effects of lateral viscosity variations on postglacial rebound implications for recent sea level trends global sea level changes have been occurring since the last pleistocene deglaciation recently the contribution of current postglacial rebound to local sea level variations has been removed by means of a radially stratified earth model in order to assess the amount of sea level rise due to recent climatic changes the authors have studied the effects of lateral variations in the viscosity structure on the postglacial uplift rates along the continental margins finite element calculations in cylindrical geometry show that the spread in the cumulative vertical displacement may be affected by 20 in the presence of lateral rheological contrasts but the vertical rates of deformation can attain differences of up to 50 for the different models horizontal deformation rates along continental margins are much more affected by the lateral variations in viscosity and can be used in the future as constraints on mantle rheology

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

deformation maps in grain size stress space as a tool to investigate mantle rheology abstract deformation maps have found widespread use in the study of the rheology of earth materials the usual procedure is to construct a map in stress temperature space from which predominant creep mechanisms strain rates and viscosities can be determined for a given grain size the rheology of polycrystals however depends strongly on grain size and consequently a fuller description would entail the construction of three dimensional stress grain size temperature diagrams as these do not display clearly the parameters of interest it is proposed to use deformation maps in stress grain size space at a given temperature in several situations of geodynamic interest grain size is a more critical parameter than temperature furthermore stress grain size maps have the advantages of simplicity and clarity for instance the boundaries between nabarro herring coble and power law creep are straight lines and so are constant strain rate and constant viscosity contours the triple point at which the three boundaries converge is under reasonable assumptions a function of temperature only and the slopes of the boundaries are independent of temperature it is therefore possible to project on a single map data for various temperatures the coordinates of the triple point contain all the necessary information for a synoptic representation of the rheology as an example deformation maps in stress grain size space are presented for olivine under laboratory and upper mantle conditions

dynamics of izmit earthquake postseismic deformation and loading of the duzce earthquake hypocenter we have developed dynamic finite element models of izmit earthquake postseismic deformation to evaluate whether this deformation is better explained by afterslip via either velocity strengthening frictional slip or linear viscous creep or by distributed linear viscoelastic relaxation of the lower crust we find that velocity strengthening frictional afterslip driven by coseismic shear stress loading can reproduce time dependent global positioning system data better than either linear viscous creep on a vertical shear zone below the rupture or lower crustal viscoelastic relaxation our best frictional afterslip model fits the main features of postseismic slip inversions in particular high slip patches at and below the hypocenter and on the western karadere segment and limited afterslip west of the hersek delta burgmann et al 2002 the model requires a weakly velocity strengthening fault that is either low effective normal stress in the slipping regions or a smaller value for the parameter describing rate dependence of friction a b than is indicated by laboratory experiments our best afterslip model suggests that the coulomb stress at the duzce hypocenter increased by 0 14 mpa 1 4 bars during the izmit earthquake assuming right lateral slip on a surface dipping 50 to the north and by another 0 1 mpa during the 87 days between the izmit and duzce earthquakes in the marmara sea region within about 160 km of the izmit earthquake rupture this model indicates that the coulomb stresses increased by 15 25 of the coseismic amount during the first 300 days after the earthquake three hundred days after the earthquake postseismic contributions to coulomb stressing rate on the maramara region faults had fallen to values equal to or less than the inferred secular stress accumulation rate our estimates of postseismic coulomb stress are highly model dependent in the marmara region the linear viscous shear zone and viscoelastic lower crust models predict greater postseismic coulomb stresses than the frictional afterslip model near field stress and fault zone rheology estimates are sensitive to the earth9s elastic structure when a layered elastic structure is incorporated in our model it yields a coulomb stress of 0 24 mpa at the duzce hypocenter significantly more than the 0 14 mpa estimated from the uniform elastic model because of the higher near field coseismic stresses the layered elastic model requires a higher value of velocity strengthening parameter a b a b times effective normal stress r to produce comparable postseismic slip a b is estimated at 0 4 and 0 2 mpa respectively for the layered and uniform elastic models these results highlight the importance of understanding the earth9s elastic structure and the mechanism for postseismic deformation if we wish to accurately model coseismic and postseismic crustal stresses

upper limits of power law creep of rocks at depths below a few kilometers most rocks probably flow in the steady state dominantly by diffusion assisted dislocation creep mechanisms whereby strain rate is proportional to a low power of deviatoric stress at high stress levels for metals ceramics and rocks the power law breaks down plb to some other strain rate stress functional relationship and this paper explores four methods by which the breakdown stress b may be estimated for metals alloys and ceramics b 10 3 for non silicate rocks b 5 10 3 and for silicates b 10 2 for silicates the low stress exponent power law breaks down at high stresses to an exponential dependence of strain rate upon stress or to a dorn law the governing flow mechanisms generally being lattice resistance controlled dislocation glide and mechanical twinning plb breakdown stresses for silicates are nearly strain rate insensitive and extrapolated values of b are so high as to suggest that low temperature high stress plasticity may be suppressed entirely in geological deformations plb for olivine rich rocks is discussed in some detail and new three dimensional deformation mechanism surfaces for wet and dry olivine polycrystals are presented

thermal and mechanical structure of the central iberian peninsula lithosphere abstract the central iberian peninsula spain is made up of three main tectonic units a mountain range the spanish central system and two tertiary basins those of the rivers duero and tajo these units are the result of widespread foreland deformation of the iberian plate interior in response to alpine convergence of european and african plates the present study was designed to investigate thermal structure and rheological stratification in this region of central spain surface heat flow has been described to range from 80 to 60 mw m 2 highest surface heat flow values correspond to the central system and northern part of the tajo basin the relationship between elevation and thermal state was used to construct a one dimensional thermal model mantle heat flow drops from 34 mw m 2 duero basin to 27 mw m 2 tajo basin and increases with diminishing surface heat flow strength predictions made by extrapolating experimental data indicate varying rheological stratification throughout the area in general in compression ductile fields predominate in the middle and lower crusts and lithospheric mantle brittle behaviour is restricted to the first 8 km of the upper crust and to a thin layer at the top of the middle crust in tension brittle layers are slightly more extended while the lower crust and lithospheric mantle remain ductile in the case of a wet peridotite composition discontinuities in brittle and ductile layer thickness determine lateral rheological anisotropy tectonic units roughly correspond to rheological domains brittle layers reach their maximum thickness beneath the duero basin and are of least thickness under the tajo basin especially its northern area estimated total lithospheric strength shows a range from 2 5 10 12 to 8 10 12 n m 1 in compression and from 1 3 10 12 to 1 6 10 12 n m 1 in tension highest values were estimated for the duero basin depth versus frequency of earthquakes correlates well with strength predictions earthquake foci concentrate mainly in the upper crust showing a peak close to maximum strength depth most earthquakes occur in the southern margin of the central system and southeast tajo basin seismicity is related to major faults some bounding rheological domains the duero basin is a relative quiescence zone characterised by higher total lithospheric strength than the remaining units

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

contrasting styles of rifting models and examples from the eastern canadian margin we present the results of a dynamical model of lithospheric rifting and rupture which show that a wide range of crustal thinning patterns across rifted passive margins can be produced by varying the steady state geotherm lithospheric composition dry versus wet materials and strain rate the basic mechanism of continental rupture is assumed to be passive rifting and necking we use a numerical thermomechanical model of lithosphere extension based on a finite element approach when plasticity is significant i e at lower temperatures or for harder materials deformation is unstable and thinning takes place abruptly over a narrow area conversely a progressive thinning across the margin is observed when creep is dominant i e in warm or ductile conditions cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterally in these circumstances rupture is likely to take place asymmetrically along one edge of the thinned area producing a narrow margin and a very wide conjugate the eastern margins of canada and their conjugates across the north atlantic provide examples which cover this range of theoretical profiles the crustal thinning patterns inferred from deep seismic data and the duration of rifting compare well with model results we discuss also the constraints that these geodynamical models provide on such current issues as the seismic reflectivity of the lower crust or the location of the ocean continent boundary in wide areas supposedly underlain by 5 km thin continental crust

evolution of mechanical state in carrara marble during deformation at 400 to 700 c the evolution of the mechanical state of carrara marble during deformation up to strains of 0 2 at 400 to 700 c and strain rates of 10 4 to 10 6 s 1 has been investigated by applying hart s 1976 state variable description of inelastic deformation to the results of isothermal constant displacement rate experiments performed at these conditions at t 600 c the magnitude of the mechanical state increases over the full range of strains investigated even when the deformation occurs at almost constant stress and strain rate although the rate of increase at given strain decreases with increasing temperature and decreasing strain rate the deformation microstructures suggest that under these conditions mechanical twinning controls the deformation behavior with twin boundary migration becoming significant at t 450 c as a dynamic recovery process twin boundary migration is as efficient as strain induced grain boundary migration at t 600 c the magnitude of the mechanical states attained is difficult to evaluate reliably using the method employed but the continuing evolution of the deformation microstructures suggests that a steady state is not attained by strains of 0 2 at these conditions mechanical twinning is replaced by other intracrystalline slip processes and by grain boundary bulge nucleated dynamic recrystallization hart s description of inelastic deformation as previously evaluated for carrara marble at t 400 c may be extrapolated to 600 c provided the mechanical state evolution equation is modified to accommodate twin boundary migration and the influence of annealing type recovery processes extrapolation to higher temperatures may require modifications to the equation of state in that description although at the conditions investigated steady state appears to be a poor approximation steady state flow laws of exponential and power law type do provide a good empirical description of the data however the observation that steady state is not attained indicates that any microphysical interpretation of the magnitude of the material parameters in such steady state flow law fits should be treated with caution

40ar 39ar and rbsr analyses from ductile shear zones from the atacama fault zone northern chile the age of deformation abstract the influence of deformation on the k ar and the rb sr isotope system is investigated it is assumed that due to the diffusion processes involved deformation has a similar effect on isotopic equilibrium as has temperature in order to examine the influence of deformation on the k ar and the rb sr isotope systems two shear zones from the atacama fault zone afz situated in the north chilean coastal cordillera have been investigated the afz which was active as a sinistral strike slip fault during the mesozoic has two sets of shear zones one formed under amphibolite sz1 one under greenschist facies conditions sz2 rb sr and 40ar 39ar age determinations were conducted on samples from cross sections of each set in sz1 the hornblendes and bioties from a weakly deformed sample reveal cooling ages of 153 152 and 150 1 ma respectively biotite from the center of the shear zone of sz1 gave an isochron of 143 9 0 3 ma mswd 0 04 which is interpreted as the age of deformation which produced resetting of the mineral system in sz2 hornblendes yielded 40ar 39ar plateau cooling ages of 138 ma biotites from undeformed samples gave rb sr and 40ar 39ar total degassing ages of 130 1 ma whereas biotite from the mylonitic rocks yielded 126 125 ma which dates the time of deformation sr isotope homogenization occurred in the mylonitic rocks and is most likely a result of deformation the formation of sz1 can be correlated to the araucanian nevadan phase the deformation in sz2 is related to the onset of uplift and cooling of the coastal cordilleran magmatic arc

high energy inner shelf carbonate facies along a tide dominated non rimmed margin northwestern australia abstract high energy inner shelf carbonates occur within 1 to 2 km of the mainland along the southern part of the northwest shelf a non rimmed carbonate shelf along northwestern australia coarse grained sands and gravels are deposited on and adjacent to a chain of subtidal and intertidal rock platforms which lies oblique to regional ne sw directed tidal currents intertidal and subtidal sand sheets less than 50 cm thick predominate along the margins and interior of the chain and subtidal linear to sinuous sand bodies up to 10 m thick and 4 km in length form banner banks to the northeast of rock platforms and reefs other features include narrow fringing and patch coral reefs calcareous red algal reefs and intertidal through subtidal coral shingle ridges fleshy algae are abundant on open sand covered and bare rock platforms as well as forming a mantle on relict coral reef frameworks supratidal through subtidal sediments are comprised of skeletal molluscs foraminifera bryozoans corals calcareous red algae and rock fragments the latter derived from rock platforms calcareous chlorophytes e g penicillus and halimeda are not important sediment contributors in general sediments are not representative of the present day turbid and fleshy algal dominated subtidal environment the predominantly coarse grained lithofacies

problems in the extrapolation of laboratory rheological data abstract the many types of variables and deformation regimes that need to be taken into account in extrapolating rheological behaviour from the laboratory to the earth are reviewed the problems of extrapolation are then illustrated with two particular cases in the case of divine rich rocks recent experimental work indicates that within present uncertainties of extrapolation the flow in the upper mantle could be either grain size dependent and near newtonian or grain size independent and distinctly non newtonian both types of behaviour would be influenced by the present of trace amounts of water in the case of quartz rich rocks the uncertainties are even greater and it is still premature to attempt any extrapolation to geological conditions except as an upper bound the fugacity and the scale of dispersion of the water are probably two important variables but the quantitative laws governing their influence are not yet clear

rheological structure and dynamical response of the dss profile baltic in the se fennoscandian shield abstract numerical modelling was applied to study the present day state of stress and deformation under different tectonic loading conditions at the seismic baltic skj profile in south eastern finland and in estonia the finite element method was used to solve the numerical problem the two dimensional model was constructed using the results from both seismic and thermal studies along the profile the model is 700 km long and 200 km deep and is roughly divided into an inhomogeneous laterally layered crust and a homogeneous mantle lithosphere both the linear elastic and non linear elasto plastic rheologies were used elasto plasticity was achieved by calculating a rheological strength as a function of depth along the profile different tectonic load cases were analysed with displacement force and pressure type boundary conditions also the effect of different strain rates was investigated the results suggest that even with relatively low compressive stress levels the lower crust deforms in a plastic manner for a wet crustal rheology when applying a dry crustal rheology plastic yielding is attained only with much higher stress fields

large scale deformation associated with ridge subduction summary continuum models are used to investigate the large scale deformation associated with the subduction of aseismic ridges formulated in the horizontal plane using thin viscous sheet theory these models measure the horizontal transmission of stress through the arc lithosphere accompanying ridge subduction modelling was used to compare the tonga arc and louisville ridge collision with the new hebrides arc and d entrecasteaux ridge collision which have disparate arc ridge intersection speeds but otherwise similar characteristics models of both systems indicate that diffuse deformation low values of the effective stress strain exponent n are required to explain the observed deformation deformation is somewhat insensitive to the vertically integrated strength of the arc inversely proportional to the argand number ar but indicates that the arc lithosphere is not extremely weak ar 100 low values of both ar and n suggest that the thermal structure is typical of cold or normal arcs and that deformation is dominated by flow in the lower crust and mantle in addition low values of n approaching newtonian flow may indicate that specific deformation mechanisms dictate deformation of the arc lithosphere possible mechanisms include low stress grain size dependent creep pyroxenite controlled rheology and mechanisms associated with water weakening changes in the boundary conditions greatly affect deformation within island arcs high rates of arc ridge intersection speed tonga louisville system yield arc parallel tension and crustal thickening in the wake of ridge subduction in contrast low rates of arc ridge intersection speed new hebrides d entrecasteaux system yield compressional deformation directly arcward of the collision zone and transverse strike slip faulting adjacent to the region of compressional deformation localized regions of extensional deformation along the frontal part of the arc adjacent to the collision zone may contribute to the formation of re entrants

lithospheric pressure depth relationship in compressive regions of thickened crust we investigate the pressure distribution with depth in regions undergoing horizontal shortening and experiencing crustal thickening both analytically and numerically our results show that in a convergent tectonic setting pressure can be considerably higher than lithostatic the pressure resulting from the weight of the overburden increases in pressure with respect to lithostatic conditions result from both the contribution of horizontal stresses and the flexural vertical loads the latter generated by the deflection of the upper crust and of the mantle because of the presence of topographic relief and a root respectively the contribution of horizontal stresses is particularly relevant to the upper crust and uppermost mantle where rocks are thought to deform brittlely in these domains pressure gradients twice lithostatic can be achieved the contribution of horizontal stresses is less important in the ductile domains as diverential stresses are progressively relaxed nevertheless the evects are still noteworthy especially close to the brittle ductile transition flexural vertical loads generated by the deflection of the upper crust and lithospheric mantle are relevant for rocks of the weaker lower crust as a result of the combination of the two mechanisms the pressure gradient varies vertically through the lithosphere ranging from negative inverted gradients to gradients up to several times the lithostatic gradient the pressure values range from one to two times the lithostatic values 1rgz to 2rgz

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

coastal forms and quaternary processes along the arid pilbara coast of northwestern australia abstract the pilbara coast is set in the most arid part of coastal australia this coast and coastal plain composed of riverine plains deltas tidal flats coastal dunes and limestone barriers relic deltas and archipelago ria shores is predominantly a terrigenous and carbonate sediment complex the coastal landforms therein formed during the quaternary through influence of ancestral landforms and fluvial and shoreline accretion coastal erosion and cementation the pre holocene features controlled development of holocene coastal forms and a variety of holocene depositional styles climate also plays a significant part in the coastal processes and in the generation of sedimentary products through high evaporation rates coupled with the limited rainfall cyclonic storms wind wind waves and the limited sediment delivery to the coastal zone this results in a distinctive range of stratigraphic sequences and chemical products as a result this arid coast is characterised by a range of features such as construction of arid zone deltas delta destruction and sediment redistribution during times of sediment depletion cyclone induced erosion and sedimentation mangroves and their associated sedimentary deposits evolution of coastal groundwater hypersalinity formation of salt flats and precipitation and cementation to form beachrocks high tidal crusts and gypsum precipitates the pilbara coastal stratigraphy and geomorphology suggests aridity was intricately involved in the sedimentation geomorphic evolution and pedogenic and diagenetic alteration of this coastal zone throughout the whole of the holocene and pleistocene

genetic links between ash flow calderas and associated ore deposits as revealed by large scale thermo mechanical modeling abstract in addition to a number of epithermal ore deposits some porphyry type ore deposits are also hosted by ash flow calderas spatial correlation between ash flow calderas and these deposits is observed in several parts of the world but temporal correlation between the mineralization and the episode of caldera formation remains unclear especially because geochronological data indicate that the mineralization may date to several million years before or after caldera formation because caldera collapse represents only a very short event in a long time scale evolution between a silicic magma chamber and its host rocks we have developed a thermo mechanical model of upper crustal and magmatic system behavior during the pre and post caldera formation stages major petrophysical properties of ash flow caldera related rocks are incorporated into the model which accounts for brittle elasto ductile rheology and heat transfer and reproduces fault initiation and various caldera collapse scenarios large scale mechanisms of initiation of thermal and structural traps for epithermal ore deposits are described in particular we found that the outer sides of caldera border faults are associated with both anomalous heat transfer and a high fracture density the finite size of the insulating intracaldera units forces the accumulated heat excess to be laterally transferred across the border faults in most cases the model predicts y shaped fault geometry which is surprisingly similar to that of mineralized faults in the field variations in lithostatic pressure on the magma chamber roof and in the vicinity of the border faults can reach 10 to 20 mpa with deep overpressured zones close to the inner edge of the chamber corners despite the fact that hydrothermal convection and fluid geochemistry are not included in the model these thermal and mechanical results are believed to constrain the conditions for mineralization processes our results show that in the presence of regional extension the development of caldera border faults may be suppressed to the advantage of deep seated fractures clustered above the central part of the magma chamber thus favoring conditions for the formation of porphyry type ore deposits finally we demonstrate that well before the eruption the gradual emplacement of a hot and large silicic magma chamber may create significant extensional stress in the brittle upper crust resulting in breakup and pre eruptive normal faulting and thus allowing for conditions of pre caldera mineralization

erosional control on the dynamics of low convergence rate continental plateau margins s u m m a r y over the last decade thermomechanical models have revealed the control of both tectonics and erosion on the morphology of continental plateaus margins however unravelling the specific effects of these two coupled processes has been difficult in practice here to assess the control of erosion we investigate the dynamics of the eastern and the northern borders of the tibetan plateau which are characterized by a low convergence rate and a steep topographic escarpment adjacent to the sichuan and tarim basins respectively thermomechanical modelling of continental lithosphere coupled with fluvial denudation reveals that important crustal deformation with large scale horizontal displacements can occur without any convergence as a response to mass transfer due to gravitational collapse and to erosional unloading these processes are sensitive to crustal structure geothermal gradient as well as surface erosion at a timescale of several million years our results suggest that this denudation triggered deformation exerts a primary control on the evolution of those plateau margins by counterbalancing the mass removal due to erosion and stabilizing the topographic escarpment this finding supports a possible explanation for the morphology of the longmen shan eastern tibet in which the paradoxical combination of persistent high topographic gradients close to the foreland and low convergence rates can be related to the influence of erosion on deformation patterns

fluid dynamics of local martian magma oceans the evolution of a melt region produced by a large impact during mars formation is addressed while some impact induced melt is redistributed during crater excavation sufficiently large impacts much larger than basin forming impacts generate an intact melt region which is retained beneath the excavation zone i e a local magma ocean local magma ocean evolution depends on the effective rheology controlling large scale deformation of the solid part of the planet mechanism of crystallization and melt region size within the uncertainties of various parameters two scenarios are possible for sufficiently weak rheology or large melt region size evolution is characterized by rapid extrusion and formation of a global magma ocean for sufficiently strong rheology or small melt region size in situ crystallization to a partially molten solid state occurs prior to isostatic adjustment subsequent to in situ crystallization local magma ocean evolution depends on melt region size and efficiency of lateral redistribution compared to bulk conductive cooling for large melt regions lateral spreading occurs via plastic deformation and results in an asymmetric global partial melt layer for small melt region size viscous spreading viscous can result in bulk cooling below the solidus prior to formation of a global layer a hypothesis for the origin of the hemispherical crustal dichotomy and tharsis rise is suggested the dichotomy is associated with a global partial melt layer produced by evolution of a large local magma ocean after dichotomy formation evolution of a second smaller local magma ocean is related to tharsis development

on topotaxy and compaction during antigorite and chlorite dehydration an experimental and natural study dehydration reactions result in minerals replacement and a transient fluid filled porosity these reactions involve interface coupled dissolution precipitation and might therefore lead to fixed crystallographic orientation relations between reactant protolith and product phases i e topotaxy we investigate these two phenomena in the dehydration of a foliated antigorite atg serpentinite by comparing the crystallographic preferred orientation cpo developed by olivine ol orthopyroxene opx and chlorite chl during high pressure antigorite and chlorite dehydration in piston cylinder experiments and in natural samples recording the dehydration of antigorite cerro del almirez betic cordillera spain experiments were performed under undrained conditions resulting in fluid filled porosity and in strong cpo of the prograde minerals controlled by the pre existing antigorite cpo in the reactant foliated serpentinite the orientation of aol opx and c text chl is parallel to c text atg from the protolith the cerro del almirez samples show similar locally well developed topotactic relations between orthopyroxene chlorite and antigorite but the product cpos are weaker and more complex at the thin section scale in contrast to the experiments olivine from natural samples shows a weak correlation between bol and the former c text atg we relate the strengthening of local topotactic relations and the weakening of the inherited cpo at a larger scale in natural samples to compaction and associated fluid migration microstructural features that might be related to compaction in the natural samples include 1 smooth bending of the former foliation 2 gradual crystallographic misorientation up to 16 of prismatic orthopyroxene due to buckling by dislocation creep 3 inversion of enstatite to low clinoenstatite p21 c along lamellae and 4 brittle fracturing of prismatic orthopyroxene enclosed by plastically deformed chlorite the coexistence of orthopyroxene buckling and clinoenstatite lamellae enables estimating the local strain rates and shear stresses generated during compaction an lower bound for the strain rates in the order of 10 12 to 10 13 s 1 and shear stresses of 60 70 mpa are estimated based on creep data lower shear stresses 20 40 mpa are retrieved using a theoretical approach these data point to slow compaction and fluid extraction in nature if the system is not perturbed by external forces with rates only marginally higher than the viscoplastic deformation of the solid matrix

numerical modelling of quaternary deformation and post rifting displacement in the asal ghoubbet rift djibouti africa over the last three decades a host of information on rifting process relating to the geological and thermal structure long time scale deformation quaternary and holocene and rifting cycle displacement across the asal ghoubbet rift has been made available these data are interpreted with a two dimensional thermo mechanical model that incorporates rheological layering of the lithosphere dyke inflation and faulting active fault locations and geometry are mainly controlled by both thermal structure and magma intrusion into the crust the distributed slip throughout the inner rift is related to the closeness of magma chamber leading to additional stress into the upper thinned crust assuming a constant arabia somalia motion of 11 mm year the variation of subsidence rate between the last 100 and 9 ka is associated with a decrease of the average injection rate from 10 to 5 mm year these values about equal to the regional opening rate suggest that both volcanism and tectonic play an equivalent role in the rifting process our modelled sequence of events gives one possible explanation for both vertical and horizontal displacements observed since the 1978 seismovolcanic crisis although part of the post rifting deformation could be due to viscous relaxation the high opening rate in the first years after the event and the abrupt velocity change in 1984 1986 argue for a large dyke inflation of 12 cm year ending in 1985 the asymmetric and constant pattern of the gps velocity since 1991 suggests that present post rifting deformation is mainly controlled by fault creep and regional stretching this study demonstrates the internal consistency of the data set highlights the role of magmatism in the mechanics of crustal stretching and reveals a complex post rifting process including magma injection fault creep and regional stretching d 2005 elsevier b v all rights reserved

viscoelastic behaviour and temporal fractal properties of lherzolite and marble possible extrapolation from experimental results to the geological time scale the viscoelastic relaxation behaviour of lherzolite and marble and their temporal fractal properties are studied based on the high temperature viscoelastic behaviour for these rock types derived from the time temperature superposition principle the relaxation function for the viscoelastic behaviour of lherzolite and marble can be expressed as a flow law from a power law function of temperature reduced time given by normalizing the various temperature behaviours with a non integer exponent the relaxation function is thus time scale invariant i e there is a temporal fractal property to the temperature reduced time their rheological behaviour over geological time scales at low temperature can be extrapolated from the experimental high temperature behaviour based on the temporal fractal property our derived flow law satisfies not only transient behaviour but also steady state behaviour

a seismic cross section through the east european continent summary a 2 d pro le for the shear wave velocity and anisotropybetween egypt and spitsbergen is presented the pro le is constructed using fundamental and higher mode love and rayleigh waves recorded by stations of the nars deep iris and geofon networks the surface wave data have been inverted for shear velocity and anisotropy using a waveform inversion in the eastern mediterranean lithosphere we observe a large diierence 7 per cent between sh and sv velocities we interpret this anomaly as an anisotropic oceanic lithosphere beneath the eastern mediterranean an interpretation which is consistent with tectonic reconstructions of the region the east european continent is imaged as a high velocity body whose thickness increases with the estimated age of the lithosphere the continental root of the ukrainian and baltic shields and east european platform extends to a depth of 200 km this is in contrast to the surrounding younger continental regions which appear to be less than 100 km thick we further studied the structure of the continental lithosphere by investigating a possible relation between seismic velocities and tectonic age both a logarithmic and a square root relation have been tted to the average seismic velocities in each tectonic region the data slightly favour a logarithmic relation but a square root relation cannot be excluded

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

effects of a third phase on the mechanical and microstructural evolution of a granulite to help constrain the rheology of and strain partitioning in composite silicates from lower contintental crust lcc opx hbl plag granulite samples were shortened coaxially 15 40 in a solid pressure medium nacl apparatus at pc 1 0 gpa t 650 950 c and strain rates from 10 5 to 10 7 s 1 the resultant mechanical behaviour and microstructures are compared with those of pikwitonei amphibolite 58 hbl 42 plag deformed by wilks and carter 1990 under similar conditions the opx hbl plag rock is stronger than the hbl plag rock at all conditions all samples strain soften beyond 10 strain and stress levels do not reach steady state once softening begins at low t and high strain rates relative mineral strength is plag opx hbl whereas at higher t and lower strain rates relative mineral strength is opx plag hbl opx grains are extensively kinked by glide on 010 001 at all conditions plagioclase deforms by translation glide and mechanical twinning that becomes more abundant with higher t and amphibole deforms by slip on 100 001 microcracking in all phases partially accommodates strain under all conditions recovery mechanisms are limited even at the highest temperature and lowest strain rates tubules along grain boundaries and within grains suggest that a fluid phase possibly related to amphibole breakdown was present during experiments semibrittle shear zones sszs comprising s c fabrics defined by elongate grains of all phases develop in all samples sszs initiation coincides with the onset of mechanical softening sszs initiate along grain boundaries and widen with increasing strain which accounts for progressive softening with time similar s c fabrics have been duplicated in simple shear tests under indentical conditions which in turn are comparable to fabrics in naturally deformed lcc rocks increasing the number of co existing hard phases in natural composite silicates promotes strain localisation by enhancing ductility contrast s between constituent phases

the pilbara coast a riverine coastal plain in a tropical arid setting northwestern australia abstract the unifying feature of the pilbara coast of northwestern australia is that it is a sedimentary repository for a range of rivers that drains a high relief precambrian rocky hinterland and discharges sediments along a coastal plain which fronts a wave dominated environment in a tropical arid climate the combination of fluvial and shoreline accretion processes coastal cementation coastal erosion and ancestral landform architecture such as residual pleistocene limestone ridges and large outcrops of precambrian bedrock has produced a complex coastal system during the quaternary as a result the coast is dominated by active deltas beach dune shores inactive eroding parts of deltas and their barriers limestone barrier coasts bays associated with eroded limestone barriers and archipelago ria coasts quaternary sediments throughout the area while varied in their distribution and history at the smaller scales exhibit a recurring pattern of lithotopes and lithologies in the region there are three main quaternary suites pleistocene red siliciclastic sediments alluvium deltaic sediments and aeolian sand that form an inland zone pleistocene limestones that form local barriers and a holocene system within which are the sedimentary suites of deltas beach dunes tidal flats and tidal embayments

the timing of sediment transport down monterey submarine canyon offshore california while submarine canyons are the major conduits through which sediments are transported from the continents out into the deep sea the time it takes for sediment to pass down through a submarine canyon system is poorly constrained here we report on the first study to couple optically stimulated luminescence osl ages of quartz sand deposits and accelerator mass spectrometry 14 c ages measured on benthic foraminifera to examine the timing of sediment transport through the axial channel of monterey submarine canyon and fan offshore california the osl ages date the timing of sediment entry into the canyon head while the 14 c ages of benthic foraminifera record the deposition of hemipelagic sediments that bound the sand horizons we use both single grain and small 2 mm area single aliquot regeneration approaches on vibracore samples from fining upward sequences at various water depths to demonstrate relatively rapid decadal scale sand transport to at least 1 1 km depth and more variable decadal to millennial scale transport to a least 3 5 km depth on the fan significant differences between the time sand was last exposed at the canyon head osl age and the timing of deposition of the sand from 14 c ages of benthic foraminifera in bracketing hemipelagic sediments are interpreted as indicating that the sand does not pass through the entire canyon instantly in large individual events but rather moves multiple times before emerging onto the fan the increased spread in single grain osl dates with water depth provides evidence of mixing and temporary storage of sediment as it moves through the canyon system the ages also indicate that the frequency of sediment transport events decreases with distance down the canyon channel system the amalgamated sands near the canyon head yield osl ages that are consistent with a sub decadal recurrence frequency while the fining upward sand sequences on the fan indicate that the channel is still experiencing events with a 150 250 year recurrence frequency out to 3 5 km water depths

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

large scale deformation related to the collision of the aleutian arc with kamchatka the far western aleutian island arc is actively colliding with kamchatka westward motion of the aleutian arc is brought about by the tangential relative motion of the pacific plate transferred to major right lateral shear zones north and south of the arc early geologic mapping of cape kamchatka a promontory of kamchatka along strike with the aleutian arc revealed many similarities to the geology of the aleutian islands later studies support the notion that cape kamchatka is the farthest west aleutian island and that il has been accreted to kamchatka by the process of arc continent collision deformation associated with the collision onshore kamchatka includes gravimetrically determined crustal thickening and formation of a narrow thrust belt of intensely deformed rucks directly west of cape kamchatka the trend of the thrust faults is concave toward the collision zone indicating a radial distribution of maximum horizontal compressive stress offshore major crustal faults trend either oblique to the kamchatka margin or parallel to major aleutian shear zones these offshore faults are complex accommodating both strike slip and thrust displacements as documented by focal mechtmisms and seismic reflection data earthquake activity is much higher in the offshore region within a zone bounded to the north by the northernmost aleutian shear zone and to the west by an apparent aseismic front analysis of focal mechanisms in the region indicate that the present day arc continent contact zone is located directly east of cape kamchatka in modeling the dynamics of the coiiision zone using thin viscous sheet theory the rhealogical parameters are only partially constrained to values of n the effective power law exponent 2 3 and ar the argand number s 30 these values are consistent with a fnrcarc thermal profile of kamchatka previously determined from heat flow modeling the thin viscous sheet modeling also indicates that onshore thrust faulting is a consequence not only of compressive stresses resulting from the west directed collision but also of sediment induced coupling of the subducting pacific plate

the tectonic evolution of the southern alps new zealand insights from fully thermally coupled dynamical modelling summary we present here the results of numerical models of the southern alps in which the thermal and dynamical developments of the orogen are fully coupled to one another this interlinkage allows the use of a wide range of thermal and physical features of the orogen as constraints on the applicability of our models in particular the thermal aspect of this method enables the prediction of the distribution of thermochronological ages at the surface of the model perturbation of the geothermal structure due to rapid uplift and exhumation causes an intrinsic weakening and concentration of strain along the equivalent of the alpine fault zone in these models this thermal weakening of the crust also produces a zone of high strain antithetic to the alpine fault in the upper crust that is comparable to the main divide fault zone within the southern alps the introduction of orographic rainfall and erosional processes into the model leads to the development of high asymmetric topography comparable to that of the southern alps this topographic profile results from the capture of available precipitation by the windward side of the orogen and the resultant rain shadow on the leeward side due to the time required to accumulate suycient topography for this rain capture evect to become significant the establishment of this high asymmetric topography lags the initiation of surface uplift by several million years comparison of the observed thermal history of the southern alps with that seen in models based on divering hypotheses of the tectonic evolution of the south island shows that the present tectonic regime of the orogen most probably developed in a single rapid reorganization of plate motions at approximately 5 ma with relative stability of the tectonic regime since that time the variation in isotopic ages along the southern alps is consistent with that expected from variation in accumulated uplift and exhumation along the orogen arising from the obliquity of convergence of the australian and pacific plates

the stress distribution related to the boudinage of a visco elastic material examples from a polar outlet glacier abstract quantitative stress measurements related to the development of mesoscale structures in rock are difficult if not impossible a method for determining the stress distribution and history during the development of mesoscale boudinage structures in ice is introduced here boudinage structures in fracture trace ice have been observed in the outlets glaciers of the framnes mountains east antarctica fracture traces are preserved when creasses fill with surface melt water which freezes to form coarse grained columnar ice a 4 0 km flow parallel traverse across the central ice stream of the framnes mountains is presented to illustrate the boudinage of fracture traces with a mean width of 0 30 m field based measurement of the geometric evolution of boudinage structures has been combined with surface flow rate measurements to quantitatively determine the strain rate at which the boudinage structures formed the strain rate measurements provide boundary constraints on several two dimensional finite difference models that have been used to analyse the stress distribution related to the formation of boudinage structures in a visco elastic solid the results reveal the development of pressure shadows during the boundinage of layered rocks and demonstrate the degree of refraction of stress across rheological boundaries which have important ramifications for the analysis of planar and linear fabrics in rocks

rifting through a stack of inhomogeneous thrusts the dipping pie concept 1 orogenic building leaves a complex heritage consisting of a stack of nappes that may have contrasting lithologic structures resulting in heterogeneous mechanical behavior of the system during the postorogenic stages while the thermal state of the region is reequilibrating strong lateral variations of the depth to the brittle ductile transition develop as a consequence of these preexisting heterogeneities we use a thermomechanical model to quantify how an inherited weak nappe influences the development of fault patterns resulting from postorogenic extension the competence contrast between the nappe and the rest of the upper crust as well as the strength of the crust itself are the principal variable parameters of our experiments the results suggest that a dipping weak nappe introduces a lateral velocity discontinuity and serves as a localization factor for deformation the presence of a preexisting nappe with a low competence contrast is sufficient to localize strain along the nappe leading to the formation of a flexural rolling hinge in this case the migration of the basin is slow continuous and limited by gravity driven processes that lead to the rise of hot weak material under the subsiding basin in case of a high competence contrast overall crustal strength is reduced by a dipping pie effect assuming overall high crustal strength the presence of a contrasting nappe leads to a bimodal fault pattern governed by two types of faults crustal scale planar faults rooting on the brittle ductile transition of the crust and thin skinned listric faults rooting on the nappe itself this bimodality results in a jump like migration of the basin downward along the dipping weak nappe applying this model to the case of the gulf of corinth greece allows us to explain in the case of assumed high competence contrast the observed microseismicity patterns the asymmetry of the gulf and the kinematics of fault migration within the basin

brittle ductile transition semi brittle behavior abstract in central british columbia mesoscopic and microscopic deformation mechanisms within metamorphic rocks ranging from greenschist facies through upper amphibolite facies have been examined to evaluate the applicability of single mechanism flow laws at different crustal levels throughout the entire sequence regardless of metamorphic grade the processes of regional strain accommodation were dominantly semi brittle in the presence of an aqueous fluid phase no sign was found of abrupt brittle ductile transition and caution against calculating such a transition through the intersection of single mechanism flow laws with friction law s that on the regional and local scales do not appear to describe the mechanisms whereby regional strain is accommodated is advised

rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock mechanics based yield criteria for the inelastic deformations at the top and base at mid plate depths stresses are likely to be supported elastically because bending strains and elastic stresses are low the collisional tectonic regime however is far more complex because very large permanent strains are sustained at mid plate depths and this requires us to include the broad transition between brittle and ductile flow moreover important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid plate depths two specific contributions of laboratory rock rheology research are considered in this paper first the high temperature steady state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere the thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm tc above which at geologic strain rates the high temperature ductile strength falls below some arbitrary strength isobar e g 100 mpa for olivine tc is about 700 800 c but for other crustal silicates tc may be as low as 400 600 c suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the moho as suggested by a number of workers on independent grounds put another way the moho is a rheological discontinuity a second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant strain rate tests or as an accelerating creep rate stage at constant differential stress a number of physical mechanisms have been identified that may be responsible for ductile faulting including the onset of dynamic recrystallization phase changes hydrothermal alteration and hydrolytic weakening microscopic evidence for these processes as well as larger scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere

creep of rocksalt abstract a review is presented of the fundamental flow properties and processes in experimentally deformed natural and synthetic halite single crystals and polycrystalline aggregates included in the summary are discussions of a microstructures induced during steady state creep b creep rupture of rocksalt c experiments associated with project salt vault and more recent field studies and d brine migration a representative steady state flow law determined for natural aggregates and maximum natural deviatoric stresses deduced from subgrain sizes are applied briefly to considerations of creep in waste repositories and of salt dome dynamics while the mechanical behavior of rocksalt is probably better understood than for all other rock types further investigations especially on load path stress history and creep rupture are clearly mandated furthermore additional investigations of brine migration and of bench and field scale deformations are needed the latter incorporating realistic rocksalt flow properties into numerical simulations of natural rock mass response

dynamics of continental rift propagation the end member modes abstract an important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation in regions of rift propagation specifically transition zones from continental rifting to seafloor spreading it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones the focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere more specifically this study explores how modeled rift behavior depends on the value of rheological parameters of the crust a three dimensional finite element model is used to simulate lithosphere deformation in an extensional regime the chosen approach emphasizes understanding the tectonic forces involved in rift propagation dependent on plate strength two end member modes are distinguished the stalled rift phase is characterized by absence of rift propagation for a certain amount of time extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone which requires a buildup of shear stresses near the rift tip and significant intra plate deformation this stage represents a situation in which a rift meets a locked zone localized deformation changes to distributed deformation in the locked zone and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend in the alternative rift propagation mode rift propagation is a continuous process when the initial crust is weak the extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses model predictions address aspects of previously unexplained rift evolution in the laptev sea and its contrast with the tectonic evolution of for example the gulf of aden and woodlark basin

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

development of en echelon magmatic segments along oblique spreading ridges en echelon magmatic segments commonly develop along obliquely spreading oceanic ridges to clarify some of the dynamic aspects of this plate boundary we performed a series of thermo mechanical numerical tests when extension of oceanic lithosphere becomes oblique deformation within the axial region localizes into distinct upwelling centers temperatures are elevated in the upwelling cells which are shallow mantle features that form the new plate boundary the predicted features are similar to the axial volcanic ridges documented at mohns and reykjanes ridges and we conclude that they become the new loci of extensional deforma tion upwelling and magmatic activity these ridges suborthogonal to the plate spreading direction only develop when the axis rift zone is weak the subsegment length and spacing depend primarily on obliquity and axial width predicted crustal thickness along the subsegmented axis varies discernibly this might explain the morphology and satellite gravity of the flanks of oblique spreading ridges

elevated shear zone loading rate during an earthquake cluster in eastern california we compare geodetic velocity to geologic fault slip rates to show that tectonic loading was doubled across the eastern california shear zone ecsz during a cluster of major earthquake activity new slip rates are presented for six dextral faults that compose the ecsz in the central mojave desert these rates were determined from displaced alluvial fans dated with cosmogenic 10 be and from a displaced lava flow dated with 40 ar 39 ar we find that the sum geologic mojave ecsz slip rate 6 2 1 9 mm yr is only half the present day geodetically measured velocity of 12 2 mm yr these rates account for cumulative fault slip and geodetic observations that span the 60 km wide shear zone therefore this difference cannot be attributed to postseismic relaxation redistribution of tectonic loading over the earthquake cycle at a regional scale suggests that earthquake clustering may be enhanced via feedback with weakening of ductile shear zones

method for correction of differential stress calculations from experiments using the solid salt assembly in a griggs type deformation apparatus abstract it is necessary to perform deformation experiments in appropriate temperature and pressure conditions equivalent to the inside of the earth to characterize rheological properties of rocks there are several types of deformation apparatus using different confining media such as gases or weak solids solid medium apparatus is suitable to generate high pressures and temperatures to investigate mechanical properties of rocks in crust or mantle however stress accuracy of solid medium apparatus was not high recent calibration allowed us to obtain steady state stresses accurately within 30 mpa however it was not able to reproduce initial linear region transient and post yield behaviors because the calibration was obtained from the comparison of stresses only at 5 strain in this study we performed several axial compression experiments on metals to measure differential stresses using a griggs type deformation apparatus with solid salt assembly ssa measured stresses are consistent with results of the prior research within 30 mpa under the identical conditions while the measured stresses increased with the confining pressures obtained mechanical data were analyzed based on the viscoelastic constitutive law master curves were constructed from identical materials between the griggs and gas apparatuses under normalized temperatures strains and confining pressures therefore it is considered that differences between both master curves are derived from distinction of rheology components of two apparatuses from the comparison between master curves a correction method for the stresses obtained in the griggs apparatus with ssa was derived applying the correction method to stress measurements of metals using the griggs apparatus with ssa it became possible to reproduce the stress equivalent to ones measured using gas apparatus not only at steady state but also at initial linear region transient and post yield behaviors within an error of 30 mpa moreover the correction can be extended to higher confining pressure up to 1500 mpa

lithospheric rheology and flexure at artemis chasma venus artemis chasma is an arcuate 2000 km diameter zone of convergence and lithospheric underthrusting on venus inelastic flexure modeling of the topography at artemis combined with observations of trench tectonics allow us to document local temperature gradients below 4 k km 1 and an immense compressive in plane force at the trench lithospheric rheology on venus may be stronger than would be predicted from conventional extrapolations of rock mechanics experiments in particular the brittle surface strength must reach a few tens of megapascals to satisfy the observed lack of flexurally induced surface faulting elastic plate bending models provide an adequate estimate of the bending moment at artemis but they fail to constrain or even recognize the in plane force the inelastic analysis implies an in plane force of the order of 1 1014 n m 1 a potential driving force is thermal thinning of regionally thick lithosphre in the highlands to the north the low heat flux at artemis which is a comparatively young structure is compatible with the notion that venus has experienced a prolonged period of cooling in the last several hundred million years the inference of such exceptionally low thermal gradients embraces three end member possibilities 1 the surface age is 600 ma 2 the lithospheric thermal age is greater than the surface age and or 3 the upper mantle temperature is anomalously low 1550 k

thermomechanical evolution of the south alpine rifted margin north italy constraints on the strength of passive continental margins abstract we have reconstructed the kinematic thermal and rheological evolution of the triassic cretaceous south alpine passive continental margin northern italy and estimated lateral strength variations associated with rifting rifting initially caused the formation of a roughly symmetrical ca 100 km wide graben located 150 200 km away from the future break up site 30 myr after the onset of rifting the rift axis shifted laterally and eventually led to oceanic crust formation the thermal evolution of the margin was first controlled by the waning of a thermal anomaly which was emplaced shortly before rifting and was cooling during the first extensional stages numerical modelling of rifting and drifting constrained by field data is used to trace thermal and rheological changes affecting the rifted margin the effects of the thermal anomaly are restricted to the first 10 20 myr of rifting subsequently isotherms are slightly affected by lithospheric thinning and show very little distortion after break up rheological modelling demonstrates that the south alpine lithosphere preserved substantial strength even shortly before break up and became stronger thereafter the modelling also shows that at break up and 100 myr later thinned domains of the margin are stronger than the less thinned ones

clues to the lithospheric structure of mars from wrinkle ridge sets and localization instability 1 wrinkle ridges are a manifestation of horizontal shortening in planetary lithospheres for which deformation is localized on faults that underlie individual ridges in ridged plains of mars such as solis planum or lunae planum wrinkle ridges are spaced 40 km apart whereas in the martian northern lowlands where ridges are identified only in mars observed laser altimeter mola altimetric data the ridge spacing is at least 80 km we attribute ridge spacing to an instability of the lithosphere under horizontal compression the localization instability which results in periodically spaced faults montesi and zuber 2003a links the difference of ridge spacing in the northern lowlands and in the highland ridged plains to the difference of crustal thickness via the depth of the brittle ductile transition bdt in solis and lunae plana where the crust is 50 to 60 km thick the crust may be ductile at depth limiting faulting to the bdt of crustal rocks in the lowlands the crust is only about 30 km thick and may be brittle throughout thus the depth of faulting may be controlled by the bdt of mantle rocks which is roughly a factor of two deeper than that of crustal rocks the geotherm can be identical in both regions at 12 3 k km 1 although differences of a few k km 1 can be accommodated within this model the heat flux implied by this geotherm is similar to the heat produced by radiogenic decay 3 gyr ago our analysis provides a rheological explanation for the difference in spacing between ridges in the highlands and the lowlands in contrast to the suggestion of head et al 2002 who proposed that alternating lowlands ridges are buried by sediments in addition finite element models that use the lithospheric structure deduced from ridge spacing show that modest gradients of crustal thickness or heat flux across a ridged plains favor the formation of faults dipping toward high elevation areas as may be the case in solis planum golombek et al 2001

numerical models for continental break up implications for the south atlantic we propose a mechanism that explains in one unified framework the presence of continental break up features such as failed rift arms and high velocity and high density bodies that occur along the south atlantic rifted continental margins we used 2d and 3d numerical models to investigate the impact of thermo rheological structure of the continental lithosphere and initial plume position on continental rifting and break up processes 2d experiments show that break up can be 1 central mantle plume induced and directly located above the centre of the mantle anomaly 2 shifted mantle plume induced and 50 to 200 km shifted from the initial plume location or 3 distant self induced due to convection and or slab subduction delamination and 300 to 800 km off set from the original plume location with a 3d perfectly symmetrical and laterally homogeneous setup the location of continental break up can be shifted hundreds of kilometres from the initial position of the mantle anomaly we demonstrate that in case of shifted or distant continental break up with respect to the original plume location multiple features can be explained its deep seated source can remain below the continent at one or both sides of the newly formed ocean this mantle material glued underneath the margins at lower crustal levels resembles the geometry and location of high velocity high density bodies observed along the south atlantic conjugate margins impingement of vertically up welled plume material on the base of the lithosphere results in pre break up topography variations that are located just above this initial anomaly impingement this can be interpreted as aborted rift features that are also observed along the rifted margins when extension continues after continental break up high strain rates can relocalize this relocation has been so far attributed to rift jumps most importantly this study shows that there is not one single rift mode for plume induced crustal break up

3d hybrid tectono stochastic modeling of naturally fractured reservoir application of finite element method and stochastic simulation technique abstract simulation of naturally fractured reservoirs offers significant challenges due to the lack of a methodology that can utilize field data to date several methods have been proposed by authors to characterize naturally fractured reservoirs among them is the unfolding folding method which offers some degree of accuracy in estimating the probability of the existence of fractures in a reservoir also there are statistical approaches which integrate all levels of field data to simulate the fracture network this approach however is dependent on the availability of data sources such as seismic attributes core descriptions well logs etc which often make it difficult to obtain field wide in this study a hybrid tectono stochastic simulation is proposed to characterize a naturally fractured reservoir a finite element based model is used to simulate the tectonic event of folding and unfolding of a geological structure a nested neuro stochastic technique is used to develop the inter relationship between the data and at the same time it utilizes the sequential gaussian approach to analyze field data along with fracture probability data this approach has the ability to overcome commonly experienced discontinuity of the data in both horizontal and vertical directions this hybrid technique is used to generate a discrete fracture network of a specific australian gas reservoir palm valley in the northern territory results of this study have significant benefit in accurately describing fluid flow simulation and well placement for maximal hydrocarbon recovery

deformation of debris ice mixtures mixtures of rock debris and ice are common in high latitude and high altitude environments and are thought to be widespread elsewhere in our solar system in the form of permafrost soils glaciers and rock glaciers these debris ice mixtures are often not static but slide and creep generating many of the landforms and landscapes associated with the cryosphere in this review a broad range of field observations theory and experimental work relevant to the mechanical interactions between ice and rock debris are evaluated with emphasis on the temperature and stress regimes common in terrestrial surface and near surface environments the first order variables governing the deformation of debris ice mixtures in these environments are debris concentration particle size temperature solute concentration salinity and stress a key observation from prior studies consistent with expectations is that debris ice mixtures are usually more resistant to deformation at low temperatures than their pure end member components however at temperatures closer to melting the growth of unfrozen water films at ice particle interfaces begins to reduce the strengthening effect and can even lead to profound weakening using existing quantitative relationships from theoretical and experimental work in permafrost engineering ice mechanics and glaciology combined with theory adapted from metallurgy and materials science a simple constitutive framework is assembled that is capable of capturing most of the observed dynamics this framework highlights the competition between the role of debris in impeding ice creep and the mitigating effects of unfrozen water at debris ice interfaces

the role of pre existing faults in basin evolution constraints from 2d finite element and 3d flexure models the lithospheric driving forces which cause intraplate basin deformation are relatively constant over large areas consequently lateral variations in deformation and stress and strain concentrations seem to be primarily caused by pre existing heterogenities in the rheological signature of the continental lithosphere underlying the sedimentary basins in this paper we explore the weak character of upper crustal faults and their control on basin shape for a number of case studies on intraplate phanerozoic basin settings using a 2d finite element and a 3d flexure model of key importance is the integration of seismic data and field observations with the tectonic modelling allowing the investigation of deformation processes and their expressions on different scales operating on different levels of the lithosphere finite element models for sub basin scales incorporating weak upper crustal faults predict strong control of these weak zones on local stress distributions and subsequent deformation in agreement with observed deformation patterns slip along upper crustal faults control stress distribution and subsequent faulting in overlying sedimentary rocks the effect of weak upper crustal fault movements on basin wide regional upper crustal flexure is looked at in two case studies on 1 extensional tectonics in the lake tanganyika rift zone east africa and 2 compressional tectonics in the central system and tajo basin central spain both settings indicate that basement warpings are controlled by large amounts of slip along so called weak crustal scale border faults which are mostly planar adopting border fault displacements in the 3d flexure model the results indicate low effective elastic thickness eet values in a range of 3 7 km for induced basement deflection patterns in accordance with observations the low eet values most likely reflect a partly decoupling of upper crustal and subcrustal deformation facilitated by the weak lower crust and in agreement with standard rheological assumptions for phanerozoic lithosphere in contrast the inferred weakness of upper crustal faults relative to surrounding rock is not evident from uniform rheological assumptions however observations of reactivations of faults which are not preferably aligned with the stress field and reactivations of basin deformation on long time scales are in support of this feature

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

stress magnitude estimates from earthquakes in oceanic plate interiors we propose a method to estimate stress magnitudes in oceanic plate interiors from focal depths and focal mechanisms using a depth dependent rheology we show it is possible to estimate the differential stress 1 3 averaged over some reference lithospheric thickness the resolving power of the method is investigated by evaluating the effect of uncertainties in parameters that are involved in the analysis we apply the method to the central indian ocean where intraplate seismicity is high from well studied earthquakes we estimate differential stresses of the order of hundreds of rnegapascals this result is consistent with the high level of stress that was found from numerical model calculations by cloetingh and wortel 1985 1986 from the few intraplate events in the pacific plate we also estimate differential stresses in this area

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1

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