic self consistent calculations used to model the texture evolution required slip on f 10 1 1 and c 1 2 1 0 to match the cpo of pure samples results were consistent with the inclusion of slip on r 1 2 1 0 in samples containing 10 vol spheres deformed in torsion slip activity on the harder systems was decreased the exact cause of the strength reduction in these experiments is not understood but it is possible that adding smooth spheres enhanced grain boundary sliding thereby decreasing activity on f 10 1 1 thus shape and volume fraction of second phases may be important in determining cpo and strength and it is important to consider the loading geometry

quartz deformation in a very fine grained quartzo feldspathic mylonite a lack of evidence for dominant grain boundary sliding deformation abstract microstructural and microtextural investigations by transmission electron microscopy tem have been performed on very fine grained quartzo feldspathic mylonites from the red bank deformed zone central australia the study of the quartz reveals a mean grain size 2 3 0 8 m and microstructural features indicative of dislocation creep processes within the very fine grained quartz analysis of local crystallographic preferred orientation cpo by tem showed a strong cpo for both the c and a axes the crystallographic relationships between adjacent grains and subgrains are consistent with dominant c a and a slip microstructural and microtextural studies also show that the recrystallization mechanism included local grain boundary migration and the formation of high angle grain boundaries from low angle sub grain boundaries by progressive misorientation we found no abnormal grain rotations as would be expected to result from grain boundary sliding processes this suggests that grain boundary sliding was not significant and the dominant deformation mechanism was dislocation creep

monitoring cenozoic climate evolution of northeastern tibet stable isotope constraints from the western qaidam basin china carbon and oxygen stable isotopic composition of cenozoic lacustrine carbonates from the intramontane qaidam basin yields cycles of variable length and shows several distinct events driven by tectonics and climate changes from eocene to oligocene the over all trend in the 13c composition of lacustrine carbonates shows a shift toward higher values possibly related to higher proportions of dissolved inorganic carbon transported to the lake or lower input of soil derived co2 at the same time the 18o composition of lacustrine carbonates is decreasing in accordance with the global cooling trend and northwards drifting of the whole region during the miocene distinct isotopic events can be recognized although their interpretation and linkage to a certain tectonic event remains difficult these events may be related to uplift in the himalayas to the strongest phase of uplift in the altyn mountains to pronounced subsidence of the qaidam basin or to the expansion of c4 plants on land generally cold highly evaporative conditions can be deduced from enrichment of 18o isotopic compositions during pliocene and quaternary times

pennsylvanian continental cyclothem development no evidence of direct climatic control in the upper freeport formation allegheny group of pennsylvania northern appalachian basin abstract the occurrence of lacustrine carbonates in the allegheny group upper middle pennsylvanian continental cyclothems of the northern appalachian basin has been attributed to climatic alternation of wet coal deposition and dry carbonate deposition periods however we present evidence from facies associations coal and carbonate petrography and carbonate stable isotope geochemistry for a uniformly wet climate throughout deposition of one of the earliest continental cyclothems the upper freeport formation uff we propose a depositional model comprising a large wetland complex drained by an anastomosed fluvial system containing a mosaic of channels freshwater siliciclastic and carbonate lakes and peat bogs swamps on a siliciclastic floodplain carbonate phases represent bio induced precipitates modified by lake dynamics and not evaporative deposits absence of covariant isotopic trends between 18o and 13c and a relatively small range of 18o values 1 9 to 6 4 pdb that are close to or lighter than predicted paleoprecipitation for the pennsylvanian equatorial regions indicate hydrologically open short residence time lake systems in support of our sedimentary model combined sedimentologic and isotopic data do not indicate reduced effective moisture precipitation evaporation during intervals of limestone deposition the uff lakes developed in a relatively wet climate zone without significant episodic closure due to climatically induced water deficit lithologic patterns within the uff cyclothem are primarily attributed to the evolution of anastomosed fluvial systems controlled by subsidence in an active foreland basin rather than to short term climatic change or eustacy

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

dynamic recrystallization and phase mixing in experimentally deformed peridotite article history ductile shear zones evolve through complex feedbacks between microstructure and rheology in the mantle shear zones often display mylonitic or ultramylonitic microstructures characterized by extensive grain size reduction and well mixed polymineralic domains the feedback between the formation of these microstructures and grain size sensitive deformation is often considered an important weakening mechanism in high temperature mantle shear zones to understand better the processes that transform coarse grained tectonites into fine grained and well mixed mylonites we have performed triaxial deformation experiments on synthetic peridotite samples comprised of mm scale olivine and orthopyroxene clasts experiments were conducted in a griggs apparatus at a confining pressure of 1 gpa temperatures of 1400 to 1550 k and strain rates of 10 5 10 6 s 1 under nominally dry conditions experiments yield deformed samples with macroscopic natural strains ranging from 0 31 to 0 74 samples are partially recrystallized with neoblasts ranging in grain size from 2 25 m at these deformation conditions unrecrystallized mm sized clasts deform by grain size insensitive dislocation creep while recrystallized grains deform by grain size sensitive dislocation accommodated grain boundary sliding disgbs using electron backscatter diffraction ebsd we investigate the interfaces between recrystallized olivine and orthopyroxene domains at the majority of these interfaces no mixing is observed however many boundaries are serrated which is interpreted to be the result of surface tension driven phase boundary migration in a few cases we observe small degrees of mixing with enclaves of one phase completely surrounded by the other phase this mixing occurs only within a few recrystallized grain length scales of the interface these results demonstrate that the serial processes of dynamic recrystallization phase boundary migration and disgbs deformation are a plausible mechanism for solid state phase mixing however more extensive deformation may be needed to reproduce the near steady state microstructure observed in highly deformed mantle shear zones

bulk flow strength of forsterite enstatite composites as a function of forsterite content creep experiments have been conducted to investigate the effect of varying forsterite content vfo on the bulk flow strength of dry forsterite enstatite fo en aggregates in order to evaluate the applicability of existing theoretical models to two phase rocks as well as to understand the rheology of polyphase systems in general the experiments were performed at temperatures of 1423 1593 k stresses of 18 100 mpa oxygen fugacities of 10 14 10 2 5 mpa and 0 1 mpa total pressure the fine grained fo 10 17 mm en 14 31 mm composites of various fo volume fractions vfo 0 0 2 0 4 0 5 0 6 0 8 and 1 were synthesized by isostatically hot pressing in a gas medium apparatus at 1523 and 350 mpa our experiments show that flow strength contrasts between fo and en are in the range of 3 8 at the given experimental conditions with fo as the stronger phase the measured stress exponent n and activation energy q values of the fo en composites fall between those of the endmembers the n values show a nearly linear increase from 1 3 to 2 0 while the q values display a non linear increase from 472 to 584 kj mol with en volume fraction from 0 to 1 0 there is no clear dependence of creep rates on oxygen fugacity for the fo en composites the mechanical data and tem microstructural observations suggest no change in deformation mechanism of each phase when in the composites compared to when in a single phase aggregate the en deformed mainly by dislocation creep while the fo deformed by dislocation accommodated diffusion creep for our grain sizes and experimental conditions comparisons between the measured composite strengths and various theoretical models indicate that none of the existing theoretical models can give a precise predication over the entire vfo range from 0 to 1 however the theoretical models based on weak phase supported structures wps yield a good prediction for the flow strengths of the composites with vfo 0 6 no model gives a good prediction for the bulk strength of two phase composites in the transitional regime vfo 0 4 0 6 applications of the wps and sps based models in the transitional regime result in under and over estimations for the composite flow strength respectively thus the effect of rock microstructure should be taken into consideration in modeling the bulk flow strengths of the crust and upper mantle using laboratory determined flow laws of single phase aggregates d 2001 elsevier science b v all rights reserved

rheology of synthetic anorthite diopside aggregates implications for ductile shear zones 1 we investigated the high temperature creep strength of fine grained anorthite diopside rocks at temperatures ranging from 1323 k to 1523 k and at 300 mpa confining pressure in a paterson type gas medium deformation apparatus flow stress varied between 20 and 450 mpa resulting in strain rates between 6 1 10 7 s 1 and 7 5 10 4 s 1 pure diopside and anorthite samples were hot pressed from crushed natural single crystals and glass powders respectively two phase samples were produced by hot isostatic pressing of mechanically mixed powders of anorthite glass with 25 50 and 75 vol diopside particles arithmetic mean grain size of the anorthite matrix is dan 3 5 m three different ranges of diopside particle size were used ddi 25 m 35 m and 45 m water content of as is samples was about 0 05 0 02 wt h2o and predried samples contain about 0 004 0 001 wt h2o at experimental conditions as is samples are assumed to be water saturated water content of predried samples is about 3 times less than that of starting diopside single crystals the specimens contain about 1 vol glass located at fluid inclusions and some multiple grain junctions two grain boundaries examined by high resolution transmission electron microscopy did not show amorphous layers to a resolution of 1 nm at experimental conditions pure diopside aggregates are about 2 3 orders of magnitude stronger than pure anorthite samples for as is and predried specimens respectively in general strength of the two phase aggregates increases with increasing diopside content but remains between isostress and isostrain rate bounds aggregate strengths predicted from continuum mechanics models are in good agreement with the experimental data for dilute diopside particle mixtures and high volume fractions when diopside particles form a load bearing framework at low stresses 100 200 mpa the stress exponent is n 1 suggesting diffusion controlled creep at higher stresses mechanical data and microstructures suggest that samples deformed in the transition region between diffusion controlled creep and dislocation creep for pure anorthite and diopside aggregates deforming in dislocation creep we estimated stress exponents of n 3 and n 5 5 respectively for the two phase aggregates n is between n 3 and n 5 depending on diopside content at low stresses deformation microstructures indicate load transfer from a weak anorthite matrix to stronger diopside particles creep activation energies for pure diopside and anorthite mixtures range from 286 kj mol 1 for wet anorthite deformed at low stresses to 691 kj mol 1 for dry diopside deformed at high stresses activation energies of two phase mixtures are between or close to those of the end members as is samples have significantly lower activation energies than predried samples

large strain deformation and strain partitioning in polyphase rocks dislocation creep of olivine magnesiowustite aggregates aggregates composed of olivine and magnesiowustite have been deformed to large strains at high pressure and temperature to investigate stress and strain partitioning phase segregation and possible localization of deformation in a polyphase material samples with 20 vol of natural olivine and 80 vol of mg0 7fe0 3 o were synthesized and deformed in a gas medium torsion apparatus at temperatures of 1127 c and 1250 c a confining pressure of 300 mpa and constant angular displacement rates equivalent to constant shear strain rates of 1 3 3 10 4 s 1 the samples deformed homogeneously to total shear strains of up to 15 during constant strain rate measurements the flow stress remained approximately stable at 1250 c while it progressively decreased after the initial yield stress at the lower temperature mechanical data microstructures and textures indicate that both phases were deforming in the dislocation creep regime the weaker component magnesiowustite controlled the rheological behavior of the bulk material and accommodated most of the strain deformation and dynamic recrystallization lead to grain refinement and to textures that were not previously observed in pure magnesiowustite and may have developed due to the presence of the second phase at 1127 c olivine grains behaved as semi rigid inclusions rotating in a viscous matrix at 1250 c some olivine grains remained largely undeformed while deformation and recrystallization of other grains oriented for a slip on 010 resulted in a weak foliation and a texture typical for pure dry olivine aggregates both a slip and c slip on 010 were activated in olivine even though the nominal stresses were up to 2 orders of magnitude lower than those needed to activate these slip systems in pure olivine at the same conditions

flow behavior of fine grained synthetic dunite in the presence of 0 5 wt h2o the deformation behavior of synthetic dunite mg forsterite plus 2 5 vol enstatite with a grain size of 1 m has been investigated in combined constant displacement rate and temperature stepping experiments at temperatures of 850 to 1000 c strain rates of 10 7 to 10 5 s 1 and a confining pressure of 600 mpa nominally dry material behaved purely elastically in contrast samples with 0 5 wt added water flowed at stresses between 9 and 81 mpa reaching strains up to 12 fitting a dorn type power law to the wet data yielded stress exponents n of 1 7 0 4 and apparent activation energies in the range 302 22 kj mol 1 the wet deformed samples showed polygonal grains no crystallographic preferred orientation no subgrains low dislocation densities and evidence for grain boundary cavitation grain growth was minor it is concluded that the wet samples probably deformed by a grain size sensitive grain boundary sliding dominated mechanism though the nature of the accommodation process remains unclear and the possibility of transitional behavior between dislocation creep and diffusion creep cannot be eliminated application of the results to upper mantle shear zones supports previous speculation that the presence of such zones can lead to a significant weakening of the top 10 to 20 km of the upper mantle during extension of the continental lithosphere

mantle shear zones revisited the connection between the cratons and mantle dynamics new geodynamic models based on teleseismic data sets propose that the westward motion of the north and south american plates is driven by asthenospheric flow coupled to the overlying lithosphere implicit in these models is the existence of a zone of partial coupling at the boundary between the lithosphere and asthenosphere we propose that porphyroclastic peridotite xenoliths from kimberlite are samples of deep mantle shear zones these samples are a direct manifestation of the mechanical coupling that arises from the drag applied to the lithosphere by the underlying asthenospheric flow we postulate that the formation of shear zones at the lithosphere asthenosphere boundary is a quasisteady state process attending plate movement beneath all cratons

paleohydrology of the late miocene ridge basin lake california uppermost miocene lacustrine deposits of the ridge basin california contain benthic microbial carbonates as fragmentary crusts on clasts in resedimented conglomerates of the violin breccia deposited adjacent to the fault controlled southwestern margin of the basin and as fragments and more extensive in situ stromatolitic units within the ridge route formation around the northeastern part of the basin ridge route microbialites formed in the open waters of the ridge basin paleolake and have 18 o values that are significantly higher than those from the violin breccia the latter may have formed in warm littoral pools or in streams maintained by local runoff their 18 o compositions are consistent with carbonate precipitation from meteoric waters supplied by rainfall dominated by 67 75 winter precipitation at temperatures of 16 17 c ridge route formation microbialites have a wide range of 13 c and 18 o values and indicate that the lake fluctuated between hydrologically open and closed conditions light carbon isotopic compositions for carbonate that precipitated during periods of open basin conditions suggest that inflow waters came from a fluvial system with a well vegetated flood or delta plain relatively high 18 o values imply that the lake was not maintained by local rainfall but depended on inflow that came either from an upstream lake or from a region to the east or northeast where the rainfall regime was dominated by isotopically heavy summer precipitation

superplasticity in hydrous melt bearing dunite implications for shear localization in earth s upper mantle abstract deformation experiments on hydrous melt bearing dunite olivine 4 vol orthopyroxene 4 vol clinopyroxene with less than 2 5 vol of the melt phase were conducted at pressures of 1 3 5 7 gpa and temperatures of 1270 1490 k in order to explore the effect of intergranular fluids on the plastic flow of olivine in earth s upper mantle the strain rate was proportional to steady state creep strength to the 2 1 power and the creep strength markedly increased with increase in grain size developments of the crystallographic preferred orientation of olivine and flattening of olivine grains were hardly observed even after 33 55 shortening of the samples these observations show that grain boundary sliding gbs dominated the deformation of olivine i e superplasticity the creep strength of hydrous melt bearing dunite was 2 5 times lower than that of melt free dunite the dependence of creep rate on melt fraction is known to be expressed empirically as e e 0 exp where is a constant and is the melt fraction the experimentally obtained value of was in the range of 150 230 corresponding to 5 7 times the reported values for the olivine basalt system at 0 3 gpa i e creep strength of dunite was efficiently reduced by the hydrous melt superplasticity is the dominant creep mechanism of olivine in fluid bearing fine grained peridotites under low temperature and high stress conditions i e peridotite shear zones in the upper mantle superplasticity induced by geological fluids would play an important role in the shear localization and thus initiation of subduction in the upper mantle

carbonate deposition in an ephemeral siliciclastic alluvial system jurassic shuttle meadow formation newark supergroup hartford basin usa abstract nonmarine carbonate accumulation from a sedimentologic perspective has multiple possible origins clastic load springs groundwater discharge soil development biochemical seasonal changes cave speleothems and eolian influx many of these carbonates contain diagnostic textures from which origin is easy to ascertain however differentiating between clastic and spring groundwater carbonates especially in depositional systems dominated by siliciclastics is not always simple here criteria for recognition are developed subsurface hydrology and tectonic origin of the depositional basin source rocks surrounding the basin sedimentologic packaging and extent of the carbonates in question as well as their sedimentary structures and fossil content and stable isotopic analyses of the carbonates can all aid in determining carbonate origin and correctly assessing sedimentary and basin evolution the present study focuses on a nonmarine carbonate deposit found within the jurassic shuttle meadow formation of the hartford basin one of many rift basins which formed as a result of the opening of the north atlantic ocean during the late triassic to early jurassic the upper 50 m of the formation at plainville connecticut is interpreted as continental in origin and is composed of five facies red ripple cross laminated muddy siltstones to sandstones fr sr massive red siliciclastic mudstones fm buff fine to medium grained horizontally laminated sandstones sh red to white disrupted calcareous shales fl3 and rare micritic carbonates pm facies fr sr sh and fm are interpreted as an alluvial plain system in an ephemeral setting with bedload sheetflooding as the dominant depositional mechanism similar to the channel country drainage system of central australia in addition preservation of lateral accretion sandstone sheets accretionary benches indicate that the shuttle meadow alluvial system was rapidly accreting the anomalous existence of carbonate beds in facies fl3 and pm in the smf bedload dominated siliciclastic sequence is attributed mainly to the unique nature of the clastic sediment load in the basin no direct evidence for a spring groundwater input is apparent the shallow carbonate lake margin or a pond with vegetation and periodic exposure is envisioned as the depositional environment the intraclastic texture of the micrites with rare primary relict lamination indicates early diagenetic alteration from root penetration desiccation cracks and burrowing

jurassic and cretaceous apparent polar wander relative to south america some tectonic implications late mesozoic apparent polar wander apw with respect to stable south america has been difficult to define it appears that this observation has a simple explanation very little apw occurred and none at all during a 50 m y period 165 to 115 ma it is possible to compute a stillstand pole for this interval 88 8 s 72 4 e k 244 7 a95 4 9 from five reliable studies comprising a total of 523 sites a second stillstand of much shorter duration 70 80 ma is defined by four poles 52 sites this is located at 80 8 s 346 7 e k 457 7 a95 4 3 apw of 9 2 occurred during the interval 115 80 ma a comparable stillstand is found in the north american record e g van fossen and kent 1992 this has a similar degree of scatter k 421 and spans at least 36 m y 124 to 88 ma thus north america and south america both were rotating due westward during a large part of the cretaceous this has some important implications for global tectonics

sedimentology and isotopic composition of upper palaeocene to eocene non marine stromatolites eastern ebro basin ne spain stromatolites are abundant in upper palaeocene to eocene fluvial deposits from the eastern ebro basin whereas they are scarce in the lacustrine facies the fluvial stromatolites display a variety of growth forms oncoids domes laminated crusts and locally irregular bioherms these morphologies are related to the hydrological behaviour of the fluvial systems elongate domes dome heads and ovoid to subspherical oncoids accreted in flowing water laminated crusts and small sized cylindrical to lenticular oncoids formed during low discharge and ponding phases in the lacustrine environments only small sized ovoid oncoids occur the main characteristics of the ebro basin non marine stromatolites are 1 laminations and concentric banding exhibited by all morphologies 2 the occurrence of well preserved cyanobacterial filaments in several laminae suggesting that these microbiotas are the major contributors in growth 3 the asymmetrical growth of cortices in domes and oncoids with polar thickening on the upper side suggesting in situ growth under low energy conditions 13c values of carbonate stromatolites range from 11 3 o late palaeocene to 4 4 o middle late eocene during the late palaeocene under tectonically quiescent conditions the 13c values mainly reflect the influence of soil derived co2 during the eocene the progressive uplift of the surrounding catchment areas led to an increase of exposures of mesozoic rocks consequently the 13c values of the eocene stromatolites reflect the influence of the co2 derived from the dissolution of mesozoic marine carbonates there is a 2 5 o shift in 18o values of stromatolites from the late palaeocene to middle late eocene which is consistent with the variation in 18o of precipitation due to changes in altitude of the catchment area during this time the isotopic values of all middle upper eocene laminated crust samples show distinct covariant trends suggesting that these stromatolites were formed either in ponding zones of fluvial channels or in disconnected pools developed during low discharge episodes

using u pb ages of miocene tufa for correlation in a terrestrial succession barstow formation california sedimentary rocks and associated fossils are the direct record of past life and its environmental context few methods allow direct and precise dating of this important archive making it diffi cult to correlate climate biologic and tectonic events preserved in terrestrial records we sampled lacustrine tufa calcite from the middle member of the miocene barstow formation to further test the feasibility of uranium lead dating of terrestrial carbonates five samples yield u pb ages 2 of 14 81 0 39 ma 15 30 0 25 ma 15 39 0 15 ma 16 14 0 40 ma and 16 24 0 23 ma these precise ages are consistent with existing ages from k ar and 40 ar 39 ar dating of intercalated volcanic ashes our most precise u pb age on tufa has an uncertainty of 1 150 ka which is equivalent to uncertainties for k ar ages from the same time period our results indicate that u pb dating of carefully sampled and screened sedimentary carbonates provides much needed age constraints in purely terrestrial sequences and can be a powerful tool for correlation even in deposits with structural complexity and rapid facies changes

rheology of fine grained forsterite aggregate at deep upper mantle conditions high pressure and high temperature deformation experiments on fine grained synthetic dunite forsterite aggregate were conducted to determine the dominant deformation mechanism in the deep upper mantle the sintered starting material has 90 forsterite 10 enstatite and an average grain size of 1 m deformation experiments were performed using a deformation dia apparatus at pressures of 3 03 5 36 gpa temperatures of 1473 1573 k and uniaxial strain rates of 0 91 10 5 to 18 6 10 5 s 1 at dry circumstances 50 h 106si the steady state flow stress was determined at each deformation condition derived stress strain rate data is analyzed together with that reported from similar but low pressure deformation experiments using flow law equations for diffusion creep stress exponent of n 1 grain size exponent of p 2 and for dislocation accommodated grain boundary sliding gbs disl n 3 p 1 the activation volume for diffusion creep v dif and for gbs disl v gbs of dunite is determined to be 8 2 0 9 and 7 5 1 0 cm3 mol respectively calculations based on these results suggest that both diffusion creep and dislocation creep play an important role for material flow at typical deformation conditions in the earth s asthenospheric upper mantle whereas the contribution of gbs disl is very limited and dislocation creep is the dominant deformation mechanism during the deformation of olivine in sheared peridotite xenolith though these conclusions are not definitive these are the first results on potential deformation mechanisms of forsterite aggregate based on extrapolation in the pressure temperature stress and grain size space

creep behavior of fe bearing olivine under hydrous conditions to understand the effect of iron content on the creep behavior of olivine mgxfe 1 x 2sio4 under hydrous conditions we have conducted tri axial compressive creep experiments on samples of polycrystalline olivine with mg contents of x 0 53 0 77 0 90 and 1 samples were deformed at stresses of 25 to 320 mpa temperatures of 1050 to 1200 c a confining pressure of 300 mpa and a water fugacity of 300 mpa using a gas medium high pressure apparatus under hydrous conditions our results yield the following expression for strain rate as a function of iron content for 0 53 x 0 90 in the dislocation creep regime e e 0 901 x0 11 2exp226 1030 9 xrt in this equation the strain rate of san carlos olivine e 0 90 is a function of t and fh2o as previously shown for anhydrous conditions an increase in iron content directly increases creep rate in addition an increase in iron content increases hydrogen solubility and therefore indirectly increases creep rate this flow law allows us to extrapolate our results to a wide range of mantle conditions not only for earth s mantle but also for the mantle of mars

carbon and oxygen stable isotopic signatures of methanogenesis temperature and water column stratification in holocene siderite varves abstract manganoan siderite fe mn co 3 occurs in abundance of up to 19 dry weight as the sole endogenic carbonate within a succession of holocene organic rich varved sediments from freshwater otter lake ol michigan radiocarbon dating and varve counts from a 7 m piston core constrain periods of major siderite accumulation to sediments older than 1200 cal yr bp sediment petrography suggests that siderite was a seasonal precipitate confined to the summer layer of the varve couplet bulk sediment chemistry reveals cycles in abundance of manganese and aluminum coincident with centennial scale cycles of siderite accumulation siderite 13 c and 18 o are enriched when the mineral is abundant and depleted when it is least abundant samples with high abundance of siderite precipitated in oxygen isotopic equilibrium with modern lake water but are 13 c enriched relative to modern waters samples from intervals of low siderite abundance are 13 c and 18 o depleted relative to equilibrium with modern lake water these data suggest that abundant siderite precipitation occurred when the ol water column was ferruginous iron meromictic allowing for enhanced ferrous iron concentrations and dissolved inorganic carbon dic enriched in 13 c below the chemocline where methanogenesis in waters and sediments influenced dic composition seasonal siderite precipitation was triggered by water column alkalinity fluctuations driven by summer calcite dissolution manganese substitution in siderite lowered kinetic barriers to low temperature mineral precipitation climate changes and basin filling influenced lake stratification and the rate of siderite precipitation the siderite occurrence reported here displays remarkable similarity to its occurrence in paleogene maar lake deposits suggesting that further studies of holocene lacustrine siderites may provide insight into ancient sedimentary systems and environments

geochemical evaluation of fenghuoshan group lacustrine carbonates north central tibet implications for the paleoaltimetry of the eocene tibetan plateau abstract a sedimentologic petrologic and geochemical evaluation of lacustrine carbonates from the eocene oligocene fenghuoshan group of north central tibet provides insight into the paleoenvironmental and paleolimnological setting of the hoh xil basin during the mid tertiary fenghuoshan lacustrine rocks consist primarily of carbonate mudstones and wackestones these carbonates are generally less than 3 m in thickness and are intercalated with siliciclastic lacustrine and fluvial alluvial plain deposits individual limestone beds coarsen and contain increasing amounts of siliciclastic material upward fenghuoshan carbonates also contain abundant ostracode bivalve and charaphyte fossils sedimentologic evidence suggests that carbonate deposition occurred in shallow relatively short lived lacustrine systems that were subsequently filled by alluvial deposits of coeval fluvial systems stable c and o isotopic analysis of fenghuoshan carbonates show 18o values ranging from 11 7 to 10 3 vienna pee de

carbonate lake deposits associated with distal siliciclastic perennial river systems abstract the genesis of thick accumulations of carbonate lake deposits on the siliciclastic floodplains of perennial rivers situated below the regional spring line requires 1 a significant source for calcium in the watershed and 2 a protected area to accumulate calcite the phanerozoic record of fluvial deposits shows that anastomosing rivers with carbonate rich provenance situated in distal areas below the spring line are the best environments for floodplain accumulation of significant carbonate lake deposits anastomosing rivers exist below the regional spring line and have isolated flood basins generally protected from bedload by high levees receiving suspended and dissolved load during flooding events numerical modeling to test the rate of accumulation for these carbonate sediments was performed using stratigraphic data from the pennsylvanian succession containing floodplain limestones of the stewart quadrangle of athens county in ohio usa limestone volumes for six limestone units upper pittsburgh fishpot sewickley benwood arnoldsburg uniontown and waynesburg limestones were calculated with the assumption of overland input of dissolved calcite from carbonate source areas in comparing the calcium input rate to the amount of calcium calculated from limestone unit volumes overland transport of calcium ions not spring input can supply sufficient ions to accumulate thick carbonates over the short lifetime of floodplain lakes within anastomosing river systems

influence of water fugacity and activation volume on the flow properties of fine grained anorthite aggregates 1 to specify quantitatively the effect of pressure and water weakening on the flow strength of feldspar we performed triaxial creep experiments in a gas deformation apparatus at temperatures of 1000 1150 c confining pressures of 100 450 mpa and axial stresses of 10 400 mpa resulting in strain rates of 6 10 7 to 3 10 3 s 1 dense samples with a grain size of 3 m were prepared by hot isostatic pressing of anorthite glass powder hydrous samples contain about 0 33 0 14 wt h2o and dry specimens 0 0005 0 02 wt h2o the estimated residual glass content of wet samples is 2 vol samples deformed by grain boundary diffusion controlled creep at low stresses and dislocation creep at stresses 150 mpa we estimate an activation volume of v 24 cm3mol 1 for anhydrous samples deforming in diffusion creep for wet samples deformed in hydrous conditions with varying buffers fixing oxygen fugacity the activation volume is about 38 cm3 mol 1 creep rate of hydrous anorthite aggregates depends on water fugacity raised to a power of r 1 0 0 3 suggesting hydrolysis of oxygen bonds considering the effect of activation volume and water fugacity on extrapolation of constitutive laws to conditions prevailing in the continental lower crust viscosities of hydrous feldspar aggregates increase by a factor of 3

regional paleohydrologic and paleoclimatic settings of wetland lacustrine depositional systems in the morrison formation upper jurassic western interior usa abstract during deposition of the upper jurassic morrison formation water that originated as precipitation in uplands to the west of the western interior depositional basin infiltrated regional aquifers that underlay the basin this regional groundwater system delivered water into the otherwise dry continental interior basin where it discharged to form two major wetland lacustrine successions a freshwater carbonate wetland lacustrine succession formed in the distal reaches of the basin where regional groundwater discharged into the denver julesburg basin which was a smaller structural basin within the more extensive western interior depositional basin an alkaline saline wetland lacustrine complex lake t oo dichi formed farther upstream where shallower aquifers discharged into the san juan paradox basin which was another small structural basin in the western interior depositional basin these were both wetlands in the sense that groundwater was the major source of water input from surface and meteoric water was limited in both basins lacustrine conditions developed during episodes of increased input of surface water inclusion of wetlands in our interpretation of what had previously been considered largely lacustrine systems has important implications for paleohydrology and paleoclimatology the distal carbonate wetland lacustrine deposits are well developed in the morrison formation of east central colorado occupying a stratigraphic interval that is equivalent to the lower morrison but extends into the upper morrison formation sedimentologic paleontologic and isotopic evidence indicate that regional groundwater discharge maintained shallow hydrologically open well oxygenated perennial carbonate wetlands and lakes despite the semi arid climate wetland deposits include charophyte rich wackestone and green mudstone lacustrine episodes in which surface water input was significant were times of carbonate and siliciclastic deposition in scarce deltaic and shoreline deposits marginal lacustrine deposits include ooid and skeletal packstone grainstone siltstone and sandstone distal lacustrine units are skeletal mudstone wackestone microbialites and laminated siliciclastic mudstone differentiation between wetlands and distal lacustrine units is not always possible palustrine features magadi type chert mtc and evaporites record episodes of increased aridity and exposure farther upstream during deposition of the upper part of the brushy basin member the ancestral uncompahgre uplift imposed a barrier to shallow eastward flowing groundwater that discharged into the san juan paradox basin on the upstream side of the uplift this created the closed hydrologic setting necessary for development of an alkaline saline wetland lacustrine complex lake t oo dichi silicic volcanic ash delivered by prevailing winds from calderas west and southwest of the basin contributed to the pore water evolution in the sediments a distinctive lateral hydrogeochemical gradient reflecting increasing salinity and alkalinity in the pore waters altered the ash to a variety of authigenic minerals that define concentric zones within the basin the basinward progression of diagenetic mineral zones is smectite clinoptilolite analcime potassium feldspar albite the groundwater fed wetlands were shallow and frequently evaporated to dryness scarce laminated gray mudstone beds record distinct episodes of freshwater lacustrine deposition that resulted from intermittent streams that carried detritus well out into the basin

influence of mineral fraction on the rheological properties of forsterite enstatite during grain size sensitive creep 2 deformation experiments 1 in order to understand the effects of secondary minerals on the flow properties of rocks we have conducted uniaxial compression experiments on polycrystalline forsterite fo enstatite en samples at constant temperature and strain rate the flow stress of the samples decreases with increasing enstatite volume fraction fen for samples with 0 fen 0 5 and increases with increasing fen for samples with 0 5 fen 1 the values of the preexponential term stress and grain size exponents and activation energy in the constitutive equation for a wide range of fen were determined samples with a low fen 0 03 deformed at strain rates of 2 10 5 to 2 10 4 s exhibit creep characteristics that correspond to dislocation accommodated grain boundary sliding creep i e stress exponent n 3 whereas diffusion accommodated grain boundary sliding creep is typical of high fen samples i e stress exponent n 1 the change of flow strength as a function of fen during grain size sensitive creep is primarily due to changes in grain size of both phases and secondarily due to changes in the volume fraction of phases with different flow strengths viscosities of all samples can be reproduced in a viscosity model that takes into account 1 the grain sizes estimated by the grain growth laws established in our part 1 paper and 2 flow laws determined for the individual phases in this case forsterite and enstatite furthermore we demonstrate that our model can be extended to make predictions of viscosity in other mineral assemblages

first occurrence of charophyte algae from a triassic paleolake in argentina and their paleoenvironmental context abstract this paper presents the first record of charophytes from the triassic sedimentary rocks of argentina and the entire southern hemisphere algal remains were found in the cerro puntudo formation dated as anisian 243 8 1 9 ma the formation is part of the cuyana basin which formed as consequence of the triassic rifting of pangaea the paleoenvironments represented by this succession include from bottom to top 1 a proximal to medial alluvial fan system 2 a distal alluvial fan deposit 3 a braided fluvial system with small channelized bodies 4 a distal fluvial system containing rare carbonates and 5 a carbonate rich lacustrine system the lacustrine facies association corresponds mainly to carbonate microbialites with three recognized facies namely disrupted micritic limestones lmd stromatolitic limestones ls and oncolitic limestones lo the lo facies is rich in oncolites formed by filamentous algae in the cortex and charophyte remains gyrogonites in the nucleus and is interpreted as being deposited in a carbonate rich pond environment perhaps sourced from springs filamentous algae are thought to cause the precipitation of carbonates and trigger the development of micritic coatings in the ponds sedimentary and paleobiologic evidence suggest minor movement of the oncolites in a ponded environment where charophytes developed the charophyte remains found in the cerro puntudo formation are gyrogonites attributed to the porocharaceae family all the previous records for triassic charophyta correspond to laurasia in the northern hemisphere therefore the paleolatitudinal location of this finding allows the expansion of the paleobiogeographic distribution of the group into gondwana

effect of varying enstatite content on the deformation behavior of fine grained synthetic peridotite under wet conditions the effect of varying enstatite content on the deformation behavior of synthetic fine grained 1 to 2 m forsterite enstatite rock with 0 5 wt added water was investigated at temperatures of 900 to 1000 c strain rates between 10 7 and 10 5 s 1 and a confining pressure of 600 mpa the samples exhibited approached steady state flow at stresses 60 mpa the results show that at constant strain rate increasing enstatite content is associated with a sharp decrease in flow strength in the range 0 2 5 vol enstatite with little further change up to 20 vol the observed power law n value of 1 7 and microstructures are similar to those obtained in previous work on material with 2 5 wt enstatite and are consistent with a water enhanced grain boundary sliding gbs dominated deformation mechanism significantly a negative correlation was found between grain size and enstatite content indicating that enstatite content played a role in controlling the grain size of the starting materials moreover a high correlation between measured flow strength and grain size was found consistent with a grain size exponent of 3 in a conventional grain size sensitive flow equation a water enhanced deformation mechanism involving gbs accommodated probably by grain boundary diffusion and or dislocation activity is therefore implied with the effect of enstatite content on flow strength being an indirect physical effect caused by grain size control other effects of second phase content such as weakening caused by interphase boundary diffusion and or migration processes seem to be unimportant under the conditions investigated

stable isotope evidence for topographic growth and basin segmentation implications for the evolution of the ne tibetan plateau lithologic magnetostratigraphic and stable isotope records from the neogene xunhua and linxia basins along the tibetan plateau9s northeastern margin suggest that topography in the intervening jishi shan mountain range began to develop between 16 and 11 ma perturbations to local climate patterns resulting from the evolution of local topography are tracked through comparison of stable isotope compositions of calcareous basin fill materials across the jishi shan similarity of isotopic compositions is interpreted to reflect the presence of integrated basins whereas distinct isotopic compositions reflect unique basin hydrologies divergent isotope trends develop between ca 16 and 11 ma and are indicative of hydrologic separation in the adjacent xunhua and linxia basins and increased aridity in the leeward xunhua basin the development of aridity in the lee of the growing topography along the plateau9s northeast margin highlights the importance of evaporative enrichment in this extremely continental setting and explains the presence of anomalously positive 18 o values in modern rainfall our findings add to a growing body of evidence for deformation along the plateau9s north and northeastern margins in the middle to late miocene

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

constraints on the mechanism of deformation of olivine imposed by defect chemistry abstract more is known about diffusion electrical conductivity and creep in forsterite and olivine than any other mineral this paper examines these data to see what constraints are placed on the mechanisms of deformation in these minerals experimental observations show a creep rate for forsterite independent of the activity of oxygen a o 2 in olivine the creep rate is proportional to a 1 6 o 2 and in addition is ten times faster for the enstatite excess situation than for the magnesiowustite excess situation these data are consistent with positively charged kinks being the rate controlling species in olivine the data for forsterite are also compatible with a kink controlled mechanism although some other jog and diffusion controlled mechanisms remain as possibilities the activation energy for creep is not related to that for diffusion of silicon or oxygen for kink controlled mechanisms the activation energy is approximately given by the sum of the fermi level and the activation energy for charged kink migration minus the ionization energy of the kink the fermi level can be changed by minor changes in impurities and by changes in the imposed thermodynamic environment so unless these parameters are controlled during deformation variations in observed activation energies for creep are to be expected from one set of experiments to another

flow laws of multiphase materials and rocks from end member flow laws abstract rheological properties of polyphase rocks play an important role in the dynamics of the lithosphere and asthenosphere however flow laws for large portions of the polyphase rocks in the earth s crust and mantle have not been well determined an analysis based on the theory of mixtures has been made to calculate the general flow laws of coarse 5 m nearly equant grained aspect ratio 3 and massive polyphase rocks and materials for which only the flow laws and volume fractions of the constituents are taken into consideration in the modeling of the bulk rheology and effects of microstructure could be ignored the theoretical analysis is based on three assumptions 1 the polyphase composite and the monophase aggregates of its constituents obey the same kind of flow laws linear power or exponential 2 there is no change in the operative deformation mechanism of each phase when it is in the composite as compared to when it is in a monophase aggregate and 3 neither chemical metamorphic reactions take place among the constituent phases nor eutectic melting occurs due to the phase mixing the proposed iterative process allows to predict to the first approximation the flow laws for a large number of polyphase rocks in terms of the experimentally determined flow laws of a relatively small number of monomineralic aggregates applications of this approach to typical polyphase rocks such as granite diorite diabase aplite and websterite as well as to synthetic two phase materials such as forsterite enstatite mixtures and water ice ammonia dehydrate aggregates yield quite accurate approximations to the experimental values