crystallographic fabrics of omphacite rutile and quartz in vendee eclogites armorican massif france consequences for deformation mechanisms and regimes abstract this study aims at further understanding of the mechanisms how lattice preferred orientations lpo develop during deformation in the main eclogite minerals microstructures and textures of deformed eclogites from the les essarts complex western france were investigated using optical microscopy and electron backscatter diffraction ebsd in the scanning electron microscope microfabric analyses of eclogite facies minerals are used to identify their deformation mechanisms which define the rheology at high pressure metamorphic conditions mechanisms of intracrystalline deformation by dislocation movement dislocation creep result usually in a non linear flow law typically power law while diffusive processes diffusion creep correspond to linear flow laws general microstructural observations may suggest intracrystalline deformation dislocation creep of omphacite the omphacite lpo vary between s and l type and correlate with oblate or prolate grain shape fabrics respectively until now these lpo types have not been understood by plasticity models based on dislocation glide on the known slip systems in clinopyroxene an alternative interpretation is given in terms of anisotropic growth and dissolution with grain boundary diffusion as the rate controlling process there are further indications suggesting diffusion creep with concomitant anisotropic growth and dissolution as a main deformation mechanism in omphacite in omphacite around a hollow garnet crystallographic and shape fabrics align with the c 001 axes parallel to the grain elongations defining the mineral lineation which rotates locally with the inferred flow direction in this part the grain sizes of omphacite and rutile are larger than in the surrounding matrix the geometry of both the shape and crystallographic fabrics is interpreted to represent the local stress regime directions and ratios of the principal stresses the lpo of rutile duplicate the lpo of omphacite and a similar distinction between s and l type was used rutile deformation mechanisms probably involve dislocation creep as well as diffusion creep quartz mainly occurs as an interstitial phase with weak lpo patterns interpreted as random no representative obliquity of the lpo in omphacite nor rutile with respect to foliation and lineation was observed to be used as potential shear sense criteria however the rutile lpo was slightly rotated relative to the omphacite lpo consistently in most samples the results suggest that diffusion processes are strongly involved in the deformation of eclogites a linear flow law should be taken into account in tectonic models where eclogites are incorporated

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

ductile fractures and magma migration from source mechanisms proposed to explain effi cient melt transport away from hot ductile source regions are problematic brittle elastic fracturing is a well known mechanism that allows fast magma migration as dikes through cold crust ductile fractures have been propo sed as an alternative for ductile environments where brittle elastic diking is inhibited ductile fracturing results from rock creep and growth of microscale voids that become interconnected leading to rock failure in this paper we present observations and numerical models supporting the hypothesis that ductile fracture controls early steps in magma migration we postulate that once developed ductile fracture dikes may reach a critical length where magma stresses at dike tips overcome fracture toughness and lead to brittle elastic diking which subsequently controls magma migration

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

brittle and semibrittle creep of tavel limestone deformed at room temperature deformation and failure mode of carbonate rocks depend on the confining pressure in this study the mechanical behaviour of a limestone with an initial porosity of 14 7 is investigated at constant stress at confining pressures below 55 mpa dilatancy associated with micro fracturing occurs during constant stress steps ultimately leading to failure similar to creep in other brittle media at confining pressures higher than 55 mpa depending on applied differential stress inelastic compaction occurs accommodated by crystal plasticity and characterized by constant ultrasonic wave velocities or dilatancy resulting from nucleation and propagation of cracks due to local stress concentrations associated with dislocation pile ups ultimately causing failure strain rates during secondary creep preceding dilative brittle failure are sensitive to stress while rates during compactive creep exhibit an insensitivity to stress indicative of the operation of crystal plasticity in agreement with elastic wave velocity evolution and microstructural observations

deformation mechanisms in a high temperature quartz feldspar mylonite evidence for superplastic flow in the lower continental crust abstract microstructures and crystallographic preferred orientations in a fine grained banded quartz feldspar mylonite were studied by optical microscopy sem and tem mylonite formation occurred in retrograde amphibolite facies metamorphism interpretation of the microstructures in terms of deformation mechanisms provides evidence for millimetre scale partitioning of crystal plasticity and superplasticity strain incompatibilities during grain sliding in the superplastic quartz feldspar bands are mainly accommodated by boundary diffusion of potassic feldspar the rate of which probably controls the rate of superplastic deformation there is evidence for equal flow stress levels in the superplastic and crystal plastic domains in this case mechanism partitioning results in strain rate partitioning fast deformation in the superplastic bands therefore dominates flow and deformation is probably best modelled by a superplastic law if this deformational behaviour is typical shearing in mylonite zones of the lower continental crust may proceed at exceptionally high rates for a given differential stress or at low differential stresses in case of fixed strain rates

lower crustal structure in northern california implications from strain rate variations following the 1906 san francisco earthquake 1 it is well known that geodetic data from a single instant in time cannot uniquely characterize structure or rheology beneath active seismogenic zones nevertheless comparison of spatial and temporal variations in deformation rate with time dependent mechanical models can place valuable constraints on fault zone geometry and rheology we consider postseismic strain rate transients by comparing geodetic data from north of san francisco bay obtained between 1906 and 1995 to predictions from viscoelastic finite element models models include 1 an elastic plate over a viscoelastic half space 2 distributed shear within a viscoelastic layer 3 discrete shear zones within an otherwise elastic layer 4 discrete shear zones in combination with distributed viscoelastic shear and 5 midcrustal detachment surfaces we vary as applicable locking depth elastic thickness depth to the top and bottom of the distributed shear layer distributed shear relaxation time discrete shear zone relaxation time and discrete shear zone width the best fitting physically reasonable elastic plate over viscoelastic half space models 1 do a poor job simultaneously predicting spatial and temporal variations in the data the best fitting distributed shear models 2 do a poor job predicting spatial variations in the deformation rate although they fit the geodetic data recent findings from seismic reflection refraction studies in northern california argue against models with shallow subhorizontal detachments 5 models incorporating discrete shear zones 3 4 provide the best fit to the geodetic data and are consistent with seismic studies that argue for discrete fault zones extending through the entire crust

geohazard map of cover collapse sinkholes in the tournaisis area southern belgium abstract this paper reports the methodology developed to draw up a geohazard map of cover collapse sinkhole occurrences in the tournaisis area in this area carboniferous limestones are overlain by a mesocenozoic cover mainly consisting of marls sand and clay the thickness of this cover ranges from a few meters to more than 100 m the surficial morphology of the area does not show any karstic evidence except for the occurrence of these collapses from a paleogeographical point of view a developed quaternary karst is not conceivable in the area recent works suggested that the collapses are set off from reactivated paleokarsts the paleokarsts studied in the area proved to be the result of a particular weathering of the limestone the organization of these paleokarsts seems very low and mainly guided by the limestone fracturing as for most induced sinkholes the reactivation of these paleokarsts is linked to the lowering of piezometric heads in most of the area a thick cover and intensive land use mask potential surface hints of the buried paleokarsts and of the fracturing of the bedrock aerial photographs and remote sensing techniques have therefore shown little results in delineating collapse hazard zones up to now the study of the surficial morphology is also of little help in order to draw up the geohazard map in such a difficult context hydrogeological data and geological mapping information could only be used these informations are based on a limited number of boreholes and piezometers and are thus only valid on a regional scale records of former collapses were also available these records were of great interest since sinkhole distribution is obviously clustered in the area bedrock roof and cover formation floor altitudes were digitized and adapted to produce digital thematic maps piezometric heads were imported from a calibrated groundwater model of the aquifer these data and a digital elevation model of the area were integrated into a geographical information system gis to produce a coherent 3 d description of the area on a regional scale parameters such as the dewatering of the limestone and the thickness of the cover formation where sinkholes occurred were then estimated density of former collapses was also computed this showed that zones of high sinkhole occurrence coincide with zones of heavy lowering of piezometric heads combining the density of former collapses with the dewatering of the limestone enabled us to delineate zones of low moderate and high collapse hazard

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

lithologic control of the depth of earthquakes in southern california the depth distribution of southern california earthquakes indicates that areas underlain by schist basement rocks have a shallower 4 to 10 kilometers maximum depth of earthquakes than do areas with other types of basement rocks the predominant minerals in the schists become plastic at lower temperatures and thus at shallower depths than the minerals in the other basement rocks the lateral variations in lithology will control the depth extent and thus the magnitudes of potential future earthquakes these depths can be determined from the depth of the current background seismicity

high temperature shear zone formation in carrara marble the effect of loading conditions abstract rock deformation at depths in the earth s crust is often localized in high temperature shear zones occurring at different scales in a variety of lithologies the presence of material heterogeneities is known to trigger shear zone development but the mechanisms controlling initiation and evolution of localization are not fully understood to investigate the effect of loading conditions on shear zone nucleation along heterogeneities we performed torsion experiments under constant twist rate ctr and constant torque ct conditions in a paterson type deformation apparatus the sample assemblage consisted of cylindrical carrara marble specimens containing a thin plate of solnhofen limestone perpendicular to the cylinder s longitudinal axis under experimental conditions 900 c 400 mpa confining pressure samples were plastically deformed and limestone is about 9 times weaker than marble acting as a weak inclusion in a strong matrix ctr experiments were performed at maximum bulk shear strain rates of 2 10 4 s 1 yielding peak shear stresses of 20 mpa ct tests were conducted at shear stresses of 20 mpa resulting in bulk shear strain rates of 1 4 10 4 s 1 experiments were terminated at maximum bulk shear strains of 0 3 and 1 0 strain was localized within the carrara marble in front of the inclusion in an area of strongly deformed grains and intense grain size reduction locally evidences for coexisting brittle deformation are also observed regardless of the imposed loading conditions the local shear strain at the inclusion tip is up to 30 times higher than the strain in the adjacent host rock rapidly dropping to 5 times higher at larger distance from the inclusion at both bulk strains the evolution of microstructural and textural parameters is independent of loading conditions our results suggest that loading conditions do not significantly affect material heterogeneity induced strain localization during its nucleation and transient stages

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

crustal structure and seismicity distribution adjacent to the pacific and north america plate boundary in southern california new three dimensional 3 d v p and v p v s models are determined for southern california using p and s p travel times from local earthquakes and controlled sources these models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the pacific north america plate boundary the models extend from the u s mexico border in the south to the southernmost coast ranges and sierra nevada in the north and have a 15 km horizontal grid spacing and an average vertical grid spacing of 4 km down to 22 km depth the heterogeneity of the crustal structure as imaged by v p and v p v s models is larger within the pacific plate than the north american plate similarly the relocated seismicity deepens and shows more complex 3 d distribution in areas of the pacific plate exhibiting compressional tectonics the models reflect mapped changes in the lithology across major geological terranes such as the mojave desert the peninsular ranges and the transverse ranges the interface between the shallow mono of the continental borderland and the deep moho of onshore california forms a broad zone to the north beneath the western transverse ranges ventura basin and the los angeles basin and a narrow zone to the south along the peninsular ranges the near surface increase in velocity from the surface to up to 8 km depth is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins at midcrustal depths a rapid increase in v p is imaged beneath the sediments of the large sedimentary basins while beneath the adjacent mountain ranges the increase is small or absent

water weakening of clinopyroxenite in diffusion creep 1 we performed triaxial compressive creep tests to study water weakening of clinopyroxenite in the diffusion creep regime all tests were carried out on fine grained samples at confining pressures between 100 and 300 mpa at temperatures of 1321 to 1421 k for water saturated and 1398 to 1508 k for anhydrous conditions samples were prepared by hot pressing ground powder of sleaford bay clinopyroxenite water was added to the sample during the run by dehydration of a talc sleeve water saturated aggregates crept 30 times faster than aggregates under anhydrous conditions at a temperature of 1400 k and a pressure of 300 mpa the stress exponent in both cases was 1 and the activation energies under water saturated and anhydrous conditions were 340 30 and 760 20 kj mol respectively under water saturated conditions creep rate increased with increasing water fugacity to the power 1 4 0 2 with an activation volume for creep of 14 6 10 6 m3 mol comparison of the water weakening in aggregates of clinopyroxene olivine and anorthite deformed in the diffusion creep regime indicates that the creep strength of clinopyroxene has a stronger dependence on the water fugacity than do the creep strengths of olivine and anorthite in water rich environments clinopyroxene will be the weakest component in polycrystalline rocks composed of these phases

strain localization along the atlantis bank oceanic detachment fault system southwest indian ridge microstructural observations and mineral thermometry from in situ samples collected from the atlantis bank oceanic core complex sw indian ridge indicate that detachment faulting was initiated under hypersolidus conditions in the ductile regime and continued through subgreenschist temperatures through the ductile semibrittle and brittle regimes as strain localized along the exposed now subhorizontal fault surface ductile semibrittle and brittle fabrics are developed within dominantly gabbroic rocks footwall rocks exhibit crystal plastic fabrics distributed over a structural thickness up to 400 m below the denuded fault surface exposed at the seafloor whereas semibrittle and brittle fabrics are concentrated in the 80 and 30 m immediately below the principal slip surface of the detachment fault respectively sample fabrics suggest that strain localization was achieved by dynamic recrystallization of plagioclase at temperatures between 910 c and 650 c by amphibole accommodated dissolution precipitation creep at temperatures 750 c 450 c by chlorite accommodated reaction softening at temperatures 450 c 300 c and by brittle fracturing and cataclasis at temperatures 300 c

deformation induced polymorphic transformation experimental deformation of kyanite andalusite and sillimanite abstract torsion experiments were performed on the al 2 sio 5 polymorphs in the sillimanite stability field to determine basic rheological characteristics and the effect of deformation on polymorphic transformation the experiments resulted in extensive transformation of andalusite and kyanite to sillimanite no transformation occurred during the hot press no deformation stage of sample preparation which was carried out at similar p t conditions and duration as the torsion experiments experiments were conducted on fine grained 4 s to a maximum shear strain of 400 electron back scattered diffraction ebsd analysis of the experiments revealed development of lattice preferred orientations with alignment of sillimanite and andalusite 001 slightly oblique to the shear plane the kyanite experiment could not be analyzed using ebsd because of near complete transformation to sillimanite very little strain 30 is required to produce widespread transformation in kyanite and andalusite polymorphic transformation in andalusite and kyanite experiments occurred primarily along 500 m wide shear bands oriented slightly oblique and antithetic to the shear plane and dominated by sub m 100 150 nm fibrolitic sillimanite shear bands are observed across the entire strain field preserved in the torsion samples scanning transmission electron microscope imaging shows evidence for transformation away from shear bands e g fibrolitic rims on relict andalusite or kyanite relict grains typically have an asymmetry that is consistent with shear direction these experimental results show that sillimanite is by far the weakest of the polymorphs but no distinction can yet be made on the relative strengths of kyanite and andalusite these observations also suggest that attaining high bulk strain energy in strong materials such as the al 2 sio 5 polymorphs is not necessary for triggering transformation strain energy is concentrated along grain boundaries and transformation occurs by a dynamic recrystallization type process these experiments also illustrate the importance of grain size sensitive creep at high strains in a system with simultaneous reaction and deformation

deformation mechanisms and phase distribution in mafic high temperature mylonites from the jotun nappe southern norway abstract shear zones and mylonites in anorthositic to gabbroic rocks from the jotun nappe in southern norway have been studied as an example of high temperature deformation in polyphase rocks the deformation has taken place at approximately 700 c and maximum confining pressures of 900 mpa the constituent phases amphibole pyroxene and plagioclase have recrystallized synkinematically reducing the grain size of the rock to about 20 to 70 m two different microstructures develop depending on the recrystallization mechanism dynamic recrystallization of plagioclase hornblende and pyroxenes produces monophase layers which extend from porphyroclasts heterogeneous nucleation of phases due to chemical disequilibrium produces phase mixtures of hornblende and plagioclase or clinopyroxene and plagioclase these phase mixtures extend from hornblende or clinopyroxene porphyroclasts the product of the deformation and simultaneous recrystallization is a well laminated microstructure of some monophase hornblende pyroxene and plagioclase layers which alternate with abundant layers of phase mixtures different deformation mechanisms operate in the different types of microstructures dislocation substructures dynamic recrystallization microstructures and a crystallographic preferred orientation measured by ji and mainprice 1988 on samples of the same shear zone indicate crystal plastic deformation in monophase plagioclase layers dynamic recrystallization in monophase hornblende layers indicates also crystal plastic deformation the phase distribution of plagioclase and hornblende grain shapes and lower dislocation densities in the phase mixtures suggest that the deformation mechanism is not crystal plasticity but rather a granular flow grain boundary sliding probably accommodated by diffusional mass transfer in the plagioclase hornblende mixtures the phases show an anticlustered non random distribution grain boundary sliding may contribute to phase mixing but cannot be the main reason for the anticlustering of phases and the complete phase mixing directly at the boundaries of the porphyroclasts the reason for the anticlustered distribution is not clear but could be explained by preferential heterogeneous nucleation of hornblende at plagioclase grain boundaries

grain damage hysteresis and plate tectonic states abstract shear localization in the lithosphere is an essential ingredient for understanding how and why plate tectonics is generated from mantle convection on terrestrial planets the theoretical model for grain damage and pinning in two phase polycrystalline rocks provides a frame work for understanding lithospheric shear weakening and plate generation and is consistent with laboratory and field observations of mylonites grain size evolves through the competition between coarsening which drives grain growth and damage which drives grain reduction the interface between crystalline phases controls zener pinning which impedes grain growth damage to the interface enhances the zener pinning effect which then reduces grain size forcing the rheology into the grain size dependent diffusion creep regime this process thus allows damage and rheological weakening to co exist providing a necessary positive self weakening feedback moreover because pinning inhibits grain growth it promotes shear zone longevity and plate boundary inheritance however the suppression of interface damage at low interface curvature wherein inter grain mixing is inefficient and other energy sinks of deformational work are potentially more facile causes a hysteresis effect in which three possible equilibrium grain sizes for a given stress coexist 1 a stable large grain weakly deforming state 2 a stable small grain rapidly deforming state analogous to ultramylonites and 3 an unstable intermediate grain size state perhaps comparable to protomylonites a comparison of the model to field data suggests that shear localized zones of small grain mylonites and ultra mylonites exist at a lower stress than the co existing large grain porphyroclasts rather than as predicted by paleopiezometers or paleowattmeters at a much higher stress this interpretation of field data thus allows localization to relieve instead of accumulate stress the model also predicts that a lithosphere that deforms at a given stress can acquire two stable deformation regimes indicative of plate like flows i e it permits the coexistence of both slowly deforming plate interiors and rapidly deforming plate boundaries earth seems to exist squarely inside the hysteresis loop and thus can have coexisting deformation states while venus appears to straddle the end of the loop where only the weakly deforming branch exists

creep and strain dependent microstructures of synthetic anorthite diopside aggregates abstract we investigated plastic deformation of fine grained synthetic diopside and two phase anorthite diopside aggregates in triaxial compression and torsion up to 5 temperature confining pressure and stress ranged between 950 1180 c 200 400 mpa and 5 500 mpa respectively water content of samples ranged between 0 005 0 002 and 0 075 0 025 wt h 2 o all samples deformed in linear viscous creep with a stress exponent of 1 0 0 2 the activation energy ranged between 571 53 and 290 28 kj mol depending on mineralogy and water content sample strength depended on water fugacity with an exponent of 1 55 0 25 samples deformed in torsion and coaxial compression gave similar flow laws in spite of significant differences in the corresponding microstructures scanning and transmission electron microscopy of two phase samples deformed in torsion showed phase mixing cavitation and dislocation processes we suggest that linear viscous creep of fine grained two phase aggregates involved grain boundary sliding accommodated by grain boundary diffusion and significant dislocation accommodation at high strains we also observed that cavity coalescence and microcracking led to sample failure hence dynamic instabilities may exist in high strain shear zones accommodating viscous deformation in the lower continental crust

a cavitation seal mechanism for ultramylonite formation in quartzofeldspathic rocks within the semi brittle field vivero fault nw spain abstract bands of multi phase mixtures with an extreme grain size reduction called ultramylonites are a common feature in shear zones from the middle crust to upper mantle conditions indeed they are one of the most typical manifestations of strain localization in high strained polymineralic rocks in the lithosphere especially in the semi brittle field yet many questions remain on their origin and existing models either do not explain all the observations or require the introduction of physical processes that are difficult to prove particularly for the inter grain mixing of phases here we present a case study of ultramylonite bands developed in a coarse grained granite deformed at conditions near the base of the seismogenic zone i e showing coseismic deformation microstructures overprinted by thermally activated ones we found that in the mylonite to ultramylonite transition two deformation mechanisms coexist promoting inter grain phase mixing through kfs precipitation 1 dislocation accommodated grain boundary sliding in quartz and 2 diffusion accommodated grain boundary sliding in feldspar the model proposed to explain grain mixing is based on a cavitation seal mechanism which is strongly dependent on grain boundary sliding for the opening of transient micro cavities and on the diffusivity of potassium feldspar for their progressive sealing

crystal plasticity and superplasticity in quartzite a natural example plastically deformed quartzites from the betic movement zone betic cordilleras spain exhibit microstructures indicative of crystal plasticity on a mineral grain scale quartzites with dynamically recrystallized grain sizes larger than 10 m have strong crystallographic preferred orientations narrow grain boundaries little creep damage and an inverse proportionality of dislocation density and grain size mylonites with grain sizes smaller than 10 m have low crystallographic preferred orientations wide grain boundaries up to 1000 a abundant creep damage and decreasing dislocation density with diminishing grain size this is thought to reflect a clear cut shift in deformational regimes from dislocation creep to superplastic flow at 10 m grain size superplasticity can be acquired by quartzites which suffer dynamic recrystallization to grain sizes smaller than 10 m during an initial dislocation creep stage dislocation motion is the major accomodating mechanism for strain incompatibilities that arise during grain boundary sliding in the mylonites it seems reasonable to estimate flow stresses from unbound dislocation densities and dynamically recrystallized grain sizes in the tectonite specimens in the mylonites dynamically recrystallized grain size probably reflects the stress magnitude before the shift in deformational mechanisms and an estimate for late stage stresses is provided by unbound dislocation densities in both deformational regimes the flow strength appears to depend on the extent of dynamic recrystallization

strain localization during high temperature creep of marble the effect of inclusions abstract the deformation of rocks in the earth s middle and lower crust is often localized in ductile shear zones to better understand the initiation and propagation of high temperature shear zones induced by the presence of structural and material heterogeneities we performed deformation experiments in the dislocation creep regime on carrara marble samples containing weak limestone or strong novaculite second phase inclusions the samples were mostly deformed in torsion at a bulk shear strain rate of 1 9 10 4 s 1 to bulk shear strains between 0 02 and 2 9 using a paterson type gas deformation apparatus at 900 c temperature and 400 mpa confining pressure at low strain twisted specimens with weak inclusions show minor strain hardening that is replaced by strain weakening at 0 1 0 2 peak shear stress at the imposed conditions is about 20 mpa which is 8 lower than the strength of intact samples strain progressively localized within the matrix with increasing bulk strain but decayed rapidly with increasing distance from the inclusion tip microstructural analysis shows twinning and recrystallization within this process zone with a strong crystallographic preferred orientation dominated by r and c slip in recrystallization induced weakening starts at local shear strain of about 1 in the process zone corresponding to a bulk shear strain of about 0 1 in contrast torsion of a sample containing strong inclusions deformed at similar stress as inclusion free samples but do not show localization the experiments demonstrate that the presence of weak heterogeneities initiates localized creep at local stress concentrations around the inclusion tips recrystallization induced grain size reduction may only locally promote grain boundary diffusion creep accordingly the bulk strength of the twisted aggregate is close to or slightly below the lower isostress strength bound determined from the flow strength and volume fraction of matrix and inclusions

quantitative analysis of trace oh in garnet and pyroxenes to calibrate infrared ir spectroscopy for quantitative analysis of trace structural oh in specific minerals we have determined concentrations of h in pure separates of mantle derived pyrope garnet 56 6 ppm h 2o by weight augite 268 8 ppm h 2o and enstatite 217 11 ppm h 2o by manometry after heating the samples and extracting h 2 gas under vacuum ir spectroscopy confirmed the presence of intrinsic oh in these samples prior to extraction and indicated between 86 and 100 removal of h during the extraction procedure the integral specific absorption coefficients of 1 39 0 14 l 7 09 0 32 and 15 6 0 94 ppm h 2o cm 2 for pyrope augite and enstatite respectively allow precise spectroscopic determination of the oh content of upper mantle garnets and pyroxenes to concentration levels of a few parts per million uncertainties in accuracy depend on mineral composition and characteristics of the oh absorption spectrum and are estimated to range between 10 and 50

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

effects of chemical environment on dislocation creep of quartzite the water related chemical parameter that affects dislocation creep in quartzite has been determined from variations in sample strength and microstructure with chemical environment in buffered deformation and hydrostatic annealing experiments samples were weld sealed in double capsules and were buffered using solid oxygen buffers agcl or co2 black hills quartzite was deformed at 900 c and 1 5 10 5s 1 two samples were deformed at 1700 mpa confining pressure at constant and with and varying over 8 and 15 orders of magnitude respectively both samples deformed by climb accommodated dislocation creep with flow stresses of 300 mpa two additional samples were deformed at 700 mpa at constant lower than for the 1700 mpa samples with varying over 2 orders of magnitude both samples faulted with a peak strength of 800 mpa these four experiments suggest no dependence of dislocation creep strength on or instead a strong dependence of strength on is inferred previously deformed samples of heavitree quartzite were hydrostatically annealed for 4 days at 800 c and 1200 or 500 mpa confining pressure varying and over 2 5 and 1 order of magnitude respectively the microstructures of these samples show increased rates of dislocation climb and grain boundary migration with increasing but no dependence on these buffered experiments indicate that dislocation creep is affected by alone and suggest that the exponent for the term in the power law creep flow law is 2

the effect of pre tectonic reaction and annealing extent on behaviour during subsequent deformation insights from paired shear zones in the lower crust of fiordland new zealand granulite facies pargasite orthogneiss is partially to completely reacted to garnet granulite either side of narrow 20 mm felsic dykes in fiordland new zealand forming 10 80 mm wide garnet reaction zones the metamorphic reaction changed the abundance of minerals and their shape and grain size distribution the extent of reaction and annealing temperature related coarsening and nucleation is greatest close to the dykes whereas further away the reaction is incomplete as a consequence grain size and the abundance of garnet decreases away from the felsic dykes over a few centimetres the aspect ratios of clusters of s1 pyroxene and pargasite in the orthogneiss which are variably reacted to post s1 garnet decrease from high in the host to near equidimensional close to the dyke post reaction deformation localized in the fine grained partially reacted areas this produced a pattern of paired shear zones located at the outer parts of the garnet reaction zone our study shows that grain size sensitive deformation occurs where the grain size is sufficiently reduced by metamorphic reaction the weakening of the rock due to the change in grain size distribution outweighs the addition of nominally stronger garnet to the assemblage

deformation mechanism maps for feldspar rocks abstract deformation mechanism maps for feldspar rocks were constructed based on recently published constitutive laws for dislocation and grain boundary diffusion creep of wet and dry plagioclase aggregates the maps display constant temperature contours in stress grain size space for strain rates ranging from 10 16 to 10 12 s 1 two fields of dominance of grain boundary diffusion controlled creep and dislocation creep are separated by a strongly grain size sensitive transition zone for wet rocks diffusion controlled creep dominates below a grain size of about 0 1 1 mm depending on temperature stress strain rate and feldspar composition plagioclase aggregates containing up to 0 3 wt water as often found in natural feldspars are more than 2 orders of magnitude weaker than dry rocks the strength of water bearing feldspar rocks is moderately dependent on composition and water fugacity for a grain size range of about 10 50 m commonly observed in natural ultramylonites the deformation maps predict that diffusion controlled creep is dominant at greenschist to granulite facies conditions low viscosity estimates of 10 18 10 19 pa s from modeling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures 900 c or at lower temperatures diffusion creep of fine grained rocks possibly localized in abundant high strain shear zones for similar thermodynamic conditions and grain size lower crustal rocks are predicted to be less than order of magnitude weaker than upper mantle rocks

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

grain growth in synthetic marbles with added mica and water evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder through primary recrystallization and normal grain growth to a size stabilized by second phases to form the marbles reagent grade caco3 was mixed with 0 1 and 5 volume mica and heat treated under pressure with added water densification with negligible recrystallization occurred within one hour at 500 c and 500 mpa confining pressure primary recrystallization occurred at 500 550 c causing increases of grain size of factors of 2 5 resulting samples had uniform grain size gently curved grain boundaries and near equilibrium triple junctions they were used subsequently for normal grain growth studies normal grain growth occurred above 550 c at 800 c grain size d increased from 7 m d0 to 65 m in 24 hours growth rates fit the equation d n d 0 n kt where k is a constant and n 2 6 minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size dmax which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0 3 and 1 once dmax is reached normal growth continues only if second phases are mobile or coarsen or if new driving forces are introduced that cause unpinning of boundaries normal grain growth in solnhofen limestone was significantly slower than in pure synthetic marble suggesting that migration is also inhibited by second phases in the limestone

postseismic relaxation across the central nevada seismic belt 1 two gps geodetic surveys across the basin and range br province of western north america have detected an anomalous compression east of the central nevada seismic belt cnsb the first network installed by the u s geological survey along an east west trending transect across the br at approximately 40 n consisted of 63 monuments observed campaign style in 1992 1996 and 1998 the second network installed by the california institute of technology consisted of 50 continuously operating gps receivers 18 of which were roughly along the same transect as the campaign survey we have used a viscoelastic postseismic relaxation model of the four largest earthquakes in the cnsb on the pleasant valley fault in 1915 the cedar mountain fault in 1932 and on the fairview peak and dixie valley faults in 1954 to interpret the geodetic velocities observed across the cnsb a model of postseismic relaxation with a lower crustal maxwell viscosity of 5 50 1018 pa s can explain the apparent compression regardless of the assumed mantle viscosity the geodetic velocities that we determined after removing the postseismic contribution indicate that within current gps detection limits the br is stable east of the cnsb and is undergoing rapid right lateral shear west of the cnsb

transformation weakening diffusion creep in eclogites as a result of interaction of mineral reactions and deformation abstract the deformation of eclogites and the driving forces for their fabric development are an important topic potentially allowing to determine deformation rates and stresses in subduction zones where the greatest number of large earthquakes occurs here fabric studies of grain size and shape texture and chemical composition from two locations of variscan and alpine eclogites are presented all samples show a well developed crystallographic preferred orientation cpo of omphacite with a strong maximum of 001 in the lineation direction and a weaker maximum of poles to 010 normal to foliation garnet shows no systematic cpo anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde in terms of pressure change p t path the individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal misorientations chemical microstructural and cpo data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures the dominant deformation is inferred to be diffusion creep where dissolution of material took place in reacting mafic phases plagioclase pyroxene and precipitation took place in the form of new eclogite facies minerals omphacite garnet zoisite this type of diffusion creep deformation represents a transformation process involving both deformation and metamorphic reactions it is emphasized that the weakening is directly connected to the transformation and therefore transient the weakening facilitates diffusion creep deformation of otherwise strong minerals pyroxene garnet zoisite at far lower stresses than dislocation creep the results imply low stresses during the deformation of eclogite blocks in subduction zones these results can be applied to other rock types too

experimental investigation of the brittle viscous transition in mafic rocks interplay between fracturing reaction and viscous deformation rock deformation experiments are performed on fault gouge fabricated from maryland diabase rock powder to investigate the transition from dominant brittle to dominant viscous behaviour at the imposed strain rates of view the mathml source 3 10 5 3 10 6 s 1 the transition is observed in the temperature range of 600 c t 800 c at confining pressures of 0 5 gpa pc 1 5 gpa the transition thereby takes place by a switch from brittle fracturing and cataclastic flow to viscous dissolution precipitation creep and grain boundary sliding mineral reactions and resulting grain size refinement by nucleation are observed to be critical processes for the switch to viscous deformation i e grain size sensitive creep in the transitional regime the mechanical response of the sample is a mixed mode between brittle and viscous rheology and microstructures associated with both brittle and viscous deformation are observed as grain size reduction by reaction and nucleation is a time dependent process the brittle viscous transition is not only a function of t but to a large extent also of microstructural evolution

texture and elastic anisotropy of a mylonitic anorthosite from the morin shear zone quebec canada a sample of anorthosite from the granulite facies morin shear zone quebec canada was investigated for crystal preferred orientation and elastic anisotropy time of flight neutron diffraction data obtained with the hippo diffractometer at lansce were analyzed with the rietveld method to obtain orientation distribution functions of the principal phases plagioclase clinopyroxene and orthopyroxene texture and microstructures are compatible with the plastic deformation of the aggregate under high t conditions all mineral phases depict a significant preferred orientation that could be related to the general top to the north shearing history of the morin shear zone texture patterns suggest that 010 001 in plagioclase and 110 001 in clinopyroxene are likely dominant slip systems using preferred orientation data p and s waves velocities and elastic anisotropy were calculated and compared with previous studies to explore elastic properties of rocks with different pyroxene plagioclase mixtures p wave velocity s wave splitting and anisotropy increase with clinopyroxene content seismic anisotropy is linked to the texture symmetry which can lead to large deviations between actual anisotropy and that measured along cartesian xyz sample directions lineation foliation reference frame this is significant for the prediction and interpretation of seismic data particularly for monoclinic or triclinic texture symmetries

the role of deformation reaction interactions to localize strain in polymineralic rocks insights from experimentally deformed plagioclase pyroxene assemblages abstract in order to study the mutual effect of deformation and mineral reactions we have conducted shear experiments on fine grained plagioclase pyroxene assemblages in a griggs type solid medium deformation apparatus experiments were performed at a constant shear strain rate of 10 5 s 1 a confining pressure of 1 gpa and temperatures of 800 850 and 900 c while the peak stress of plagioclase orthopyroxene assemblages reaches values between those of the end member phases the strength of polymineralic materials strongly decreases after peak stress and reaches flow stresses that stabilize far below those of the weaker phase plagioclase this weakening correlates with the coeval development of high strain shear zones where new phases are preferentially produced including new pyroxene plagioclase and amphibole the reaction products mostly occur as intimately mixed phases within fine grained and interconnected shear bands together with different compositions with respect to the starting material this indicates that deformation significantly enhances the kinetics of mineral reactions which in turn strongly weaken the deforming sample here attributed to a switch to grain size sensitive diffusion creep through phase nucleation and grain size reduction such an interplay between deformation and mineral reactions may have strong implications for the initiation development and durability of shear zones in the lower crust

rock strength and texture evolution during deformation in the earth s ductile lithosphere a two phase thermodynamics model building upon the two phase and grain damage theory we propose a new formulation allowing to track the evolution of phase mixing segregation during ductile deformation of a two phased aggregate our model is based on a set of variables characterizing a rock texture the mean grain sizes and the phase proportion during ductile deformation activation of different micro mechanical processes impacts the aggregate texture dislocation and diffusion creep are the two main deformation processes considered we only account for the effect of zener pinning in slowing down grain growth and allow for active grain size reduction mechanisms in the diffusion creep domain for this purpose an equation is proposed to track the phase mixing evolution during ductile deformation numerical application using anorthite rheology shows that any grain reduction mechanisms that could be active in the diffusion creep regime requires a very high partition fraction in order to reach the grain size size predicted by the feldspar piezometer application of this model to gabbroic composition relevant for the ductile crust demonstrates that the strong coupling between phases grain sizes and interface evolution results in steady state grain sizes far below the field boundary this effect is coeval with an important increase of mixing between the two phases in addition accounting for the phase mixing results in a drop of the global aggregate stress during deformation this model allows for further comparison of mylonitized textures evolution with natural shear zones at the local and regional scales

lahars at merapi volcano central java an overview abstract merapi volcano in central java is one of the most active volcanoes in the world at least 23 of the 61 reported eruptions since the mid 1500s have produced source deposits for lahars the combined lahar deposits cover about 286 km 2 on the flanks and the surrounding piedmonts of the volcano at merapi lahars are commonly rain triggered by rainfalls having an average intensity of about 40 mm in 2 h most occur during the rainy season from november to april and have average velocities of 5 7 m s at 1000 m in elevation a wide range of facies may be generated from a single flow which may transform downvalley from debris flow to hyperconcentrated streamflow because of the high frequency and magnitude of the lahar events lahar related hazards are high below about 450 600 m elevation in each of the 13 rivers which drain the volcano hazard zone maps for lahar were produced by pardyanto et al volcanic hazard map merapi volcano central java 1 100 000 geol surv of indonesia bandung ii 4 1978 and the japanese indonesian cooperation agency master plan for land conservation and volcanic debris control in the area of mt merapi jakarta 1980 but these maps are of a very small scale to meet modern zoning requirements more recently a few large scale maps 1 10 000 and 1 2000 scale and risk assessments have been completed for a few critical river systems

the evolution of the seismic aseismic transition during the earthquake cycle constraints from the time dependent depth distribution of aftershocks 1 using the example of the 1992 m 7 3 landers earthquake we show that in the aftermath of a large earthquake the depth extent of aftershocks shows an immediate deepening from pre earthquake levels followed by a time dependent postseismic shallowing we use these seismic data to constrain the change in the depth of the seismic aseismic transition with time throughout the earthquake cycle most studies of the seismic aseismic transition have focussed on the effects of temperature and or lithology on the transition either from brittle faulting to viscous flow or from unstable to stable sliding a strain rate dependent transient deepening of the brittle ductile transition following a major earthquake is predicted by geological and laboratory observations by analyzing the time dependent depth distributions of aftershocks we identify and quantify the temporal evolution of this transition in the example of the landers earthquake its depth changes by as much as 3 km over the course of 4 years

numerical study on stress heterogeneity development in two phase rocks with large rheological contrast and implications in the lower crust development of stress heterogeneity in two phase rocks was investigated via a finite element method at 1000 1200 k and 100 mpa two groups of rock models were considered anorthite diopside and anorthite clinopyroxene with a phase volume ratio of 1 1 in each group and different dislocation creep rates between phases 4 8 orders of magnitude our numerical results indicate that the stress inside the model can be several times higher than the differential stress applied to the model and stress will tend to concentrate in hard phase especially near the sharp boundaries with soft phase moreover large stress gradient in hard phase and nearly homogeneous stress in soft phase will lead to the initialization of localized dynamic recrystallization or fracture these numerical observations suggest that the rheological contrast between two phases plays a crucial role in stress heterogeneity rather than other factors such as grain size the boundary conditions or mesh density which may eventually accelerate development of stress heterogeneity in the lower crust our study provides new insights into the dynamic processes of grain size reduction in the lower crust which may cause the transformation from dislocation creep to diffusion creep and enable the weakened shear zones

time dependent distributed afterslip on and deep below the izmit earthquake rupture surface deformation transients measured with the global positioning system during the 87 days between the 17 august 1999 izmit earthquake and the 12 november 1999 duzce earthquake indicate rapidly decaying aseismic fault slip on and well below the coseismic rupture elastic model inversions for time dependent distributed fault slip using a network inversion filter approach show that afterslip was highest between and below the regions of maximum coseismic slip and propa gated downward to or even below the base of the crust maximum afterslip rates decayed from greater than 2 m yr immediately after the i zmit earthquake to about 1 2 m yr just prior to the duzce earthquake maximum afterslip occurred below the eastern karadere rupture segment and near the i zmit hypocenter afterslip in the upper 16 km decayed more rapidly than that below the seismogenic zone these observations are consistent with a phase of rapid aseismic fault slip concentrated near the base of the seismogenic zone continued loading from the rapid deep afterslip along the eastern rupture zone is a plausible mechanism that helped trigger the nearby mw 7 2 12 november duzce earthquake

rupture processes of the 1999 august 17 izmit and november 12 duzce turkey earthquakes summary we derive the rupture history of the 1999 august 17 izmit mw 7 4 and 1999 november 12 duzce mw 7 1 earthquakes in turkey from teleseismic body waves using broad band data of the global seismograph network aftershock locations and mapped surface breaks the centroid solutions indicate strike slip mechanisms for both events the izmit earthquake was characterized by rupture propagating predominantly eastwards it consisted of a main rupture lasting about 25 s followed within 1 min by two more events of mw 6 9 and mw 7 0 with the teleseismic data we could not resolve the westward extent of rupture into the marmara sea however an upper bound of the seismic moment release west of the epicentre of the izmit event is estimated to be 1 9 1019 n m the duzce earthquake lasted about 14 s and was characterized by a bilateral mode of rupture in excellent agreement with mapped surface breaks and aftershock locations

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

a survey of hydrous species and concentrations in igneous feldspars the hydrous components in 85 feldspars from various igneous environments spanning the range of naturally occurring compositions were examined with infrared spectroscopy the feldspars contain structural oh 0 512 ppm h 2o h 2o 0 1350 ppm h 2o and nh 4 0 1500 ppm nh 4 groups as well as fluid inclusions and alteration products although composition and structure do influence the type of hydrous species that can be incorporated into a particular feldspar mineral the concentration of these species is not controlled by major element composition coarse perthitic microclines have a heterogeneous distribution of hydrous species and contain h 2o or nh 4 in k rich lamellae and fluid inclusions in na rich areas the structural oh in plagioclase feldspars is not associated with twin boundaries or exsolution lamellae all of the possible structural hydrous species are found in pegmatite feldspars whereas volcanic feldspars contain only structural oh the variation in oh concentration within a given feldspar composition suggests that fluids in the geologic environment play a role in determining the hydrogen concentration of each sample the vast majority of plutonic feldspars have undergone partial or total equilibration with meteoric fluids during low temperature 400 150 c hydrothermal exchange obliterating any structural hydrogen and creating substantial concentrations of fluid inclusions up to 4000 ppm h 2o in the exchanged regions the amount of water stored as fluid inclusions within feldspars in the upper crust 1 10 19 kg is small compared to the 1 35 10 21 kg of water in the oceans but is roughly equivalent to the reservoir of water stored in hydrous minerals in the upper crust

postseismic deformation following the 1906 san francisco earthquake we have reevaluated triangulation data from northern california following the 1906 san francisco earthquake thereby increasing the temporal and spatial resolution of postseismic deformation following that event we have calculated uniform shear strain rates and average station velocities at point arena using data from 1906 1907 1929 1930 and 1973 1975 and for the point reyes petaluma arc using data from 1929 1930 1938 1939 and 1960 1961 with the addition of recent geodetic data we infer an effective relaxation time for long term postseismic deformation following the 1906 earthquake of 36 16 years the point arena data are satisfactorily fit with accelerated afterslip at depth along the san andreas fault plane for a 10 km deep 25 km wide afterslip zone the average slip rate between 1906 and 1930 is 11 2 cm yr between 1929 and 1975 it is 4 6 cm yr deformation in the point reyes petaluma arc is clearly asymmetric with respect to the trace of the san andreas fault especially between 1929 and 1939 after inverting for the range of acceptable accelerated deep afterslip and horizontal detachment models a detailed analysis using geologically reasonable geometries revealed that both model types have trouble explaining the spatial variations in the deformation field through time in particular accelerated deep afterslip models cannot reproduce the breadth of the observed deformation field to the northeast of the san andreas fault as a result more complex time varying deformation mechanisms are required to explain the point reyes petaluma arc observations

crustal deformation in great california earthquake cycles periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model a depth l h extending downward from the earth s surface at a transform boundary between uniform elastic lithospheric plates of thickness h is locked between earthquakes it slips an amount consistent with remote plate velocity vpl after each lapse of earthquake cycle time tcy lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress the plates are coupled at their base to a maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions compatible with plate velocity are transmitted to the surface plates the coupling is described approximately through a generalized elsasser model we argue that the model gives a more realistic physical description of tectonic loading including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 san andreas ruptures we fix vpl 35 mm yr tcy 160 years and l 9 11 km based on earthquake nucleation depths the geodetic data are then found to be described reasonably within the context of a model that is locally uniform along strike and symmetric about a single san andreas fault strand by lithosphere thickness h 20 30 km and elsasser relaxation time tr 10 16 years we therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust mantle transition zone and has an effective viscosity between about 2 1018 and 1019 pa s depending on the thickness assigned to the asthenospheric layer predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands

the 1999 izmit earthquake sequence in turkey seismological and tectonic aspects the izmit duzce earthquake sequence is the largest sequence recorded by modern digital networks in the history of turkish earthquakes the m w 7 4 izmit mainshock occurred at 29 967 e and 40 729 n and 13 km depth at 00 hr 01 min 38 7 sec utc on 17 august 1999 and was preceded by at least eight immediate foreshocks of magnitude m l 3 0 aftershock locations show that a large region has become seismically very energetic following the mainshock and that most of the aftershock zone developed quickly in the first 3 days the distribution of the aftershocks illuminates numerous seismotectonic structures associated with the various branches of the north anatolian fault naf in the aftershock zone the seismicity pattern defines a rupture plane that extends about 80 km east and 75 km west of the mainshock epicenter confirming bilateral rupture during the mainshock the western aftershock activity is almost entirely located offshore extending along the axis of the izmit bay we show that the westernmost extent of the mainshock rupture reached the offshore region just 10 km south of the princes islands the dominance of strike slip motion is one of the main characteristics of the izmit sequence a number of significant aftershock clusters occur off the inferred mainshock rupture plane the spatial and temporal occurrence of these clusters show that they are triggered by the complex redistribution of stresses and dynamic strains imposed by the dislocation of the mainshock the focal mechanisms of the events located in the western end of the rupture zone show both strike slip and normal faulting mechanisms we infer that the yalova cluster was triggered by the mainshock the dominance of the normal faulting mechanisms supports the notion that dynamic extensional strains played a major role in triggering this cluster four events located off the princes islands indicate similar source mechanisms characterized by right lateral strike slip motion parallel to the strike of the northern boundary fault nbf and local trend of aftershock distribution this observation leads to a first order seismotectonic evidence that the nature of the nbf which is known to be a transtensional feature at the surface is dominated by strike slip motion at depth the coexistence of strike slip and extensional seismotectonic features in the sea of marmara highlights the importance of strain partitioning amongst shallow transtentional structures accommodating north south extension and deeper structures associated with the naf accommodating east west right lateral shear manuscript received 22 september 2000

evidence of phase nucleation during olivine diffusion creep a new perspective for mantle strain localisation for the past decades grain size reduction leading to diffusion creep in olivine is believed to be a very important process for strain localisation in the lithospheric mantle however the mechanisms of grain size reduction in this regime are still poorly understood e g platt 2015 here we show new experimental results that document grain size reduction and material weakening during wet olivine diffusion creep while occurring for both mono phase and two phase aggregates grain size reduction is coeval with strain localisation and local phase mixing in olivine pyroxene aggregates based on evidence of fluid inclusions and cracks filled with a fine grained phase mixture we conclude that grain size reduces as a result of fluid assisted nucleation that takes place in the presence of an aqueous fluid during diffusion creep cavitation induced by grain boundary sliding creep cavitation can be inferred and may play a critical role for olivine grain size reduction amongst their implications for rock rheology in general our findings highlight a key process for strain localisation in the ductile uppermost mantle

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

the initiation of strain localisation in plagioclase rich rocks insights from detailed microstructural analyses abstract in order to shed light on the cause for onset of strain localisation in plagioclase rich rocks we have performed detailed microstructural analyses on a sheared anorthosite leucogabbro using optical microscopy electron backscatter diffraction ebsd and chemical analyses the analysed sample is from an archaean unit sw greenland deformed at lower to mid crustal conditions t 675 700 c and moderate pressure the initial deformation occurred dominantly by dislocation creep and the grain size was reduced primarily by subgrain rotation recrystallisation recrystallised plagioclase grains average size 80 m are dominantly found in i clusters ii lenses and iii continuous bands subparallel to shear zone boundaries recrystallised grains in clusters and lenses display inherited crystallographic orientations their bulk crystallographic preferred orientation cpo is random however crystallographic characteristics show that parent and daughter grains have the same misorientation axes and possibly the same active slip systems recrystallised grains in continuous bands show a cpo with a single dominant active slip system 001 aligned with the structural xyz framework for these parent and daughter grains misorientation axes are random and the dominant slip system is different grain rotations of recrystallised grains are traceable back to the orientation of the adjacent porphyroclast we infer that the cause for strain localisation is recrystallisation and development of a cpo in continuous recrystallised bands microstructures in combination with misorientation and slip system analyses indicate a possible change from dislocation creep in clusters and lenses to dislocation accommodated grain boundary sliding disgbs in continuous bands this inferred shift in dominant deformation mechanism would lower the strength of the shear zone

high temperature deformation in the neoproterozoic transpressional ribeira belt southeast brazil abstract the neoproterozoic ribeira belt is subdivided in two domains with contrasting tectonic characteristics the northern domain is dominated by shallowly dipping foliations and orogen normal thrust tectonics the southern domain is characterized by a 1000 km long network of anastomosing transcurrent shear zones parallel to the belt this contrast is interpreted as reflecting continent continent convergence that is almost orthogonal to the margins in the northern domain and significantly oblique in the southern domain the central transitional domain of the ribeira belt displays the northern termination of the transcurrent shear zone network the alem paraiba padua shear zone system appss the 250 km long alem paraiba padua system involves granulites facies mylonites deformed through transpression a detailed study of the microstructure and lattice preferred orientation lpo of the rock forming minerals in these granulite mylonites allow a better understanding of deformation mechanisms active at high temperature in the crust plagioclase crystals are plastically deformed they display curved twins and cleavages mechanical twins and evidence of dynamic recrystallization lpo of plagioclase is consistent with activation of the 010 100 and 010 001 slip systems lpo of orthopyroxene and amphibole indicates that these minerals have been deformed through dislocation creep with the activation of the 100 001 slip system quartz in granulite mylonite displays evidence of extensive growth through grain boundary migration the lpo of quartz is therefore the result of a static transformation of an initial syn kinematic lpo and cannot be straightforwardly interpreted in terms of deformation mechanisms active during mylonitization

elevated stresses and creep rates beneath the brittle ductile transition caused by seismic faulting in the upper crust 1 large faults in the brittle upper continental crust are envisaged to terminate near the brittle ductile transition zone at a depth that is a complex function of material properties temperature stress and strain rate here we investigate the effect of coseismic displacement along a fault in the upper crust on the adjacent middle and lower crust using a two dimensional numerical model of faulting within an elastic frictional viscous rheology results demonstrate that a major displacement event can cause a dramatic increase in differential stress and creep strain rates in the crust near and below the lower termination of the fault a transient deflection of the brittle ductile transition zone and a zone of elevated strain rates down to the base of the crust the perturbation decays over timescales consistent with postseismic relaxation the stresses and strain rates predicted in such models are in accord with those inferred from microstructural damage analysis of exhumed metamorphic rocks indicating short term deformation at decaying stress and strain rates

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

postseismic deformation associated with the 1992 m ω 7 3 landers earthquake southern california following the 1992 m 7 3 landers earthquake a linear array of 10 geodetic monuments at roughly 5 km spacing was established across the emerson fault segment of the landers rupture the array trends perpendicular to the local strike of the fault segment and extends about 30 km on either side of it the array was surveyed by global positioning system 0 034 0 048 0 381 1 27 1 88 2 60 and 3 42 years after the landers earthquake to measure both the spatial and temporal character of the postearthquake relaxation the temporal behavior is described roughly by a short term decay time 84 23 days exponential relaxation superimposed upon an apparently linear trend because the linear trend represents motions much more rapid than the observed preseismic motions we attribute that trend to a slower decay time greater than 5 years postseismic relaxation the curvature of which cannot be resolved in the short run 3 4 years of postseismic data about 100 mm of right lateral displacement and 50 mm of fault normal displacement accumulated across the geodetic array in the 3 4 year interval covered by the postseismic surveys those displacements are attributed to postseismic right lateral slip in the depth interval 10 to 30 km on the downward extension of the rupture trace the right lateral slip amounted to about 1 m directly beneath the geodetic array and the fault normal displacement is apparently primarily a consequence of the curvature of the rupture these conclusions are based upon dislocation models fit to the observed deformation however no dislocation model was found with rms residuals as small as the expected observational error

the influence of phase and grain size distribution on the dynamics of strain localization in polymineralic rocks abstract deformation microstructures of a quartzo feldspathic pegmatite deformed at mid crustal levels allow the study of the dynamics of strain localization in polymineralic rocks strain localization results from i difference in grain sizes between phases both original and obtained during fluid present reactions and ii initial compositional banding due to original difference in grain size stress concentrates in the initially finer grained phases resulting in their intense grain size reduction via subgrain rotation recrystallization sgr when the grain size is sufficiently reduced through either deformation or interphase coupled dissolution precipitation replacement of the coarse grained feldspar aggregates start to deform by dominantly diffusion accommodated grain boundary sliding gbs phase mixing inhibits grain growth and sustains a grain size allowing gbs consequently discontinuous microscale shear zones form locally within initially coarse grained areas at the same time difference in strain rate between feldspar rich and quartz rich domains needs to be accommodated at domain boundaries this results in the formation of continuous mesoscale shear zones deformed by gbs once these are formed deformation in the coarse grained parts is arrested and strain is mainly accommodated in the mesoscale shear zones resulting in superplastic behaviour consistent with diffusion creep

stress gradients around porphyroclasts palaeopiezometric estimates and numerical modelling abstract based on strain gradients commonly found around porphyroclasts the mechanical interaction of porphyroclasts and recrystallized mylonite matrix in a retrograde amphibolite facies shear zone of the ivrea verbano zone italy was investigated differential stresses inferred from recrystallized grain size and dislocation density indicate stress concentrations at the porphyroclast matrix interface of about 1 7 2 0 with respect to the average in the matrix the palaeopiezometric stress data are compared to the results of a finite element model composed of two rheologically different phases in this model the local distribution and concentration of stresses around porphyroclasts mostly depend on clast matrix coupling and the effective viscosity contrast of the two phases the bulk strength and work hardening of the composite system increases with interface coupling both the modified stress field and the reduced average grain size around porphyroclasts may result locally in instabilities and promote strain localization and a change in the dominant deformation mechanism

modeling mechanical layering effects on stability of underground openings in jointed sedimentary rocks abstract this paper examines the significance of mechanical layering for blocky rock mass deformation around underground openings excavated through sedimentary rocks the analysis is based on an integration of geologically based discrete fracture models geodfn which incorporate mechanical layering with the numerical discrete element method the discontinuous deformation analysis dda we begin with addressing limitations of classical solutions for mine roof stability in layered and jointed rock masses via the analysis of the free standing unsupported 2000 year old underground quarry known as zedekiah s cave below the old city of jerusalem israel we show that both the clamped beam model and the voussoir beam analogue fail to predict the observed roof stability only application of discrete element modeling which allows for interactions between multiple blocks in the rock mass can capture correctly the arching mechanism which takes place in the roof and which properly explains the long term stability of this underground opening we continue with examining the effect of joint trace geometries on blocky rock mass deformation using the hybrid geodfn dda approach we show that with increasing joint length and decreasing bridge length vertical deformations in the rock mass are enhanced we explain this by the greater number of distinct blocks in the rock mass due to the greater joint intersection probability in such geometries we find that rock bridge length is particularly important when considering the stability of the immediate roof with increasing rock bridge length the number of blocks in immediate roof decreases and consequently individual block width is increased increased block width in immediate roof layers enhances stable arching development thus improving their load carrying capacity and overall stability of the underground structure

semi brittle deformation within the alpine fault zone new zealand abstract this paper reports the results of an attempt to identify the deformation micromechanisms in the brittle ductile transition zone of the alpine fault new zealand characterization of sequentially developed microstructures was carried out using optical microscopy hvem and stem analysis it was found that the transition zone corresponds to a broad zone of retrogression produced initially by microcracking and fluid infiltration the non catastropic nature of the fracturing indicates that fluid enhanced sub critical cracking is a significant crustal deformation mechanism after the initial phase of retrogression both ductile and brittle processes coexist and cyclically alternate in response to stress cycling the relative importance of macroscopic brittle processes may decrease with time as more ductile minerals especially phyllosilicates develop during continued retrogression

deformation induced clinopyroxene fabrics from eclogites abstract clinopyroxene omphacite microfabric studies in eclogites are useful in characterizing the deformation that has affected deeper parts of subduction zones we present the microfabrics and tem structures of four samples representative of the various deformation types known in eclogites the 001 axis tends to be parallel to the lineation while the 010 plane is close to the foliation but these general features undergo strong variations in relation to the shape fabric in strongly lineated samples 010 tends to be spread out in a great circle normal to the lineation whereas 001 is dispersed in the foliation in the case of strongly foliated samples the slip systems identified by tem are 001 100 001 110 and 1 2 110 110 operation of dislocation creep is testified by the presence of dislocations and dislocation walls but this process alone cannot explain all the observed features particularly some aspects of the crystallographic preferred orientation hence other deformation mechanisms must be involved these may be dynamic migration recrystallization by grain boundary migration as well as mass transfer processes this study illustrates that not every crystallographic mineral preferred orientation can be explained by dislocation glide and or twinning processes alone in addition it also demonstrates that eclogite microfabrics can be used to unravel deformation parameters in subduction and or collision zones

rheology microstructure and crystallographic preferred orientation of matrix containing a dispersed second phase insight from experimentally deformed ice abstract we utilize in situ neutron diffraction to continuously track the average grain size and crystal preferred orientation cpo development in ice during uniaxial compression of two phase and pure ice samples two phase samples are composed of ice matrix and 20 vol of second phases of two types 1 rheologically soft platy graphite and 2 rigid rhomb shaped calcite the samples were tested at 10 c below the ice melting point ambient pressures and two strain rates 1 10 5 and 2 5 10 6 s 1 to 10 and 20 strain the final cpo in the ice matrix where second phases are present is significantly weaker and ice grain size is smaller than in an ice only sample the microstructural and rheological data point to dislocation creep as the dominant deformation regime the evolution and final strength of the cpo in ice depend on the efficiency of the recrystallization processes namely grain boundary migration and nucleation these processes are markedly influenced by the strength shape and grain size of the second phase in addition cpo development in ice is further accentuated by strain partitioning into the soft second phase and the transfer of stress onto the rigid second phase

near field postseismic deformation associated with the 1992 landers and 1999 hector mine california earthquakes after the landers earthquake m w 7 3 1992 489 a linear array of 10 monuments extending about 30 km n50 e on either side of the earthquake rupture plus a nearby off trend reference monument were surveyed frequently by gps until 2003 2 the array also spans the rupture of the subsequent hector mine earthquake m w 7 1 1999 792 the pre landers velocities of monuments in the array relative to interior north america were estimated from earlier trilateration and very long baseline interferometry measurements except at the reference monument the post landers velocities of the individual monuments in the array relaxed to their preseismic values within 4 years following the hector mine earthquake the velocities of the monuments relaxed to steady rates within 1 year those steady rates for the east components are about equal to the pre landers rates as is the steady rate for the north component of the one monument east of the hector mine rupture however the steady rates for the north components of the 10 monuments west of the rupture are systematically 10 mm yr 1 larger than the pre landers rates the relaxation to a steady rate is approximately exponential with decay times of 0 50 0 10 year following the landers earthquake and 0 32 0 18 year following the hector mine earthquake the postearthquake motions of the landers array following the landers earthquake are not well approximated by the viscoelastic coupling model of pollitz et al 2000 a similar viscoelastic coupling model pollitz et al 2001 is more successful in representing the deformation after the hector mine earthquake

abandoned stone quarries in and around the izmir city centre and their geo environmental impacts turkey there are 70 abandoned quarries in and around izmir city centre of these quarries 56 are in andesites 2 are in dasitic lavas 4 are in flysch and 8 are in limestones the quarries initially located near the izmir city centre are now within the city centre because of the expansion of the urbanization due to the rapid increase of population geo environmental problems identified in the abandoned quarry sites are slope stability problems such as sliding and toppling geotechnical study of the abandoned stone quarries in and around the city centre of izmir which has been carried out involves the recording and determining the properties of discontinuities for stability assessment of already failed and standing slopes in the quarries using stereographic projection techniques and the statistical evaluation of different types of rock mass movements in these quarries the number of mass movements that had already occurred and are likely to occur in 70 quarries lying in and near izmir have been determined as 84 totally stability analyses of rock slopes have shown that rock falls toppling plane and wedge failure type rock mass movements are more common in the south of izmir bay but toppling type slope failures are more frequent in the north

influence of geomorphological constraints and exploitation techniques on stone quarry spatial organisation abstract the lutetian limestones were used to build numerous historical monuments in the north east part of france in rheims laon and soissons areas more than 400 underground quarries have been inventoried by research in the regional archives on old maps and by several months of ground prospecting actually most of these underground quarries are abandoned only 3 are now open in the soissons area in the rheims region the last active quarry located at courville village was closed in 2005 because of the safety conditions the aim of the study was to demonstrate the importance of geomorphological context on the fracturing network and on the quarry spatial organisation underground topography fracturing network measurement aerial mapping and analysis of the geomorphological features show the high connection between natural constraints and extraction choices and may explain the progressive disappearance of building stone quarries in a part of the studied area in the east of the studied area where the usable limestone has a weak thickness and important mechanical distension stress the galleries direction depends on fracturing network in contrast in the west soissons area limestone shows thick homogeneous facies that allow a rational extraction using room and pillars mining system

viscoelastic coupling model for the earthquake cycle driven from below in a linear system the earthquake cycle can be represented as the sum of a solution which reproduces the earthquake cycle itself viscoelastic coupling model and a solution that provides the driving force we consider two cases one in which the earthquake cycle is driven by stresses transmitted along the schizosphere and a second in which the cycle is driven from below by stresses transmitted along the upper mantle i e the schizosphere and upper mantle respectively act as stress guides in the lithosphere in both cases the driving stress is attributed to steady motion of the stress guide and the upper crust is assumed to be elastic the surface deformation that accumulates during the interseismic interval depends solely upon the earthquake cycle solution viscoelastic coupling model not upon the driving source solution thus geodetic observations of interseismic deformation are insensitive to the source of the driving forces in a linear system in particular the suggestion of bourne et al 1998 that the deformation that accumulates across a transform fault system in the interseismic interval is a replica of the deformation that accumulates in the upper mantle during the same interval does not appear to be correct for linear systems

shape of pinch and swell structures as a viscosity indicator application to lower crustal polyphase rocks abstract pinch and swell structures occur where a more competent layer in a weaker matrix is subjected to layer parallel extension in this contribution we use numerical models to explore the use of pinch and swell structure shape symmetry and asymmetry as a determinant of relative viscosity between layers maximum asymmetry is attained when the matrix viscosity on one side is subtly weaker than the competent layer while the other side is significantly weaker our numerical results are directly applied to asymmetrically developed pinch and swell structures in exposed lower continental crust here shape geometries observed in a shear zone comprised of plagioclase dominated garnet dominated and mixed amphibole plagioclase dominated bands reveals that the plagioclase dominated band is the most competent band and is marginally stronger 2 and significantly stronger 10 40 than the fine grained garnet dominated and mixed amphibole plagioclase dominated band respectively based on the experimentally determined viscosity of a plagioclase dominated material and quantitative microstructural analysis the viscosity range of the natural rock bands is 2 8 1015 to 1 1 1017 pa s consequently the assumption that the experimentally derived plagioclase flow law is an appropriate proxy for the middle to lower continental crust may lead to a viscosity over estimation by up to forty times