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

speciation of sulfur in the insoluble organic matter from carbonaceous chondrites by xanes spectroscopy sulfur speciation in a comprehensive set of insoluble organic matter iom samples extracted from 3 ci orgueil alais ivuna 5 cm que97990 murchison murray que99355 cold bokkeveld 1 cr renazzo and ungrouped c2 tagish lake chondrites has been studied by k edge xanes micro spectroscopy five main sulfur groups were identified 1 sulfides 2 aliphatic sulfur 3 heterocyclic organic sulfur 4 oxidized organic sulfur and 5 sulfates the iom of the 3 cis and the extensively altered cold bokkeveld cm exhibit a higher abundance of heterocyclic versus aliphatic organic sulfur compared to the other cms and compared to the renazzo and tagish lake chondrites this suggests greater thermal heating on the parent body of cis consistent with the higher temperatures experienced by these chondrites 70 c for the most altered clasts of cold bokkeveld 100 150 c for cis alais may have experienced more heating than ivuna and orgueil the iom of cis contains oxidized organic sulfur suggesting the presence of a mild temperature oxidation process on the parent body among type 2 chondrites other than cold bokkeveld no significant variation of the sulfur speciation was detected this suggests tenuous oxidation processes and a low temperature aqueous alteration 35 c on the parent body the global chemical variations between the cr and cm groups e g h c elemental ratio alkyl groups concentration reported in earlier studies appear to be more the result of chemical variations among the accreted precursors than of post accretional processes

rethinking the ancient sulfur cycle the sulfur biogeochemical cycle integrates the metabolic activity of multiple microbial pathways e g sulfate reduction disproportionation and sulfide oxidation along with abiotic reactions and geological processes that cycle sulfur through various reservoirs the sulfur cycle impacts the global carbon cycle and climate primarily through the remineralization of organic carbon over geological timescales cycling of sulfur is closely tied to the redox state of earth s exosphere through the burial of oxidized sulfate and reduced sulfide sulfur species in marine sediments biological sulfur cycling is associated with isotopic fractionations that can be used to trace the fluxes through various metabolic pathways the resulting isotopic data provide insights into sulfur cycling in both modern and ancient environments via isotopic signatures in sedimentary sulfate and sulfide phases here we review the deep time 34s record of marine sulfates and sulfides in light of recent advances in understanding h

a two phase model for compaction and damage 3 applications to shear localization and plate boundary formation a new two phase theory employing a nonequilibrium relation between interfacial surface energy pressure and viscous deformation bercovici et al this issue provides a model for damage void generation and microcracking and thus a continuum description of weakening failure and shear localization here we demonstrate applications of the theory to shear localization with simple shear flow calculations in which one phase the matrix representing for example silicate is much stronger more viscous than the other phase the fluid this calculation is motivated as a simple model of plate boundary formation in a shear zone even without shear the two phases eventually separate due to gradients in surface tension however the influence of shear on phase separation is manifest in several ways as shear velocity increases the separation rate of the phases increases demonstrating a basic feedback mechanism accumulation of the fluid phase causes focused weak zones on which shear concentrates causing more damage and void generation and thus greater accumulation of fluid beyond a critical shear velocity phase separation undergoes intense acceleration and focusing leading to a tear localization in which the porosity becomes nearly singular in space and grows rapidly like a tear or crack at an even higher value of shear velocity phase separation is inhibited such that shear localization gives way to defocusing of weak zones suggestive of uniform microcracking and failure throughout the layer our two phase damage theory thus predicts a wide variety of shear localization and failure behavior with a continuum model applications of the theory to various fields such as granular dynamics metallurgy and tectonic plate boundary formation are numerous

identifying the influence of geology land use and anthropogenic activities on riverine sulfate on a watershed scale by combining hydrometric chemical and isotopic approaches abstract throughout the last few decades sulfate concentrations in streamwater have received considerable attention due to their dominant role in anthropogenic acidification of surface waters the objectives of this study conducted in the oldman river basin in alberta canada were to determine the influence of geology land use and anthropogenic activities on sources concentrations and fluxes of riverine sulfate on a watershed scale this was achieved by combining hydrological chemical and isotopic techniques surface water samples were collected from the main stem and tributaries of the oldman river on a monthly basis between december 2000 and march 2003 and analyzed for chemical and isotopic compositions at a given sampling site sulfate sources were primarily dependent on geology and did not vary with time or flow condition with increasing flow distance a gradual shift from 34 s values 10 and 18 o values 0 of riverine sulfate indicating evaporite dissolution and soil derived sulfate in the predominantly forested headwaters to negative 34 s and 18 o values suggested that sulfide oxidation was the predominant sulfate source in the agriculturally used downstream part of the watershed significant increases in sulfate concentrations and fluxes with downstream distance were observed and were attributed to anthropogenically enhanced sulfide oxidation due to the presence of an extensive irrigation drainage network with seasonally varying water levels sulfate s exports in an artificially drained subbasin 64 kg s ha yr were found to exceed those in a naturally drained subbasin 4 kg s ha yr by an order of magnitude our dataset suggests that the naturally occurring process of sulfide oxidation has been enhanced in the oldman river basin by the presence of an extensive network of drainage and irrigation canals

contemporaneous early diagenetic formation of organic and inorganic sulfur in estuarine sediments from st andrew bay florida usa estuarine sediment samples were collected from nine stations in st andrew bay near panama city florida usa contrasting grain size and varying concentration of organic carbon allowed a comparative study of the relationships among organic degradation processes sulfur cycling and diagenetic removal of dissolved sulfide produced by bacterial sulfate reduction abundance and sulfur isotopic composition were determined for dissolved sulfide dissolved sulfate pyrite humic acid sulfur fulvic acid sulfur and elemental sulfur presence of free dissolved sulfide and enrichment in 34s of porewater sulfate in samples from organic rich muddy sediments of st andrew bay indicate high rates of sulfate reduction and anoxic conditions below 5 mm depth such conditions are not reflected in samples from organic poor sandy near shore sediments collected from highly productive areas occupied by seagrass beds in this estuarine system pyrite and fulvic acid sulfur are the largest sinks for sulfide produced by bacterial sulfate reduction strong correlation between abundances of pyrite and fulvic acid sulfur suggests coincident formation in the uppermost 10 cm of sediment large differences in isotopic composition between these two species and consistent enrichment in 34s of the fulvic acid fraction however indicate two different reaction pathways abundance and sulfur isotopic composition of humic acid and fulvic acid sulfur suggest that organic sulfur is derived from mixing between 1 34s depleted sulfur with an unknown oxidation state that was recycled from bacterial hydrogen sulfide and 2 34s enriched sulfur interpreted as primary biosynthetic sulfur originally assimilated from dissolved sulfate samples taken from sandy sediment localities where aquatic vascular plants are rooted in the anoxic zone show a strong depletion in 34s in the fulvic acid sulfur fraction suggesting recycled hydrogen sulfide as the dominant source of organic sulfur

a simple model of plate generation from mantle flow summary a simple model of non newtoniancreeping ow is used to evaluate classes of rheologieswhichallow viscous mantle ow to becomeplate like the model describes shallow layerlithosphericmotion driven by sources and sinks the sources represent spreading ridges while the sinks rep resent subduction zones the sources and sinks thus also prescribe the poloidal component ofthe surface ow eld the toroidal strike slip component of the ow eld is found via thesolution of the stokes equation with non newtonian rheology as a rst basic investigation ofthe model the horizontal divergence from the two dimensional rectangular velocity eld ofolson bercovici 1991 is used for the source sink eld the degree to which the induced uid ow reproducesthe rectangularplate is used to measure the success of differentrheologiesin generating plate like ows results indicate that power law rheologies even in the limit ofvery high power law index can only produce modest plate like ow for example the ratioof toroidal to poloidal kinetic energy for a source sink eld derived from a square plate is atbest 0 65 whereas a perfect square plate has a ratio of 1 0 moreover the power law rheologyappears to reach an asymptotic limit in its ability to produce plate like behavior this impliesthat plate tectonics is unlikely to arise from a power law rheology even in the limit of very high a class of rheologies that yield signi cantly more promisi ng results arise from the carreaupseudo plastic rheology with the power law index taken to be 0 one rheology in thisclass is the continuum model for stick slip earthquake behavior of whitehead gans 1974 which is essentially the carreau equation with 1 this class of rheologies referred to asthe stick slip rheologies induces a toroidal to poloidal kinetic energy ratio for a square plates ssource sink function which can be as high as0 9 the viscosity or strength distribution forthis class of rheologies also appears more plate like showing fairly uniform high viscosity re gions pseudo plates and sharply de ned low viscosity zones pseudo margins in contrast even the most nonlinear power law rheology produces spatially varying high viscosity regionsand relatively smooth low viscosity margins the greater success of the stick slip rheologiesin producing plates is attributed to a self lubricating mechanism in which the transfer of mo mentum from regions of high shear to low shear is inhibited in contrast even in the limit ofin nite power law index a power law rheology can retard but never prohibit momentum trans fer this feature is essential to the sharpening of velocity pro les into plate like pro les whichis illustrated with a simple boundary layertheory key words plate tectonics mantle convection non newtonian ow toroidal poloidal cou pling

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

fast ductile failure of passive margins from sediment loading sediment loading at continental margins has been interpreted as a very inefficient mechanism for initiating subduction we have re investigated this problem using a time dependent non linear ductile failure analysis in which the effects of power law plastic strain hardening are assessed the results follow classical flexural rigidity solutions until wholesale lithosphere failure occurs sediment loading causes two shear zones to develop from the bottom of the lithosphere upwards for strong strain hardening shear zones nucleate earlier but the plate may never break for small hardening the instability grows much faster and the plate can break under the sediment load with a limiting yield stress lower than 0 4 gpa when the ductile faults growing from the top and the bottom of the plate merge in the center the flexurally supported gravitational energy is released the lithosphere becomes unstable and shears off super exponentially within a few million years

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

mechanisms of shear localization in the continental lithosphere inference from the deformation microstructures of peridotites from the ivrea zone northwestern italy the ultramafic massif of balmuccia northwestern italy shows a variety of deformation fabrics including some localized shear zones that resulted in nearly complete melting pseudotachylyte a series of peridotite specimens were collected near one of the pseudotachylyte fault veins to investigate the mechanisms of shear localization the microstructural analyses show at least three deformation stages the first stage i is nearly homogeneous deformation at low stress 3 mpa and high temperature 1300 1500 k followed by localized deformation stage ii at a higher stress 60 mpa and moderate temperature 1000 1150 k and finally semi brittle deformation stage iii at a very high stress 400 mpa that resulted in the formation of pseudotachylyte the stage ii deformation resulted in relatively small strains in most areas but significant localized deformation leading to dynamic recrystallization occurred the degree of which increases systematically toward the fault zone based on this observation together with the observation that the brittle deformation post dated dynamic recrystallization we conclude that the shear localization in this locality occurred as a result of ductile deformation rather than brittle deformation various mechanisms of shear localization in the ductile regime are examined and we conclude that the grain size reduction due to dynamic recrystallization at relatively high stresses and moderate temperatures is a probable mechanism of shear localization the intermediate regime between dislocation and diffusion creep where rheology is grain size sensitive yet continuous recrystallization occurs due to dislocation creep is suggested to play an important role in shear localization in the upper mantle the conditions of shear localization due to this mechanism are examined based on the laboratory data on creep and dynamic recrystallization it is shown that shear localization occurs at relatively low temperatures and or high stresses localized deformation in the continental lithosphere will occur well below the brittle ductile transition 10 km depth down to 40 80 km depth depending upon the geothermal gradient and water fugacity

relationships between dynamically recrystallized grain size and deformation conditions in experimentally deformed olivine rocks new microstructural data on experimentally deformed wet and dry natural olivine rocks anita bay and aheim dunite together with the other reliable experimental data indicate that the experimental stress recrystallized grain size relationship in olivine rocks is largely independent of water content and temperature and is only slightly dependent on the flow properties of the material the experimental data cover a stress range of 30 300 mpa water contents from 30 ppm to 300 ppm and temperatures in the range 1100 1650 c local melt contents of up to 10 volume cannot be demonstrated to have a significant effect on the stress grain size relationship

quantitative measure of the variation in fault rheology due to fluid rock interactions we analyze friction data from two published suites of laboratory tests on granite in order to explore and quantify the effects of temperature t and pore water pressure pp on the sliding behavior of faults rate stepping sliding tests were performed on laboratory faults in granite containing gouge granite powder both dry at 23 to 845 c lockner et al 1986 and wet pp 100 mpa at 23 to 600 c blanpied et al 1991 1995 imposed slip velocities v ranged from 0 01 to 5 5 m s and effective normal stresses were near 400 mpa for dried granite at all temperatures and wet granite below 300 c the coefficient of friction shows low sensitivity to v t and pp for wet granite above 350 drops rapidly with increasing t and shows a strong positive rate dependence and protracted strength transients following steps in v presumably reflecting the activity of a water aided deformation process by inverting strength data from velocity stepping tests we determined values for parameters in three formulations of a rate and state dependent constitutive law one or two state variables were used to represent slip history effects each velocity step yielded an independent set of values for the nominal friction level five constitutive parameters transient parameters a b1 and b2 and characteristic displacements dc1 and dc2 and the velocity dependence of steady state friction ss ln v a b1 b2 below 250 data from dry and most wet tests are adequately modeled by using the slip law ruina 1983 and one state variable a 0 003 to 0 018 b 0 001 to 0 018 dc 1 to 20 m dried tests above 250 can also be fitted with one state variable in contrast wet tests above 350 require higher direct rate dependence a 0 03 to 0 12 plus a second state variable with large negative amplitude b2 0 03 to 0 14 and large characteristic displacement dc2 300 to 4000 m thus the parameters a b1 and b2 for wet granite show a pronounced change in their temperature dependence in the range 270 to 350 c which may reflect a change in underlying deformation mechanism we quantify the trends in parameter values from 25 to 600 c by piecewise linear regressions which provide a straightforward means to incorporate the full constitutive response of granite into numerical models of fault slip the modeling results suggest that the succeptibility for unstable stick slip sliding is maximized between 90 and 360 c in agreement with laboratory observations and consistent with the depth range of earthquakes on mature faults in the continental crust

sulfur enrichment of humic substances in a delaware salt marsh sediment core abstract humic sulfur operationally defined as the sulfur extracted with humic substances in 0 l n naoh solution comprises up to 51 of the total sulfur inventory in a sediment core taken from a delaware spartina alterniflora marsh pyrite sulfur is the next largest fraction except at the near surface sediments where zerovalent sulfur concentrations are significant x ray photoelectron spectroscopy indicates that the humic sulfur consists of sulfoxides or sulfones and in a more reduced state organic sulfides and or organic polysulfides a subsurface decrease in the humic acid c s atomic ratio to 56 2 suggests that the upper 4 cm of marsh sediment is the locus for humic sulfur formation s alterniflora detritus and microbial biomass cannot fully account for observed sulfur enrichment of humic c s atomic ratios therefore the enrichment of humic substances by sulfur is probably via reaction of reduced sulfur compounds with organic matter a humic sulfur formation rate of 10 6 mol s cm 3 a 1 is calculated for the surface sediments and leads to an areal production of 18 mol s cm 2 a 1 of humic sulfur humic sulfur formation and preservation is enhanced by the limited availability of iron for the rapid precipitation of iron sulfide minerals and the apparent resistance of organic sulfur compounds towards reoxidation to sulfate especially in the upper 9 cm of marsh sediment where inorganic sulfur compounds are rapidly oxidized

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

prediction of faulting from the theories of elasticity and plasticity what are the limits elasticity rigid plasticity and elasto plasticity are the simplest constitutive models used to describe the initiation and evolution of faulting however in practice the limits of their application are not always clear in this paper we test the behaviour of these different models using as examples tectonic problems of indentation of a die compression with basal shear bending of a plate and normal faulting around a dike by comparing the results of these tests we formulate some guidelines that may be useful for the selection of an appropriate constitutive model of faulting the theory of elasticity reasonably predicts the initiation of the fault pattern but gives erroneous results for large strains the theory of rigid plasticity is more appropriate for large deformations where the geometry of faults can be found by the method of characteristics this method works well for zones of failure that are not severely constrained by elastic material outside e g when faults are connected to the free surface a viscous substratum or a zone of weakness non associated elasto plasticity is the most complete theory among those considered in this paper it describes the evolution of faults from the initiation of localized deformations to the formation of a complicated fault network

a geodynamic framework for interpreting crustal scale seismic reflectivity patterns in compressional orogens summary 2 d plane strain finite element models are used to calculate crustal deformation patterns within model compressional orogens and the results are compared with natural examples the fundamental driving mechanism for model deformation is provided by asymmetric detachment and underthrusting of mantle lithosphere with deformation being rooted at the stress singularity where the mantle detaches deformation steps up into the overlying crust and spreads laterally according to the rhcological properties assumed for the crust the crust is modelled using plastic frictional and viscous rheologies and incorporates the effects of compositional layering and variable geothermal gradients overall deformation is determined by balancing the internal strength of the crust against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming orogen the amount of convergence in the models is limited to 150 km or less and so the results are not strictly comparable with larger orogens involving greater convergence nevertheless larger orogens seem to evolve through stages that may be understood in terms of the models provided that consideration is confined to smaller scale components that are limited in time and space the continuum deformation calculated by the finite element models is qualitatively converted to equivalent pattcrns of discrete faults and shear zones by associating these discrete features with the areas of highest cumulative strain using the results of sandbox experiments as guides significant styles of deformation include 1 coupling between step up shears and subhorizontal detachments at one or more levels 2 localization of upper crustal deformation by subduction of the lower crust and 3 pinning of deformation by surface denudation a partial catalogue of such model styles is presented in which the strength of basal coupling between crust and mantle the number of rhcological layers within the crust the thickness of subducted lower crust and the extent of surface denudation are varied systematically compressional orogens with a wide range of ages are characterized by crustal scale seismic reflectivity similar to patterns of fault shear zones represented in this catalogue of styles this similarity suggests that crustalscale seismic reflection data can be used to constrain the dynamic processes involved in compressional orogcnesis

earthquakes in the ductile regime pseudotachylytes from a crustal scale shear zone in central australia have developed in a cyclical manner once developed an individual pseudotachylyte is deformed in a ductile manner only to be overprinted at a later stage by a new generation of pseudotachylytes such cyclic generation and deformation of pseudotachylyte has been interpreted in the past as representing conditions at the brittleductile transition a different interpretation however is presented here it is proposed that psuedotachylytes and associated ultramylonites can develop entirely within the ductile regime as ductile instabilities such instabilities are different in nature to those previously discussed at length in the geophysical literature but are identical in principle with the instabilities that develop for velocity weakening frictional behavior in spring slider systems at a given strain rate a critical temperature tc is defined at which the transient work hardening equals the product of stress relaxation due to a thermal fluctuation and the heat generated by shearing a necessary condition for ductile instability at a given strain rate is that the temperature is belowtc then the rate of change of stress with respect to strain is negative an additional requirement is that this rate of change exceeds in magnitude the effective elastic stiffness of the loading system ductile instabilities are marginally possible at geological strain rates in quartzites but are possible at mid crustal temperatures in other rock types on the basis of these observations a new interpretation is presented for the base of the seismogenic zone in crustal regions

sulphur speciation in anoxic hypersaline sediments from the eastern mediterranean sea abstract the anoxic hypersaline tyro and bannock basins are among the most sulphidic bodies of water in the marine environment h2s 2 mm we report the distribution of elemental sulphur acid volatile sulphur avs organic polysulphides humic sulphur 0 5 m naoh extractable and pyritic sulphur in the sediments from these basins pyritic sulphur appears to be the main phase of inorganic reduced sulphur 50 80 of the total sulphur pool and is at the same level about 250 moles per gram dry weight in cores recovered from the two basins remarkably humic sulphur has been found to account for 17 28 of the total sulphur pool in the tyro basin and for 10 43 in the bannock basin sulphur isotope data show negative 34s values for both pyritic sulphur 34s 19 c to 39 and humic sulphur 34s 15 to 30 this indicates that both pyritic and humic sulphur originate from microbially produced hzs neither basin shows a significant correlation between pyritic sulphur and organic carbon suggesting syngenetic pyrite formation additionally the degree of pyritization in these sediments dop is 0 62 indicates that pyrite formation is limited by the reactivity of fe humic sulphur correlates with the pyritic sulphur distribution and seems to be related with gelatinous pellicles bacterial mats at the interface between oxic seawater and brine both pyritic and humic sulphur can be formed in the water column at the interface of oxic seawater and brine by a syngenetic pathway about 5 of the pyrite is formed in the sediments by diagenetic processes the presence of a bacterial mat and high hzs content in the brines favour such conditions however most of the humic sulphur inventory can be supported by diagenetic processes although these basins differ in their major element chemistry their reduced sulphur species chemistry appears to be similar

wrinkle ridges reverse faulting and the depth penetration of lithospheric strain in lunae planum mars abstract wrinkle ridges in the lunae planum region of mars formed in response to compressional stresses associated with the development of the tharsis rise in this study we investigate the accomodation of lithospheric strain associated with wrinkle ridge development in order to constrain the depth penetration of deformation associated with the formation of tharsis using observations that indicate that the shallow martian lithosphere in lunae planum consists of a surface plains unit underlain successively by an unconsolidated megaregolith and competent basement we construct models of an elastic lithosphere in which a ridge nucleates along a single fault in either the surface plains unit or basement we then calculate strain and displacement fields for a range of plausible lithosphere structures assuming uniform horizontal shortening of the lithosphere transmission of strain between the surface layer and basement is most effective for a thin megaregolith and small young s modulus contrast between the surface layer megaregolith and basement however results under a broad range of conditions for both models show straining of both the surface layer and basement if lithosphere structure information and strains estimated by kinematic studies of the wrinkle ridges are applicable then thatsis related compressional deformation must have involved the lithosphere rather than being restricted to the near surface

a source sink model of the generation of plate tectonics from non newtonian mantle flow a model of mantle convection which generates plate tectonics requires strain rate or stress dependent rheology in order to produce strong platelike flows with weak margins as well as strike slip deformation and plate spin i e toroidal motion here we employ a simple model of source sink driven surface flow to determine the form of such a rheology that is appropriate for earth s present day plate motions in this model lithospheric motion is treated as shallow layer flow driven by sources and sinks which correspond to spreading centers and subduction zones respectively the source sink field is derived from the horizontal divergence of plate velocities and thus directly prescribes poloidal motion the toroidal flow field is solved through the non newtonian stokes equation for shallow layer tangential flow on the surface of a sphere two plate motion models are used to derive the source sink field the first is an idealized square plate which is used to explore the basic aspects of the model the second is an analytically continuous model of earth s present day plates bercovici and wessel 1994 as originally implied in the simpler cartesian version of this model bercovici 1993 the classical power law rheologies do not generate platelike flows as well as the hypothetical whitehead gans stick slip rheology which incorporates a simple self lubrication mechanism for the idealized plate geometry the power law rheologies yield much more diffuse strike slip shear i e radial vorticity than the stick slip rheology for the present day plate geometry the power law rheologies fail to reproduce the original proportion of left and right lateral strike slip shear whereas the stick slip rheology gives almost exactly the right proportion none of the fluid rheologies examined however produce more than approximately 60 of the original maximum shear for either plate model the viscosity fields produced by the power law rheologies are diffuse and the viscosity lows over strike slip shear zones or pseudo margins are not as small as over the prescribed convergent divergent margins in contrast the stick slip rheology generates very platelike viscosity fields with sharp gradients at the plate boundaries and margins with almost uniformly low viscosity quantitative comparisons with the toroidal poloidal kinetic energy partitioning and vorticity fields of the original plate model are also examined and the stick slip rheology is generally found to yield the most favorable comparisons power law rheologies with high viscosity contrasts however lead to almost equally favorable comparisons though these also yield the least platelike viscosity fields this implies that the magnitude of toroidal flow and platelike strength distributions are not necessarily related arid thus may present independent constraints on the determination of a self consistent plate mantle rheology the results of this study however predict that if such a rheology can indeed be uniquely determined it is likely to be in the class of stick slip self lubricating rheologies

interaction of weak faults and non newtonian rheology produces plate tectonics in a 3d model of mantle flow according to the theory of plate tectonics relatively rigid plates are bounded by large faults plate motion has negligible internal strain with significant toroidal component to the velocity by contrast models of mantle flow with viscous rheology in an intact medium predict little toroidal component and substantial internal strain in surface motion it has been suggested5 that the observed characteristics of plate motion are related to faulted plate margins which are observed to be weak here we confirm this suggestion using three dimensional models of mantle flow that incorporate faults and the forces exerted on plates by subducting slabs slab pull and mid ocean ridges ridge push our models show that plate like motion results from the interaction between weak faults and a strain weakening power law rheology weak transform faults tend to guide plate motion this guiding effect and the decoupling that occurs at thrust faults may result in oblique subduction convergent margins are associated with realistic trench and fore bulge topography by simultaneously predicting surface kinematics topography and gravity the models achieve a useful degree of tectonic realism

viscous heating in heterogeneous media as applied to the thermal interaction between the crust and mantle we have studied the effects of viscous heating involving heterogeneous media which occurs in the interaction between the mantle lithosphere crust system with the mantle lithosphere in delamination high rates of viscous heating are found in corner flow regions where the deformation rate of the flow is highest the weak lower crust is sheared between the upper crust and the mantle lithosphere giving rise to a second type of viscous heating with high dissipation values several ten to hundred times the chondritic value greater dissipation rates are found in the shear interaction between heterogeneous media with different non newtonian rheologies than in the shear heating inside a single non newtonian or newtonian homogeneous medium high spatial resolution of at least 2 5 km or finer is needed to resolve the sharp variations of viscous heating near the interface of the two media

the interaction of viscous heating with grain size dependent rheology in the formation of localized slip zones the formation of localized shear zones is important for understanding many local and global processes in geodynamics we have developed a self consistent thermal mechanical model together with a rheology which depends on temperature strain rate and grain size distribution the grain size distribution has contributions from both dynamic recrystallization and grain growth processes and is governed locally by a nonlinear ordinary differential equation a one dimensional model with 104 points is employed to resolve all of the scales involving grain size and temperature we found that grain growth inhibits the development of shear zones and that there is a delicate interplay between viscous heating and grain growth process in determining whether narrow fault zones are developed quickly for realistic parameters of rheology and grain boundary processes for wet olivine the magnitude of the rate of grain growth is crucial to determine whether shear zones are stable or unstable at temperature t 1000 k or shear stress 100 mpa

rapid conversion of elastic energy into plastic shear heating during incipient necking of the lithosphere an important and novel mechanism for ductile failure of the lithosphere is identified here which is intrinsic to the thermal mechanical feedback in a temperature dependent plastic body with coupled elastic fields both a temperature dependent power law visco elasto plastic rheology and a temperature dependent elasto plastic rheology are employed to study in a self consistent fashion the deformation of the lithosphere subject to extension by means of a two dimensional finite element code a structural perturbation initially localizes elasto plastic deformation only in its immediate vicinity however after 800 000 years have elapsed the localized zone of deformation takes off in a crack like fashion and travels to the bottom of the lithosphere in about 50 000 years time when the plate is severed thermal runaway is caused by mechanical heating triggered by the rapid energy transfer of the globally stored elastic energy into localized plastic dissipation in the ductile fault

the elemental and isotopic composition of sulfur and nitrogen in chinese coals abstract coal combustion is an important atmospheric pollution source in most chinese cities so systematic studies on sulfur and nitrogen in chinese coals are needed the sulfur contents in chinese coals average 0 9 1 0 indicating that most chinese coals are low in sulfur a nearly constant mean 34 s value is observed in low sulfur ts 1 t 3 and j 3 high sulfur chinese coals os 0 8 often found at late carboniferous c 3 and late permian p 2 in southern china had two main sulfur sources original plant sulfur and secondary sulfur the wide variety of 34 s values of chinese coals 15 to 50 is a result of a complex sulfur origin the 15 n values of chinese coals ranged from 6 to 4 showing a lack of correlation with coal ages whereas nitrogen contents are higher in paleozoic coals than in mesozoic coals this may be related to their original precursor plant species high nitrogen pteridophytes for the paleozoic coals and low nitrogen gymnosperms for the mesozoic coals different to 34 s values chinese coals showed higher 15 n values in marine environments than in freshwater environments

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

tomographic evidence for localized lithospheric shear along the altyn tagh fault seismic tomography across the altyn tagh fault at the north edge of the tibetan plateau reveals a low p wave velocity anomaly below the fault down to 140 kilometers this anomaly probably reflects strike slip shear in the lithosphere slip partitioning may also induce a wedge of crust from the tarim basin to plunge into the mantle

crustal fault reactivation facilitating lithospheric folding buckling in the central indian ocean high quality normal incidence seismic reflection data confirm that tectonic deformation in the central indian ocean occurs at two spatial scales whole lithosphere folding with wavelengths varying between 100 and 300 km and compressional reactivation of crustal faults with a characteristic spacing of c 5 km faults penetrate through the crust and probably into the upper mantle both types of deformation are driven by regional large intraplate stresses originating from the indo eurasian collision numerical modelling of the spatial and temporal relationships between these two modes of deformations shows that in agreement with geophysical observations crustal faults are reactivated first with stick slip behaviour subsequent lithospheric folding does not start until horizontal loading has significantly reduced the mechanical strength of the lithosphere as predicted by elasto plastic buckling theory modelling suggests that lithospheric folding does not develop in the absence of fault reactivation crustal fault reactivation therefore appears to be a key facilitating mechanism for oceanic lithospheric buckling in the central indian ocean

composition of organic sulfur in riverine and marine sediments insights from sulfur stable isotopes and xanes spectroscopy abstract sulfur isotopes and x ray absorption near edge structure xanes spectroscopy were combined to characterize compare three operational organic sulfur os pools i e fulvic acid sulfur fa s humic acid sulfur ha s and non chromium reducible organic sulfur non cros in marine the east china sea ecs and jiaozhou bay jzb vs riverine yangtze river yr and jzb tributaries sediments xanes results indicate that in marine sediments high valency s was the dominant os functionality in both ha s and fa s while non cros was dominated by low valency os 80 92 in the riverine sediments fa s was dominated by high valency os while the average fractions of low and high valency os in ha s were comparable the isotopic composition of fa s 34 s fa s and ha s 34 s ha s indicated that a substantial fraction of sulfide was incorporated into fa via sulfurization in the marine sediments whereas terrigenous os was almost the sole important source of ha s compared with the ecs sediments jzb sediments had more depleted 34 s fa s and a higher fraction of highly reduced fa s due to eutrophication induced sulfurization relative to riverine fa s substantial sulfurization had not resulted in an increase in the fraction of highly reduced fa s in marine sediments this implies that the terrestrial systems may be much more favorable for the formation and or preservation of highly reduced biogenic fa s than the marine settings the fraction of highly reduced ha s in the jzb and its tributaries was similar whereas the fraction was much lower in the ecs than in the yr this indicates that highly reduced ha s moieties in the yr may have been subject to extensive mineralization loss during transport in the large riverine estuarine system whereas the process in the jzb tributaries may be much weakened due to a relatively small catchment area distinct differences in isotopic and structural composition between humic s fa s ha s and non cros in the ecs sediments indicate that a combination of s isotopes and s xanes is needed for characterizing the two operational pools for a better understanding the nature of os in the ocean

self consistent generation of tectonic plates in three dimensional mantle convection abstract despite the fundamental importance of plates in the earth s mantle convection plates have not generally been included in numerical convection models or analog laboratory experiments mainly because the physical properties which lead to plate tectonic behavior are not well understood strongly temperature dependent viscosity results in an immobile rigid lid so that plates where included at all in 3 d models have always been imposed by hand an important challenge is thus to develop a physically reasonable material description which allows plates to develop self consistently this paper focuses on the role of ductile shear localization in two dimensional geometry it is well established that strain rate softening non newtonian rheologies e g power law visco plastic cause weak zones and strain rate localization above up and down wellings resulting in a rudimentary approximation of plates three dimensional geometry however is fundamentally different due to the presence of transform plate boundaries with associated toroidal motion since power law and visco plastic rheologies do not have the property of producing shear localization it is not surprising that they do not produce good plate like behavior in three dimensional calculations here it is argued that a strain rate weakening rheology previously shown to produce plate like behavior in a two dimensional sheet representing the lithosphere is a reasonable generic description of various weakening processes observed in nature one and two dimensional models are used to show how this leads to shear localization and the formation of faults this rheology is then applied to the high viscosity lithosphere of 3 d mantle convection calculations and the velocity pressure viscosity solution for the entire 3 d domain lid and underlying mantle is solved self consistently it is found that the lithosphere divides into a number of very high viscosity plates separated by narrow sharply defined weak zones with a viscosity many orders of magnitude less than the plate interiors broad weak zones with dominant convergent divergent motion above up and down wellings are interconnected by a network of narrow weak zones with dominant strike slip motion passive spreading centers are formed in internally heated cases while the resulting plates are not fully realistic these results show that self consistent plate generation is a realizable goal in three dimensional mantle convection and provide a promising avenue for future research

generation of plate tectonics from lithosphere mantle flow and void volatile self lubrication abstract the formation of plate tectonics from mantle convection necessarily requires nonlinear rheological behavior recent studies suggest that self lubricating rheological mechanisms are most capable of generating plate like motion out of fluid flows the basic paradigm of self lubrication is nominally derived from the feedback between viscous heating and temperature dependent viscosity here we propose a new idealized self lubrication mechanism based on void e g pore and or microcrack generation and volatile e g water ingestion we test this void volatile self lubrication mechanism in a source sink flow model this leads to a basic nonlinear system which permits the excitation of strike slip toroidal motion a necessary ingredient of plate like motion out of purely divergent i e poloidal or characteristically convective flow with relatively inviscid void filling volatiles the void volatile mechanism yields a state of highly plate like motion i e with uniformly strong plate interiors weak margins and extremely focussed strike slip shear zones moreover the void volatile model obeys a chemical diffusion time scale that is typically much longer than the thermal convection time scale the model thus complies with the observation that plate boundaries are long lived and survive even while inactive the void volatile model of self lubrication therefore predicts self focussing shear zones plate generation and plate boundary longevity through what has long been suspected to be a key ingredient for the existence of plate tectonics i e water

influence of shear heating on microstructurally defined plate boundary shear zones abstract in plate boundary scale ductile shear zones defined by microstructural weakening shear heating may lead to a temperature increase over 5 m y of up to 80 c just below the brittle ductile transition up to 120 c just below the moho and to thermal boundary zones tens of km wide on either side of the shear zone where rock strength is highest shear zones are narrow 1 km and thermal gradients within the shear zone itself are low so there is no tendency for increased localization heating results in thermal weakening but this is partly offset by grain growth related to the decrease in stress in shear zones of the order of 10 km width shear stress strain rate and hence heat generation are lower and thermal gradients are insufficient to cause additional strain localization temperature increases in the mid crust are of the order of 10 c insufficient to cause partial melting or an increase in metamorphic grade in the upper mantle shear zones may be 50 km or more wide and the temperature increase is less than 20 c in 5 m y but temperature differences between center and margin may enhance the strain rate at the center by up to 18

hydrochemistry and isotope geochemistry of the upper danube river abstract the upper danube river and 19 of its major tributaries were monitored for 1046 km downriver and seasonally for ca mg sr na k hco3 co2 cl po4 sio2 no3 no2 nh4 so4 13cdic 18oh2o dh2o 34sso4 18oso4 and 87sr 86sr hydrological considerations and 18o d data show that the water balance in the river particularly after its confluence with the inn is controlled by the southern tributaries draining the mesozoic carbonate complexes of the alps as a result chemical balance at bratislava is mostly a conservative product of its tributaries the concentrations of ca mg and sr in the danube are 6 to 9 times higher than in pristine rivers although all these cations are derived from dissolution of mesozoic carbonates the ultimate cause is likely the enhanced generation of soil co2 due to agricultural and forestry practices dissolution of triassic sulfates is an additional factor for ca enrichment in the headwater section of the river dissolved sulfate with a comparable enrichment factor to that of alkaline earths appears to be derived mostly from atmospheric deposition a proposition based on consideration of its sulfur and oxygen isotopic compositions na k and cl are enriched by a factor of 2 5 to 4 times mostly as a result of industrial and municipal pollution sources in contrast to the above components which behave mostly conservatively during the downriver flow of the danube biogenic elements such as nutrients and si are influenced by in river processes the photosynthesis respiration balance that impacts the carbon cycle and the oxygen balance has been discussed elsewhere pawellek and veizer 1994 no3 no2 and nh4 are enriched by a factor of 10 to 16 times from point and diffuse sources along the watercourses generating at times downflow nitrification plumes from nh4 to no3 po4 varies seasonally chiefly as a result of biologic demand during the warm periods for sio2 the biological uptake mostly for secretion of diatom frustules combined with the deficiency of this compound that results from the predominantly carbonate lithology of the catchment results in concentrations that are below those of pristine rivers overall the present day salted characteristics of the river are chiefly a consequence of the long habitation history of the upper danube watershed

using a dual isotopic approach to trace sources and mixing of sulphate in changjiang estuary china the dual isotopic compositions of dissolved so42 in aquatic systems are commonly used to ascertain so42 sources and possible biogeochemical processes in this study the physical parameters major anions and isotopic compositions of so42 in water samples from changjiang river nanjin to the east sea in changjiang estuary were determined the salinity ranged from 0 to 32 3 in the estuary water samples the cl so42 concentrations and 18o h2o values followed the salinity variations from freshwater to seawater which indicated that mixing processes might be a major factor involved in the distribution of water and solutes the contents and isotopic compositions of so42 suggested that atmospheric deposition evaporite dissolution and sulphide oxidation were the major sources of dissolved so42 in the freshwater of changjiang river in addition the mixing model calculated by contents and isotopic compositions of so42 indicated that the mixing of freshwater and sea water was the major factor involved in so42 distribution in changjiang estuary however slightly elevated 18o so4 values were observed in the turbidity maximum zone which suggested that biological processes might affect the o isotopic compositions of so42 there

observations models and mechanisms of failure of surface rocks surrounding planetary surface loads geophysical models of flexural stresses in an elastic lithosphere due to an axisymmetric surface load typically predict a transition with increased distance from the center of the load of radial thrust faults to strike slip faults to concentric normal faults these model predictions are in conflict with the absence of annular zones of strike slip faults around prominent loads such as lunar maria martian volcanoes and the martian tharsis rise we suggest that this paradox arises from difficulties in relating failure criteria for brittle rocks to the stress models indications that model stresses are inappropriate for use in fault type prediction include 1 tensile principal stresses larger than realistic values of rock tensile strength and or 2 stress differences significantly larger than those allowed by rock strength criteria predictions of surface faulting that are consistent with observations can be obtained instead by using tensile and shear failure criteria along with calculated stress differences and trajectories with model stress states not greatly in excess of the maximum allowed by rock fracture criteria

humic sulfur in eutrophic bay sediments characterization by sulfur stable isotopes and k edge xanes spectroscopy abstract organic sulfur os is an important sedimentary sulfur pool in marine sediments and chemical extractions are often used for quantification of various os pools however os sources and mechanisms of os formation are not well understood in this study sulfur stable isotope and sulfur x ray absorption near edge structure xanes spectroscopy were combined to investigate the sources and speciation of humic acid sulfur ha s and fulvic acid sulfur fa s in sediments of eutrophic jiaozhou bay whilst there may be some indication that eutrophication has enhanced fa s burial in the sediment this has not substantially modified the characteristically low humic sulfur i e ha s fa s contents of the sediments sulfur isotopic compositions indicate that both ha s and fa s are mixtures of diagenetic and biosynthetic os in origin ha s is dominated by biosynthetic sulfur and fa s by diagenetic source sulfur isotopic compositions and contents of pyrite and diagenetic os indicate that inhibition of sulfurization by pyrite formation if any appears insignificant xanes analysis suggests that the contents of high oxidized os i e sulfones and ester sulfates and strongly reduced os species are comparable in the ha s whereas the fa s is dominated by strongly reduced os as a result of enhanced sulfurization

organic carbon and nitrogen contents and their isotopic compositions in surficial sediments from the east china sea shelf and the southern okinawa trough abstract the southern okinawa trough has been noted as an important depocenter for the particulate organic matter from the east china sea shelf in order to determine the major sources of sedimentary organic matter in the southern okinawa trough we analyzed surficial sediments from the east china sea shelf and the southern okinawa trough for organic carbon and nitrogen contents and their isotopic compositions in the decarbonated fraction distributions of total organic carbon content 13corg and nitrogen content all show a similar spatial pattern resembling the distribution of fine grained sediments the coastal belt of elevated organic carbon content extends southward from the changjiang mouth and veers offshore towards the southern okinawa trough just north of taiwan implying a pathway for channeling fine grained sediments from the inner shelf to the depocenter this distribution pattern is consistent with the shelf circulation the isotope compositions 13corg 15n of sediments from the southern okinawa trough fall between those of riverine particulate organic matter and the mid outer shelf sediments but overlap with those of the inner shelf sediments in addition the previously reported 13corg values of sinking particles collected by sediment traps in the southern okinawa trough are also close to those of the inner shelf sediments especially during high flux conditions therefore the isotopic evidence strongly supports the notion that a major fraction of the sedimentary organic matter in the southern okinawa trough may originate from the inner shelf

a simple parameterization of strain localization in the ductile regime due to grain size reduction a case study for olivine we propose a simple parameterization of the transition between dislocation creep and grain size sensitive creep under conditions characteristic of the lithospheric mantle and derived from the results of laboratory experiments on olivine rich rocks through numerical modeling and linear stability analysis we determine the conditions under which this transition takes place and potentially leads to strain localization we pay particular attention to the effect of cooling rate and strain rate which are likely to be dominant parameters in actively deforming tectonic areas we conclude that at constant temperature strain localization can only take place if the rheology of the material is nonlinearly related to grain size that strain localization is facilitated by syndeformation cooling that there is only a narrow region in the strain rate versus cooling rate parameter space where localization is likely to take place and that grain growth inhibits strain localization at fast cooling rates but may lead to grain growth localization at low cooling rates we draw attention to the potential consequences of our analysis of strain localization for the style of plate motions at the earth s surface

dilatancy during rock deformation at high temperatures and pressures the dilatancy during deformation at high pressure and temperature and at various pore pressures has been measured in carrara marble solnhofen limestone and gosford sandstone the method consists of monitoring the volume of pore fluid argon that flows into or out of the specimen at constant pore pressure the confining pressure was 300 mpa temperatures were up to 873 k and pore pressure ranged from 30 to 280 mpa positive dilatancy was observed in some cases in all three rocks in the ductile regime up to the highest temperature provided the difference between confining pressure and pore pressure was relatively small thus indicating the occurrence of microcracking even under these conditions however the behavior was complicated by the collapse of initial porosity in the limestone and sandstone and by microcracking due to intergranular thermal stresses upon heating especially in the marble the dilatancy observations have important implications for kinetics of fluid rock interactions under geological conditions

mechanics of graben formation in crustal rocks a finite element analysis small 1 10 km wide grabens are ubiquitous tectonic structures that have been observed on the surfaces of all the terrestrial planets and satellites in spite of their frequent occurrence there is no fundamental understanding of how they form other than that they are characteristic of extensional terranes this paper examines the mechanics of the initial stages of graben formation and demonstrates that the configuration of a pair of antithetically dipping normal faults a graben is the energetically most favorable fault configuration in elastic brittle rocks subjected to pure extension a corollary to this hypothesis is that the occurrence of synthetic normal faults is indicative of a stress field other than purely extensional or of nonhomogeneous characteristics of the rock the analysis proceeds by computing the stress field in the vicinity of a single initial normal fault using a two dimensional finite element model it is shown that slip on this fault substantially lowers the proximity to failure of the surrounding rocks except for two small regions one near the surface of the downthrown block and the other near the fault tip the region on the downthrown block is located a horizontal distance from the initial fault slightly less than the fault s depth the proximity to failure actually increases here and a secondary fault develops an antithetic secondary fault would link this region with the other failure zone near the fault tip to demonstrate the preference for antithetic secondary faults more rigorously we show that this fault configuration results in a smaller total strain energy than the synthetic configuration in addition a two layer model is used to show that the graben s width is only weakly dependent upon the presence of mechanically distinct layers in the crust both weak over strong and strong over weak configurations are investigated the depth of the initial fault and the thickness of the top layer are varied and the width of the resulting graben is evaluated by computing the distance between the initial fault and the region of highest proximity to failure near the surface this analysis indicates that the depth of the initial fault is the major factor controlling graben width

a 300 to 550 year history of slip on the imperial fault near the u s mexico border missing slip at the imperial fault bottleneck subsurface investigation of the imperial fault zone approximately 50 60 m south of the u s mexico international border in mexicali valley reveals that only the 1940 imperial valley earthquake m l 7 1 has produced significant surface slip in the past 300 years the 1940 earthquake produced about 5 m of slip in this vicinity as indicated by an offset tree line we excavated a buried channel adjacent to these trees and determined that it is also offset 5 m these data indicate that at least the upper meter of sediments has only been displaced by the 1940 event one other slip event of undetermined magnitude is recorded within the lacustrine highstand deposits of the last lake about 300 years ago there is no evidence of earlier faulting events in sediments that possibly may correlate to the penultimate lake approximately 550 years b p below an unconformity 4000 year old deltaic deposits are intensely faulted by previous slip along the imperial fault these data suggest that the slip rate on the imperial fault for the past 300 550 years is only about 15 20 mm yr substantially less than that expected from geodetic measurements and regional structural models this in turn suggests that either slip is accommodated by other faults in the border region or that strain release on the imperial fault is characterized by episodic clustered behavior

ductile structures and instabilities their implication for variscan tectonics in the ardennes abstract increasing consideration of instabilities may explain the development of structures at all scales in naturally deformed rocks this address is a review of geometric scale free concepts applied to the tectonic study of ductile structures the notion of instabilities is first given to introduce non equilibrium dynamics such as strain localisation and single deformation events producing superimposed structures these concepts lead to the idea that orogens are regions of accumulated strain in zones dominated by non coaxial deformation consequently stretching lineations depend on finite strain and do not represent the plate motions repeated phases of deformation represent local perturbations during a kinematic and dynamic event rather than being correlatable as regional phases the fold and thrust belt of the northern variscan belt in the ardennes is taken as a natural example where superposed structures represent the instable response to forces among which plate tectonic forces play a subsidiary role

on the relationship between the formation of shear zones and the form of the flow law for rocks undergoing dynamic recrystallization abstract the formation and maintenance of shear zones in rocks at high temperatures when viscous flow takes place at constant volume requires strain weakening several processes can lead to a strain weakening characteristic in the flow of such rocks to high strains but conventional experimental rock mechanics techniques cannot normally provide high quality mechanical data to high strains recent experimental data for calcite marble have demonstrated strain weakening accompanied by dynamic recrystallization a power law with strain exponent r can be used to describe the dependence of the flow stress on strain together with the conventional description of the dependence of strain rate on stress described by a stress exponent n the stable localization of strain into shear zones is favoured by more negative values of r e g r 0 8 and more positive values of n e g n 5 to 7 a further important factor is the critical strain e0 for the onset of recrystallization and hence strain weakening shear zone formation is favoured by smaller values for e0 shear zone formation weakens the crustal section in which they occur but the amount of weakening may not be great perhaps no more than 50 of the previous steady state flow stress certain rock types such as mafic igneous rocks may through their mechanical properties be more susceptible to shear zone formation than others such as mica schists it is emphasized that the analysis presented is based upon a strain dependent flow law that describes single phase dynamic recrystallization by grain boundary migration that leads to enhanced recovery without any change in deformation mechanism no consideration is given to possibly important effects arising from the activation of grain size sensitive flow or to potential weakening effects arising from syntectonic metamorphic reactions

sulfur speciation in the upper black sea sediments abstract we report solid phase sulfur speciation of six cores from sediments underlying oxic suboxic and anoxic sulfidic waters of the black sea our dataset includes the five sulfur species pyrite sulfur acid volatile sulfides avs zerovalent sulfur s 0 organic polysulfides rsx humic sulfur together with reactive iron and manganese as quantified by dithionite extraction and total organic carbon pyrite sulfur was the major phase in all cores 200 400 mol g dry wt 1 except for the suboxic core however zerovalent sulfur and humic sulfur also reached very significant levels up to about 109 and 80 mol g dry wt 1 respectively humic sulfur enrichment was observed in the surface fluff layers of the eastern central basin sediments where unit 1 type depositional conditions prevail elemental sulfur accumulated as a result of porewater sulfide oxidation by reactive iron oxides in turbidities from the anoxic basin margin and western central basin sediments the accumulation of elemental sulfur to a level close to that of pyrite s in any part of central black sea sediments has never been reported before and our finding indicates deep basin turbidites prevent the build up of dissolved sulfide in the sediment this process also contributes to diagenetic pyrite formation whereas in the non turbiditic parts of the deep basin water column formed syngenetic pyrite dominates the sulfur inventory in slope sediments under suboxic waters organic sulfur humic sulfur organic polysulfides account for 33 42 of total solid phase s indicating that the suboxic conditions favor organosulfur formation our study shows that the interactions between depositional patterns unit 1 vs turbidite redox state of overlying waters oxic suboxic sulfidic and organic matter content determine sulfur speciation and enable the accumulation of elemental sulfur and organic sulfur species close to a level of pyrite s

high temperature creep of partially molten plagioclase aggregates we have investigated the high temperature creep of synthetic labradorite an60ab40 between 1323 k and 1523 k at atmospheric pressure and low stresses 2 65 mpa average grain size varies from 12 to 16 m samples contained up to 12 vol melt an60ab40 glass fourier transform infrared measurements indicate that trace amounts of water 0 1 0 15 wt h2o were incorporated during crystallization this water was only partly released at high temperature and atmospheric pressure at low stresses and temperatures deformation of labradorite was controlled by grain boundary diffusion creep yielding a stress exponent of n 1 and an activation energy of 365 25 kj mol regime i with increasing stress and temperature mechanical data and microstructural observations indicate a transition to power law creep with n 3 regime ii this transition occurs in both partially molten and melt free aggregates in regime i the strain rate of samples containing as much as 12 vol melt is between 3 and 5 times higher than in fully crystalline specimens the activation energy is only slightly affected by the melt content between 0 and 10 2 vol transmission electron microscopy shows that the amorphous phase resides in connected triple junctions or in unconnected pockets grain boundaries are not wetted except surrounding large melt pockets the melt distribution of undeformed and deformed samples is similar the strength and activation energy of synthetic labradorite are comparable to those for pure anorthite with trace amounts of water both plagioclase compositions are slightly weaker than synthetic diopside

borrego springs santa rosa mylonite zone a late cretaceous west directed thrust in southern california the borrego springs shear zone section of the eastern peninsular ranges mylonite zone eprmz contains unequivocal evidence for ductile shearing with an east over west sense of movement field evidence includes the presence of s and c planes and shear bands all lines of microstructural evidence are internally consistent and support the field observations it appears that the data are compatible only with post mid cretaceous west directed thrusting on the eprmz

abyssal peridotite mylonites implications for grain size sensitive flow and strain localization in the oceanic lithosphere abstract microstructures preserved in abyssal peridotites dredged from the oceans record several different physical regimes of deformation fabrics associated with deformation processes at slow spreading mid ocean ridges form two major classes of abyssal peridotites based on detailed microstructural observations the most abundant class are medium to coarse grained tectonites with microstructures that reflect deformation processes during mantle upwelling and emplacement to the base of the lithosphere these tectonites give geothermometric temperatures of 755 c or higher interpreted to represent lower temperature limits for diffusive exchange in coarse grained abyssal peridotites during cooling this conclusion is consistent with flow laws for olivine at these temperatures the second class of abyssal peridotites previously largely undescribed for the mid ocean ridge environment include fine grained mylonites associated with faulting and shear zones that develop during extension and cooling of the oceanic lithosphere when the brittle plastic transition extends into mantle rocks these mylonites give temperatures of 600 c which we suggest represent a lower temperature limit for plastic deformation reduced grain size in mylonites allows for diffusive exchange to continue to these low temperatures relict augen in the mylonitic samples preserve equilibration temperatures similar to those exhibited by the coarse grained tectonites based on flow laws for olivine we suggest that deformation in some fine grained mylonites occurred by diffusion creep down to 600 c rheological data for olivine indicate that dislocation creep is not likely to occur at this temperature we conclude that a reduction in grain size by cataclasis or dynamic recrystallization resulted in a transition in deformation mechanisms from dislocation to diffusion creep during uplift and or cooling the observation of these fine grained mylonites indicates that shear zones that extend into the upper mantle will be weaker than expected if deformation was accommodated by brittle processes or dislocation creep weak faults may promote the development of long lived detachments in the upper mantle this inference supports mid ocean ridge tectonic models that suggest that ultramafic rocks exposed at the inside corners of ridge axis discontinuities are exhumed along long lived detachment faults

earthquake nucleation on faults with rate and state dependent strength abstract dieterich j h 1992 earthquake nucleation on faults with rate and state dependent strength in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 115 134 faults with rate and state dependent constitutive properties reproduce a range of observed fault slip phenomena including spontaneous nucleation of slip instabilities at stresses above some critical stress level and recovery of strength following slip instability calculations with a plane strain fault model with spatially varying properties demonstrate that accelerating slip precedes instability and becomes localized to a fault patch the dimensions of the fault patch follow scaling relations for the minimum critical length for unstable fault slip the critical length is a function of normal stress loading conditions and constitutive parameters which include dc the characteristic slip distance if slip starts on a patch that exceeds the critical size the length of the rapidly accelerating zone tends to shrink to the characteristic size as the time of instability approaches solutions have been obtained for a uniform fixed patch model that are in good agreement with results from the plane strain model over a wide range of conditions above the steady state stress the logarithm of the time to instability linearly decreases as the initial stress increases because nucleation patch length and premonitory displacement are proportional to dc the moment of premonitory slip scales by d3c the scaling of dc is currently an open question unless dc for earthquake faults is significantly greater than that observed on laboratory faults premonitory strain arising from the nucleation process for earthquakes may by too small to detect using current observation methods excluding the possibility that dc in the nucleation zone controls the magnitude of the subsequent earthquake then the source dimensions of the smallest earthquakes in a region provide an upper limit for the size of the nucleation patch

frictional weakening and slip complexity in earthquake faults previous work has shown that velocity weakening friction produces slip complexity in simple dynamical models of earthquake faults carlson and langer 1989 here i show that a different type of dynamical instability caused by slip weakening friction also produces slip complexity the deterministically chaotic slip complexity produced by slip weakening friction in a simple one dimensional model is studied and the scaling of the distribution of sizes of events with the parameters in the model examined in addition a possible physical origin of frictional weakening is examined through a very simplified mathematical representation of a physical process proposed by sibson 1973 whereby frictional heating causes an increase in pore fluid temperature and pressure thereby reducing the effective normal stress and friction the two different types of frictional weakening are derived from two opposing limits with slip weakening occurring when the dissipation of heating is slow compared to the rupture timescale as lachenbruch 1980 has shown while velocity weakening is shown to occur when the dissipation is fast compared to the rupture timescale since both end member cases of frictional weakening are seen to produce slip complexity slip complexity is argued to be a generic feature of frictional weakening and elastodynamics on a fault

the early diagenetic formation of organic sulfur in the sediments of mangrove lake bermuda due to a low mineral content the sapropelic sediments depositing in mangrove lake bermuda provide an excellent opportunity to explore for possible additions of sulfur to organic matter during the early stages of diagenesis we evaluated early diagenetic organic sulfur transformations by monitoring the concentrations and stable isotopic compositions of a number of inorganic and organic sulfur pools thereby accounting for all of the sulfur in the sediments we have identified and quantified the following sulfur pools porewater sulfate porewater sulfide elemental sulfur pyrite sulfur hydrolyzable organic sulfur hyos chromium reducible organic sulfur cros and nonchromium reducible organic sulfur non cros of the organic sulfur pools the non cros pool is by far the largest followed by cros and finally hyos by 60 cm depth these pools contribute respectively to 85 7 9 and 3 6 of the total solid phase sulfur the hyos pool is probably of biological origin and shows no interaction with the sulfur compounds produced during diagenesis by contrast cros is produced most likely from the diagenetic addition of polysulfides to functionalized lipids in the upper h2s poor elemental sulfur rich region of the sediment a portion of this sulfur pool is unstable and decomposes on contact with the h2s rich porewaters the portion of cros that remains in the sulfidic waters appears to readily exchange sulfur isotopes with h2s while some of the non cros pool is of biological origin some is also formed by the diagenetic addition of sulfur to organic compounds in the upper h2s poor region of the sediment by contrast with cros non cros is not diagenetically active in the h2s rich porewaters overall somewhere between 27 and 53 of the organic sulfur buried in mangrove lake sediments is of diagenetic origin with the remaining organic sulfur derived from biosynthesis we extrapolate our mangrove lake results and calculate that in typical coastal marine sediments between 11 and 29 mol g 1 of organic sulfur will form during early diagenesis of which 2 5 mol g 1 will be chromium reducible

abyssal hills formed by stretching oceanic lithosphere tectonic plates are formed and move apart at mid ocean ridges some portion of this plate separation process can occur by stretching of the crust resulting in a complex pattern of extensional faults abyssal hills the most ubiquitous topographic features on earth1 are thought to be a product of this faulting2 3 here we report the results of a self consistent numerical model of lithospheric formation and stretching that includes spontaneous fault creation in this model an axial valley develops where the fault activity is most concentrated the frozen fault generated topography rafted out of the axial valley is visually and statistically similar to observed abyssal hills formed at many slower spreading ridges faults appear to be replaced by new faults because their offset changes the local stress field we accordingly need no temporal variation in magmatism as required by some previous models4 5 6 to control the spacing or offset of faults our model results suggest instead that the irregularity of abyssal hill relief may result from a self organized critical stress state at spreading centres

isotopic composition and speciation of sulfur in the miocene monterey formation reevaluation of sulfur reactions during early diagenesis in marine environments the timing and pathways of early diagenetic sulfur transfer from dissolved species in pore waters to solid inorganic and organic compounds in sediments have been studied in the miocene monterey formation santa maria basin onshore california correlation between concentrations of total organic carbon toc and total sulfur ts in addition to concentrations of titanium aluminum total iron and reactive iron have been used to infer organic matter reactivity redox conditions and relative rates of clastic and biogenic input for each lithofacies isotopic compositions of six sulfur species acid volatile disulfide kerogen bitumen sulfate and elemental have provided information regarding relative timing of sulfur incorporation sulfate diffusivity in the upper centimeters of the sediments and the sources of sulfur for individual species isotopically the disulfide species expresses the greatest fractionation relative to estimated values of miocene seawater sulfate 22 cdt on average disulfide is depleted in 34s by 10 4 relative to kerogen and by 9 9 relative to acid volatile sulfide the 34s of bitumen shows no systematic change relative to 34skeregon suggesting the presence of migrated bitumen isotopic similarity of sulfate and elemental sulfur to sulfides and bitumen indicates that sulfate and elemental sulfur are chemical and or biological oxidation products derived from sulfides and bitumen consistent ordering of isotopic values for sulfur species disulfide acid volatile sulfide kerogen indicates that pyrite precipitated nearest to the sediment water interface under mildly reducing conditions and with little or no decrease in sulfate concentration relative to seawater enrichment of 34s in acid volatile sulfide and kerogen sulfur resulted from formation of these species at greater depths or in restricted micro environments under more reducing conditions and with low concentrations of porewater sulfate the formation of acid volatile sulfide after pyrite but during early diagenesis is significant because it implies more strongly reducing conditions than are generally recognized in deep water marine sediments

speciation and spatial distribution of solid phase iron in surface sediments of the east china sea continental shelf abstract speciation and reactivity characterization of solid phase fe in marine sediments are of significance to understanding its heterogeneous mineralogy and crystallinity the diagenetic cycling of fe and its regulating roles on many other elements in sediments in this study a combination of sequential and single step extractions was used for the determination of seven fe pools in surface sediments of the east china sea ecs continental shelf 1 carbonate associated fe fe ii carb plus acid volatile sulfide fe fe ii avs 2 easily reducible amorphous poorly crystalline fe oxides fe ox1 3 reducible crystalline fe oxides fe ox2 4 magnetite fe mag 5 poorly reactive sheet silicate fe fe prs 6 pyrite fe fe py and 7 unreactive silicate fe fe u total fe fe t in the sediments is largely determined by terrestrial aluminosilicate particles as indicated by a great similarity of the fe t with that of the yangtze river and global riverine particulates the size of fe prs is found to be the largest pool followed by fe u fe ox2 fe mag fe ii avs carb fe ox1 and fe py the large fe prs may result from neoformation of fe rich clay minerals via reverse weathering and subsequent ageing the small sizes of fe ii avs carb and fe py pools is believed to be the result of low so 4 reduction due to generally low labile organic matter together with the oxic suboxic dynamic environments of the surface sediments the occurrence of fe ox1 fe ox2 and fe prs in the sediments is closely associated with the clay fraction as indicated by a high spatial correlation between the former and the latter highly reactive fe fe hr in the sediments is comparable to that in global marine sediments but apparently lower than in the yangtze river and global riverine particulates due probably to sequestration in the yangtze estuary the ratios of fe hr fe t fe pr fe t and fe u fe t in the ecs surface sediments consistently show more similarity to those in the yangtze river particulates than in the global continental margin or deep sea sediments the surface sediments maintain a high level of buffering capacity toward sulfidation suggested by a large fraction of highly reactive fe iii oxides fe iii hr in fe hr

a study of sulphur enrichment in the humic fraction of marine sediments during early diagenesis abstract the evolution of the sulphur content of humic acids extracted from a near shore sediment core was investigated special attention was taken to avoid s contamination of the humic materials during extraction the s c ratios increased continuously with depth to values which strongly suggest s addition to the humic matrix during early diagenesis by reactions between organic matter and h2s or its oxidation products their light isotopic composition supports this view however subsequent isotopic exchange has obscured the mechanism initially involved since a large fraction of the enrichment occurred above the sulphidic zone redox boundaries such as the interface of anoxic microniches within more oxidized zones or the sulphidic suboxic boundary of the sediment column must have been important sites for s addition bioturbation by increasing the contact between sedimentary zones of a different redox regime will likely enhance such processes

a unified description of localization for application to large scale tectonics 1 localized regions of deformation such as faults and shear zones are ubiquitous in the earth s lithosphere however we lack a simple unified framework of localization that is independent of the mechanism or scale of localization we address this issue by introducing the effective stress exponent ne a parameter that describes how a material responds to a local perturbation of an internal variable being tested for localization the value of ne is based on micromechanics a localizing regime has a negative ne indicating a weakening behavior and localization is stronger for more negative 1 ne we present expressions for the effective stress exponent associated with several mechanisms that trigger localization at large scale brittle failure with loss of cohesion elastoplasticity rate and state dependent friction shear heating and grain size feedback in ductile rocks in most cases localization does not arise solely from the relation between stress and deformation but instead requires a positive feedback between the rheology and internal variables brittle mechanisms failure and friction are generally described by ne of the order of 100 shear heating requires an already localized forcing which could be provided by a brittle fault at shallower levels of the lithosphere grain size reduction combined with a transition from dislocation to diffusion creep leads to localization only if the grain size departs significantly from its equilibrium value because either large scale flow moves rocks through different thermodynamic environments or new grains are nucleated when shear heating or grain size feedback produce localization 1 ne can be extremely negative and can control lithospheric scale localization

long term storage and age biased export of fluvial organic carbon field evidence from west iceland terrestrial organic carbon oc plays an important role in the carbon cycle but questions remain regarding the controls and timescale s over which atmospheric co remains sequestered as particulate oc poc motivated by observations that terrestrial poc is physically stored within soils and other shallow sedimentary deposits we examined the role that sediment storage plays in the terrestrial oc cycle specifically we tested the hypothesis that sediment storage impacts the age of terrestrial poc we focused on the efri haukadalsa river catchment in iceland as it lacks ancient sedimentary bedrock that would otherwise bias radiocarbon based determinations of poc storage duration by supplying pre aged petrogenic poc our radiocarbon measurements of riverine suspended sediments and deposits implicated millennial scale storage times comparison between the sample types suspended and deposits suggested an age offset between transported suspended sediments and stored deposits poc at the time of sampling which is predicted by theory for the sediment age distribution in floodplains we also observed that poc in suspended sediments is younger than the predicted mean storage duration generated from independent geomorphological data which suggested an additional role for oc cycling consistent with this we observed interparticle heterogeneity in the composition of poc by imaging our samples at the microscale using x ray absorption spectroscopy specifically we found that particles within individual samples differed in their sulfur oxidation state which is indicative of multiple origins and or diagenetic histories altogether our results support recent coupled sediment storage and oc cycling models and indicate that the physical drivers of sediment storage are important factors controlling the cadence of carbon cycling

speciation and stable isotopic compositions of humic sulfur in mud sediment of the east china sea constraints on origins and pathways of organic sulfur formation abstract humic sulfur is commonly the most important pool of organic sulfur os in marine sediments and also important for the budget of total sedimentary sulfur in this study contents and stable sulfur isotopic compositions of hydrolyzable organic sulfur hyos humic acid sulfur ha s and fulvic acid sulfur fa s in one core collected from the east china sea ecs inner shelf were determined to investigate the sources and pathways of os formation and to compare diagenetic geochemistry of ha s and fa s hyos in the core is essentially biological in origin and diagenetically refractory with a fairly low fraction of labile sulfur bearing biomolecules ha s in the core is largely terrigenous residual bio os while fa s is a mixture of marine bio and diagenetic os diag os with the diag os fractions between 44 and 71 both ha s and fa s contents in the core are at the lower ends of the corresponding values reported in the literature the ha in the sediments is diagenetically inert subject to neither significant diagenetic sulfurization nor bio os mineralization loss while the fa is reactive prone to both sulfurization and decomposition loss of diagenetically bound os low contents of humic s i e ha s fa s in the core may suggest that os burial has only minor contribution to the burial of total sulfur in the sediments due to generally low om sulfurization

ductile creep and compaction a mechanism for transiently increasing fluid pressure in mostly sealed fault zones a simple cyclic process is proposed to explain why major strike slip fault zones including the san andreas are weak field and laboratory studies suggest that the fluid within fault zones is often mostly sealed from that in the surrounding country rock ductile creep driven by the difference between fluid pressure and lithostatic pressure within a fault zone leads to compaction that increases fluid pressure the increased fluid pressure allows frictional failure in earthquakes at shear tractions far below those required when fluid pressure is hydrostatic the frictional slip associated with earthquakes creates porosity in the fault zone the cycle adjusts so that no net porosity is created if the fault zone remains constant width the fluid pressure within the fault zone reaches long term dynamic equilibrium with the hydrostatic pressure in the country rock one dimensional models of this process lead to repeatable and predictable earthquake cycles however even modest complexity such as two parallel fault splays with different pressure histories will lead to complicated earthquake cycles two dimensional calculations allowed computation of stress and fluid pressure as a function of depth but had complicated behavior with the unacceptable feature that numerical nodes failed one at a time rather than in large earthquakes a possible way to remove this unphysical feature from the models would be to include a failure law in which the coefficient of friction increases at first with frictional slip stabilizing the fault and then decreases with further slip destabilizing it

cutting of the european continental lithosphere plasticity theory applied to the present alpine collision presently active european alpine collision has a history of more than 100 myr convergence we investigate the collisional contact problem of an apparently rigid african promontory currently the italy adria block iab steadily penetrating at 1 cm yr over at least the past 45 myr into the deformable european plate we assume that neither continental subduction nor delamination occurs presently in the 300 km wide contact zone enabling use of the indenter indentee approach a key parameter is the aspect ratio h a defined by the width of the deformable foreland over the width of the indenter itself this parameter controls the activation of two fundamentally different modes of deformation 1 classical indentation mode for h a 4 4 described by previous authors and having effect only in a portion of the deformable plate in the vicinity of the indenter and 2 cutting mode for h a 4 4 giving plane strain longitudinal splitting of the entire deformable plate and plastic separation of the two halves we use a matrix operator method to solve for slip lines in the mixed boundary value problem obliquity velocity friction stress of the european alpine case implications are as follows 1 plane stress vertical lithospheric thickening is the first indentation mode of continental collision and 2 for the present ratio of h a 3 deformation is in the cutting mode and slow plane strain intraplate extension rifting is predicted this mode explains the intraplate stress field obtained from borehole and earthquake data considering that the paleoindenter probably has been larger than the iab it is inferred that h a has been below 4 4 since at least the end of the eocene and that consequently the intraplate graben system lower rhine rhine bresse limagne is due to the cutting of the european plate by alpine collision

sulfonates a novel class of organic sulfur compounds in marine sediments abstract x ray absorption near edge structure spectroscopy xanes used to measure sulfur speciation in a variety of organic rich marine sediments has established sulfonates as a novel and major component of sedimentary organic sulfur the origins of sulfonates in sediments are not clear although both biological and geochemical mechanisms are possible the accumulation of oxidized sulfonate sulfur in reducing marine sediments was not known previously hence a new perspective in sulfur geochemistry is established the biogeochemical implications of the presence of sulfonates in marine sediments are discussed

the akato tagh bend along the altyn tagh fault northwest tibet 2 active deformation and the importance of transpression and strain hardening within the altyn tagh system we investigated active deformation within the akato tagh restraining double bend to determine the age of the active altyn tagh fault relative to the altyn tagh system and thereby evaluate the extent to which this system evolved by net strain hardening or softening active structures were mapped based on their geomorphology and disruption of quaternary deposits surfaces the style of active faulting is strongly correlated with fault strike 065 070 striking segments show pure left slip whereas faults with more northward or eastward strikes are transtensional or transpressional respectively our mapping further suggests that the 065 to 070 striking western and eastern segments of the akato tagh bend are characterized by pure strike slip motion with partitioned transpression along the 090 striking central segment of the double bend it remains unclear how this active bend perpendicular shortening is absorbed in conjunction with previous work the present study fails to support the commonly held idea that tarim tibet motion is strongly oblique to the altyn tagh system estimates for the age of the akato tagh bend derived in a companion study suggest the bend is only a few million years old the current principal trace is probably similarly young because formation of the bend by recent deformation of an old trace should result in transpression along the western and eastern segments contrary to the pure left slip shown here the altyn tagh system comprises multiple fault strands in a zone 100 km wide across strike because the main trace appears to be much younger than the system in which it is embedded we speculate that this system evolved by the sequential formation and death of short lived fault strands in particular we suggest that geometrically complex strike slip fault systems such as the altyn tagh may form via system strain hardening where this net response reflects a dynamic competition between hardening and softening processes that are active simultaneously within the fault zone hardening mechanisms may include growth of restraining bend topography or material hardening of phyllosilicate rich gouge whereas softening processes might include r p shear linkage or reduction in bend angle by vertical axis rotation our analysis suggests that net strain hardening of a fault system can produce continental deformation that is spatially localized over the 1 5 m y during which an individual strand is active but distributed over the 10 100 m y corresponding to the life span of the whole fault system and the collision zone in which it is contained thus time scale is critically important in determining whether or not continental deformation is spatially distributed or localized