frequency dependent polarization measurements of long period surface waves and their implications for global phase velocity maps abstract surface wave dispersion maps provide important constraints on global models of shear wave velocity structure current surface wave dispersion maps show significant differences from researcher to researcher and it is clear that further work is required in addition to dispersion data polarization measurements obtained from long period 100 s or more three component recordings from the various global networks can also be used to constrain dispersion maps the off great circle propagation of the surface wave packets is relatively easy to interpret within a ray theoretic framework and provides sensitivity to higher order structure the polarization angles as a function of frequency are readily measured using a multi taper technique which also has the benefit of providing an error estimate for the measurements application of the technique to three component seismograms from the global geoscope array reveals large deviations from great circle propagation up to 15 for low orbit love waves and 10 for rayleigh waves in the frequency band 5 12 5 mhz on a more regional scale an analysis of seismograms from the german regional seismic network grsn reveals even larger strongly frequency dependent deviations from great circle propagation in the frequency range 10 50 mhz

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

relative strength of mafic and felsic rocks during amphibolite facies metamorphism and deformation field observations of mafic dykes intruded into felsic rocks from the lewisian complex nw scotland suggest that the mafic rocks are weaker than the felsic ones contrary to experimental results in order to resolve this conflict samples were studied to constrain the deformation mechanisms active under amphibolite facies conditions crystallographic preferred orientation cpo in plagioclase is used to infer deformation by dislocation creep on the 112 11 0 11 2 110 001 11 0 and 001 11 0 slip systems with increasing strain this cpo became weaker due to grain boundary sliding that accompanied diffusion creep in rocks where grain size reduction of plagioclase occurred by chemically dominated recrystallisation there is no cpo suggesting deformation was wholly accommodated by diffusion creep and grain boundary sliding in metamorphosed dykes plagioclase grains have orientations that are not consistent with dislocation creep deformation amphibole has a cpo consistent with either dislocation creep on 100 or deformation by diffusion creep with anisotropic dissolution and precipitation rates it is inferred that metamorphism of the dykes lead to the production of fine grained amphibole and plagioclase both of which deform by diffusion creep but that anisotropic dissolution and precipitation in the amphibole produced a cpo the mafic dykes are weaker than the felsic gneisses because grain size reduction is more extreme in the dykes even though both may be deforming primarily by grain size sensitive mechanisms this work highlights the variation in processes active in polyphase rocks and how these can lead to variations in cpo of the same mineral among different rock types

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

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

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

a recrystallized grain size piezometer for experimentally deformed feldspar aggregates a recrystallized grain size piezometer for low temperature migration recrystallization in feldspar has been experimentally calibrated hot pressed samples of fine grained albitic feldspar grain size 1 10 m and a natural albitic feldspar aggregate 150 m were deformed in simple shear and axial compression at a temperature of 900 c and a confining pressure of 1500 mpa transmission electron microscopy tem verified that deformation occurred by recrystallization accommodated dislocation creep grain size distributions were measured from scanning electron microscopy sem photographs of polished and etched samples the results yield a recrystallized grain size piezometer relationship of d 55 5 0 66 0 07 where d is the geometric mean grain size m and the differential stress mpa the exponent is higher than that determined in previous studies of other materials for rotation recrystallization and high temperature migration recrystallization indicating that different recrystallization mechanisms have distinct recrystallized grain size piezometer relationships

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

short and long term effects of gps measured crustal deformation rates along the south central andes in this study we present the contemporary crustal deformation field along the central and southern andes 17 42 s estimated from four global positioning sys tem gps campaigns conducted in 1994 97 we find that the majority of the observed crustal deformation field is relatively homogenous roughly parallel to the plate conver gence direction with decreasing velocities away from the trench we attribute this type of deformation pattern to the inter seismic phase of an earthquake deformation cycle caused by 100 locking of the thrust interface between the subducting nazca and the overrid ing south american plates we have also detected a strong post seismic deformation sig nal in the vicinity of the 1995 mw8 0 antofagasta 22 26 s and 1960 mw9 5 chile 38 43 s earthquakes this type of deformation can be described as short term in nature compared to geological timescales the above conclusions are based on the results of the 3 d andean elastic dislocation model aedm by subtracting the aedm predicted deformation rates from the observations we obtained a residual velocity field that high lights the post seismic as well as more long term deformation eects for example we find a strong evidence for the continuing crustal shortening across the back arc reach ing its maximum 4 mm yr in the very north of our study area in addition between latitudes 29 34 s there is an indication of e w oriented extension within the fore arc in accordance with the recent geologic findings for the n s oriented normal faulting

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 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

natural deformation and recrystallization of some intermediate plagioclase feldspars abstract intermediate an25 an48 plagioclase feldspars from the nordre stromfjord shear zone central west greenland have undergone extensive ductile deformation by intracrystalline slip and by twinning the plagioclases were deformed under low granulite facies conditions optical substructures include undulatory extinction deformation bands subgrains and deformational twins mainly on the albite law dynamic recrystallization occurred primarily by subgrain rotation with bulge nucleation migration of a high angle boundary as a secondary mechanism in some cases recrystallization is accompanied by a small decrease in the an content which is thought to indicate prograde metamorphic conditions the plagioclases exhibit a good crystallographic preferred orientation which is compatible with 010 001 being the principal slip system quartz has in the same rocks undergone more plastic deformation and more extensive recovery than plagioclase this is at least in part due to the fact that most dislocations in plagioclase are dissociated

mantle flow beneath a continental strike slip fault postseismic deformation after the 1999 hector mine earthquake two recent large earthquakes in the mojave desert california the magnitude 7 3 1992 landers and magnitude 7 1 1999 hector mine earthquakes have each been followed by elevated crustal strain rates over periods of months and years geodetic data collected after the hector mine earthquake exhibit a temporally decaying horizontal velocity field and a quadrant uplift pattern opposite to that expected for localized shear beneath the earthquake rupture we interpret the origin of this accelerated crustal deformation to be vigorous flow in the upper mantle in response to the stress changes generated by the earthquake our results suggest that transient flow in the upper mantle is a fundamental component of the earthquake cycle and that the lower crust is a coherent stress guide coupling the upper crust with the upper mantle

crystallographic preferred orientations of plagioclase via grain boundary sliding in a lower crustal anorthositic ultramylonite to clarify the rheological properties and deformational mechanisms that operate within lower crustal shear zones we analyzed deformation microstructures and fabrics in plagioclase grains from high strain regions of intrafolial drag folds of an anorthositic ultramylonite from langoya vesteralen northern norway these grains have developed weak crystallographic preferred orientations cpos other than deformation twinning they do not show any indications of intracrystalline plasticity or dynamic recrystallization in some domains of the drag fold plagioclase cpos are more distinct but the majority of inferred slip systems are not consistent with the main slip systems in plagioclase these suggest that neither dislocation creep nor dislocation accommodated grain boundary sliding gbs produced the plagioclase cpos but rather grain rotation via gbs along specific grain boundaries parallel or subparallel to crystallographic planes of 010 100 or 110 which led to an alignment of the easy slip grain boundaries in the flow direction that produced the observed cpo patterns the anisotropy of dissolution and growth rates may have been responsible for the development of the specific grain boundaries

an experimental study of h solubility in feldspars effect of composition oxygen fugacity temperature and pressure and implications for crustal processes abstract a series of experiments with run durations of 100 240 h have been carried out on h solubility in natural labradorite albite and anorthoclase at 2 10 kbar and 700 850 c in equilibrium with different oxygen buffer materials fe feo ni nio re reo 2 and fe 2 o 3 fe 3 o 4 attainment of equilibrium water contents were examined by conducting experiments with different run durations and by performing profile analysis along crystal core to rim paths the annealed products generally show typical oh related infrared absorption bands in the range 3700 2600 cm 1 the spectra of the labradorite and anorthoclase are dominated by broad bands at 3300 3100 cm 1 while the spectra of the albite consist of broad bands at 3100 3000 cm 1 and sharp bands at 3600 3400 cm 1 the measured h solubility ranging from 80 to 285 ppm h 2 o differs greatly between the feldspars with different chemical compositions depends strongly on oxygen fugacity and increases with increasing temperature and pressure the h solubility is significantly higher e g by a factor of 3 or even more at very reducing conditions than at oxidizing conditions the high h solubility of feldspar at reduced conditions indicates that at least locally the earth s ancient deep continental crust was probably more hydrous than the modern one crustal dehydration can be caused by a change of oxygen fugacity and a corresponding change of water solubility in the constituent feldspar under isothermal conditions without the requirement of high temperature and or additional heat sources

delayed triggering of the 1999 hector mine earthquake by viscoelastic stress transfer stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1 2 3 4 5 the 1999 hector mine earthquake in southern california magnitude 7 1 occurred only 20 km from and 7 years after the 1992 landers earthquake magnitude 7 3 this suggests that the hector mine earthquake was triggered in some fashion by the earlier event but uncertainties in the slip distribution and rock friction properties associated with the landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the hector mine hypocentre with estimates varying from 1 4 bar ref 6 to 0 5 bar ref 7 more importantly coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events here we present the results of a three dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the landers earthquake using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the landers earthquake our calculations suggest that lower crustal or upper mantle flow can lead to postseismic stress increases of up to 1 2 bar at the location of the hector mine hypocentre during this time period contributing to the eventual occurrence of the 1999 hector mine earthquake these results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8 9 10 11

investigation of the strength contrast at the moho a case study from the oman ophiolite we investigate the viscosity contrast between the crust and mantle by analysis of deformed gabbronorite dikes in the mantle section of the oman ophiolite our study provides a rare opportunity to directly constrain the rheological contrast between a plagioclase rich lithology typical of the lower crust and an olivine dominated harzburgite we constrain deformation conditions using field relationships geothermometry grain size piezometry and electron backscatter diffraction the viscosity of gabbronorite was at least two orders of magnitude lower than the viscosity of the harzburgite consistent with the rheological contrast between plagioclase and olivine calculated via experimental flow laws these flow laws predict that a significant viscosity contrast exists at the crust mantle boundary where the crustal lithology is dominated by plagioclase or where deformation localizes within plagioclase rich layers

the competition between rates of deformation and solidification in syn kinematic granitic intrusions resolving the pegmatite paradox abstract while fully crystallized granites rich in feldspar generally serve to strengthen the continental crust their precursor melts are assumed to be important agents of crustal weakening many syn tectonic granitic pegmatites are deformed within shear zones but ubiquitously preserve undeformed primary magmatic textures implying that they were largely molten during shearing yet the shapes of pegmatite bodies indicate that they deformed with a greater competence than their surroundings this co located pair of material behaviours is paradoxical we interpret field relationships in a typical pegmatite shear zone association torrisdale nw scotland and propose a mechanism by which syn tectonic granitic melts may in effect act as competent bodies while not yet fully crystallized competence was rapidly increased by preferential crystallization on intrusion margins that served to encapsulate residual melt inside stiff rinds further crystallization may have been pulsed as the concentrations of crystallization inhibitors fluxes increased in residual fluids postulating the existence of initial stiff rinds also consistent with modern estimates for rates of feldspar crystallization cms yr from undercooled hydrous silicic magma to form pegmatites these greatly outpace strain rate estimates for shear zones thus fully liquid granitic melts may only be present fleetingly and have little opportunity to weaken deforming crust before crystallization begins

computational study of deformation mechanisms and grain size evolution in granulites implications for the rheology of the lower crust abstract most of granulite terrains worldwide are characterized by large mean grain sizes of 1 mm or more an important exception are the high pressure felsic granulites in the bohemian massif the european variscan belt there recrystallization of original coarse grained ternary feldspar led to formation of a fine grained 100 m mixed matrix dominated by plagioclase and k feldspar this change occurred at temperatures of 850 c and was probably caused by chemically induced decomposition related to slight cooling and enhanced by deformation during continental collision the resulting microstructure shows indications of diffusion creep assisted by melt enhanced grain boundary sliding further on minor coarsening occurred associated with deformation by dislocation creep and aggregation of mineral phases using a thermodynamics based model of grain size evolution we show that stability of the fine grained microstructure crucially depends on zener pinning in the two phase mineral matrix pinning efficiently hinders grain growth and the small grain size that resulted from the ternary feldspar decomposition can be stable even at high temperatures the late switch from the grain size sensitive creep to dislocation creep is rather difficult to explain by temperature and strain rate or stress changes only however a simple incorporation of melt solidification can successfully simulate this behavior alternatively the switch and the associated grain size growth can be related to mineral phase aggregation at lower pressure temperature conditions resulting into a decrease of pinning efficiency this study suggests that the fine grain size of the bohemian granulites in contrast to the common coarse grained type stems from abrupt recrystallization during the high pressure high temperature conditions and pinning in the fine grained matrix such a process may in some cases significantly and suddenly reduce the strength of the lower continental crust and allow for its efficient redistribution

viscoelastic flow in the lower crust after the 1992 landers california earthquake space geodesy showed that broad scale postseismic deformation occurred after the 1992 landers earthquake three dimensional modeling shows that afterslip can only explain one horizontal component of the postseismic deformation whereas viscoelastic flow can explain the horizontal and near vertical displacements the viscosity of a weak about 10 km thick layer in the lower crust beneath the rupture zone that controls the rebound is about 10 18 pascal seconds the viscoelastic behavior of the lower crust may help to explain the extensional structures observed in the basin and range province and it may be used for the analysis of earthquake hazard

formation of orogenic wedges and crustal shear zones by thermal softening associated topographic evolution and application to natural orogens abstract the model of an orogenic wedge has been applied to explain the tectonic evolution of many orogens worldwide orogenic wedges are characterized by 1 a first order shear zone which underthrusts the mantle lithosphere and lower crust beneath the adjacent mantle lithosphere and 2 a sequence of second order upper crustal shear zones which form tectonic nappes shear zone formation in the lithosphere is however incompletely understood we perform two dimensional thermo mechanical numerical simulations of lithospheric shortening to study shear zone formation propagation and associated wedge formation the only perturbation in the model lithosphere is a different temperature at the left 1300 c and right 1400 c half of the model bottom despite this smooth and weak perturbation simulations show self consistent and spontaneous formation of first and second order shear zones generating an orogenic wedge the shear zones are caused by thermal softening and temperature dependent rock viscosity lateral spacing of upper crustal shear zones spans between 30 and 50 km and is controlled by the depth of the boundary between upper and lower crust which acts as mechanical detachment level modelled upper crustal shear zones are active for 1 to 4 my surface processes such as sedimentation and erosion influence shear zone orientation spacing and duration but do not impact fundamental processes of shear zone formation and propagation simulations produce both singly vergent and doubly vergent wedges topographic uplift rates are controlled by the applied bulk shortening rate the modelled surface uplift and subsidence associated with crustal shear zones could explain major consecutive thrusting events and related sedimentation within flexural basins during the formation of the helvetic nappe system in western switzerland furthermore results for shear zone propagation along a mid crustal detachment and associated uplift of crustal basement could potentially explain foreland basement cored uplifts in natural orogens such as the laramide orogen taiwan or the shillong plateau

earthquake cycle simulations with rate and state friction and power law viscoelasticity abstract we simulate earthquake cycles with rate and state fault friction and off fault power law viscoelasticity for the classic 2d antiplane shear problem of a vertical strike slip plate boundary fault we investigate the interaction between fault slip and bulk viscous flow with experimentally based flow laws for quartz diorite and olivine for the crust and mantle respectively simulations using three linear geotherms dt dz 20 25 and 30 k km produce different deformation styles at depth ranging from significant interseismic fault creep to purely bulk viscous flow however they have almost identical earthquake recurrence interval nucleation depth and down dip coseismic slip limit despite these similarities variations in the predicted surface deformation might permit discrimination of the deformation mechanism using geodetic observations additionally in the 25 and 30 k km simulations the crust drags the mantle the 20 k km simulation also predicts this except within 10 km of the fault where the reverse occurs however basal tractions play a minor role in the overall force balance of the lithosphere at least for the flow laws used in our study therefore the depth integrated stress on the fault is balanced primarily by shear stress on vertical fault parallel planes because strain rates are higher directly below the fault than far from it stresses are also higher thus the upper crust far from the fault bears a substantial part of the tectonic load resulting in unrealistically high stresses in the real earth this might lead to distributed plastic deformation or formation of subparallel faults alternatively fault pore pressures in excess of hydrostatic and or weakening mechanisms such as grain size reduction and thermo mechanical coupling could lower the strength of the ductile fault root in the lower crust and concomitantly off fault upper crustal stresses

microstructural evolution and rheology of quartz in a mid crustal shear zone abstract we present microstructural and crystallographic preferred orientation cpo data on quartz deformed in the middle crust to explore the interaction and feedback between dynamic recrystallization deformation processes and cpo evolution the sample investigated here is a moderately deformed quartz rich mylonite from the blue ridge in virginia we have created high resolution crystallographic orientation maps using electron backscatter diffraction ebsd of 51 isolated quartz porphyroclasts with recrystallized grain fractions ranging from 10 to 100 recrystallized grains are internally undeformed and display crystallographic orientations dispersed around the orientation of the associated parent porphyroclast we document a systematic decrease in fabric intensity with recrystallization suggesting that progressive deformation of the recrystallized domains involves processes that can weaken a pre existing cpo relationships between recrystallization fraction and shear strain suggest that complete microstructural re equilibration requires strains in excess of 5 variation in the degree of recrystallization implies that strain was accumulated heterogeneously and that a steady state microstructure and rheology were not achieved

ultra high temperature and subsolidus shear zones examples from the poe mountain anorthosite wyoming abstract ultra high temperature shear zones formed with interstitial melts present during the emplacement of the poe mountain anorthosite wyoming the shear zones are characterized by coarse grained plagioclase with flat extinction and rare deformation microstructures dissected grain microstructures suggest that plagioclase underwent recrystallization by fast grain boundary migration orthopyroxene crystallized from interstitial melts as strain free foliation parallel tablet grains plagioclase has a magmatic petrofabric characterized by poles to 010 normal to the shear foliation and 100 parallel to the foliation and movement direction poles to 001 have a broad distribution with a point maximum roughly parallel to the foliation and normal to the movement direction orthopyroxene petrofabrics result from oriented growth parallel to the shear foliation 100 defines a point maximum perpendicular to the shear foliation and 010 and 001 have great circle distributions approximately parallel to the shear foliation microstructures of the ultra high temperature shear zones contrast with those of granulite grade shear zones cutting across the intrusion the granulite grade shear zones initiated as fractures which evolved into shear zones by dynamic recrystallization deformed primary plagioclase and orthopyroxene recrystallized by grain boundary rotation into smaller polygonal grains

microstructures in feldspars from a major crustal thrust zone the grenville front ontario canada abstract samples collected across the nw grenville front in ontario canada have been used to document the change in feldspar microstructures produced during shearing under metamorphic conditions that range from upper amphibolite to lower greenschist grade the differential displacement across this 5 km thick section of the grenville front tectonic zone is at least 10 km at upper to mid amphibolite conditions both plagioclase and potassium feldspars behave plastically grainsize reduction occurs through grain boundary migration and or subgrain rotation and produces a core and mantle structure at middle to lower amphibolite grade grain size reduction is accomplished through grain boundary migration at lower amphibolite to upper greenschist grade grain size reduction occurs by the nucleation and growth of new grains with a different composition the abundance of intracrystalline deformation at this grade is inversely proportional to the amount of fine grained matrix in ultramylonite with a high matrix to porphyroclast ratio few intracrystalline strain features occur other than ubiquitous undulatory extinction myrmekite is well developed on the high stress boundaries in k feldspar at upper to mid greenshist conditions feldspars deform mainly by slip on synthetic shear fractures deformation twins undulatory extinction deformation bands and kink bands occur in plagioclase and perthite flames and myrmekite occur principally on high stress boundaries in k feldspar grain size reduction in feldspar is accomplished through cataclasis at middle to lower greenschist facies deformation in feldspars is dominated by antithetic fractures the transition from the dominance of synthetic to antithetic fractures is gradational below lower greenschist grade rocks deform by pervasive cataclasis of all minerals

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

40ar 39ar dating of cataclastic k feldspar a new approach for establishing the chronology of brittle deformation abstract constraining the timing of brittle deformation such as in high angle normal or strike slip faulting is technically challenging in this study we present a new approach of age determinations from cataclastic k feldspars of brittle faults developed in rocks of the tan lu fault zone and from louzidian and liaonan metamorphic core complexes in ne china the attempt is based on field investigations microstructural observations back scattered electron bse imaging electron microprobe analyses x ray diffraction scanning electron microscope sem results and 40ar 39ar step heating of k feldspar separated from brittle fault zones of cataclastic mylonite granite and gneiss sem and bse images of individual k feldspar crystals reveal distinct grain deformation features including grain scale faults fractures and microfractures that occur parallel or perpendicular to each other the 40ar 39ar dating results provide well defined plateau spectra for the cataclastic k feldspar grains in the tan lu fault zone 74 5 1 3 ma and of brecciated gneisses and mylonites that formed at depth in the louzidian and liaonan metamorphic core complexes 120 129 ma and 116 2 ma these data suggest complete resetting of the ar isotope system of the cataclastic k feldspars allowing to constrain the timing of their deformation

influence of melt on the creep behavior of olivine basalt aggregates under hydrous conditions the influence of melt on the creep behavior of olivine basalt aggregates under hydrous conditions has been investigated by performing a series of high temperature triaxial compression experiments samples with melt fractions of 0 02 0 12 were deformed under water saturated conditions at temperatures between 1373 and 1473 k and a confining pressure of 300 mpa in a gas medium apparatus at constant differential stress and temperature the rate of deformation increased rapidly but systematically with increasing melt fraction in the diffusion creep regime at a given differential stress samples with melt fractions of 0 02 and 0 12 deformed a factor of 2 and 20 respectively faster than a melt free sample in the dislocation creep regime a sample with a melt fraction of 0 12 deformed a factor of 40 faster than a melt free sample for partially molten olivine basalt aggregates deformed under hydrous conditions the dependence of creep rate on melt fraction can be expressed in the form 0 exp where 26 for diffusion creep and 31 for dislocation creep the results of this study combined with reasonable estimates for the spatial variation in the concentrations of water and melt as well as for the geotherm and the activation volume for creep provide constraints on the viscosity structure of earth s upper mantle as an example we present a viscosity profile for the mantle wedge above a subducting plate demonstrating that the viscosity in that region can vary by 3 orders of magnitude over a depth of 60 km due to the combined effects of water and melt weakening

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

bed material size and the riffle pool sequence relationships between the bed material size distribution and riffle pool geometry of a small coarse bedload upland stream channel are described downstream sequences of bed material size are highly variable although series analysis reveals some persistence in the data over short distances cross correlation of bed material series with bed elevation series indicates riffles tend to coincide with coarser bed material sequences comparison of the bed material size distributions of individual riffles pools and bars is however more revealing mean particle size was found to be the most useful criterion for distinguishing between bed forms and in these terms bars are the most distinctive features moreover riffles and pools can be viewed as simply downstream oscillations in elevation of a single coarser grained sedimentary unit coarse sediment supply from erosion scars created where the channel meets confining bluffs in valley fill deposits does mean that the average particle size of adjacent and downstream bed forms is increased however conclusions remained unchanged after reanalysis following the identification and subtraction of immobile particle diameters plan geometry and coarse sediment supply in the upland environment are also shown to influence bed material size distributions tightly curved pools tend to have the finest pool sediments but also the least difference between bar and pool sediments size of material in pools is also negatively related to an index of cross section asymmetry

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

melt assisted interior to margin switch from dislocation to diffusion creep in coarse grained plagioclase evidence from a deformed anorthosite pluton abstract at bolangir eastern india massive anorthosites in the pluton interior grade into anorthosites with outward dipping margin parallel foliation neighboring the pluton margin sheets and veins of ferrodiorites residual melts of anorthosite crystallization concordant and discordant to the margin parallel foliation suggest pluton deformation at near magmatic conditions t 950 c and p 6 12 kbar in the pluton interior the larger than centimeter sized magmatic plagioclase grains are replaced by aggregates of smaller 100 600 m dynamically recrystallized internally strained grains with un equilibrated boundaries neighboring the pluton margin poly sized 200 2500 m plagioclase grains in anorthosites are of two types strain free rectangular shaped plagioclase grains with high energy an richer margins indenting neighboring plagioclase grains formed by diffusion creep whereas unstrained end to end touching euhedral plagioclase grains showing tilling represent magmatic flow textures the pluton interior to margin switch in plagioclase deformation from grain boundary migration accommodated dislocation creep to grain boundary diffusion creep is attributed to the increasing melt fraction melt crystal ratio during syn deformation pluton emplacement plagioclase grains in the bolangir pluton are significantly coarser compared to plagioclase aggregates

transient creep of a composite lower crust 2 a polymineralic basis for rapidly evolving postseismic deformation modes postseismic horizontal strain and displacement following the june 28 1992 landers california earthquake mw 7 3 is broad scale and cannot be explained solely by delayed afterslip located at the rupturing fault trace both the observed strain at pinon flat observatory pfo and observed global positioning system receiver velocities evolve rapidly after the landers big bear earthquake sequence the observed exponential decay of these motions with timescales of 4 34 days may reflect a soft creep rheology in the lower crust and brittle ductile transition zone or even within the seismogenic crust itself here a simple model of a two dimensional screw dislocation in a layered maxwell viscoelastic earth is employed in conjunction with a composite rheology to demonstrate that the short timescale transient response modes 4 34 days are consistent with the behavior of a biviscous lower crust the lowest viscosity of this system is derivable from laboratory experimental data on the long term creep of natural quarztites and the highest viscosity is consistent with isostasy related lower crustal flow in a continental extensional tectonic environment the model predicts significant stress relaxation at the base of the seismogenic crust near the base of the seismogenic zone and about 4 km away from the mainshock the rate of predicted relaxation is of the order of 0 01 mpa d 1 during the first 20 days of postseismic flow oblate spheroidal inclusions at 5 concentration levels that are both aligned and fairly flat in shape and that have a viscosity of 3 4 1015 pa s are consistent with both the amplitude and decay time of horizontal crustal strain observed at pfo after the landers mainshock it is speculated that the structures exposed in cross sections and in seismic reflection profiles of the lower crust that have mylonitic associations are in part the cause of such rapid postseismic evolution in southeastern california unmylonitized quartz rich rock at sufficiently elevated temperatures could also contribute to the rapid decay modes

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

dislocation creep of fine grained recrystallized plagioclase under low temperature conditions abstract lattice preferred orientation lpo and dislocation substructure of fine grained plagioclase were investigated in ultramylonite from a shear zone along the hatagawa fault zone in northeast japan in the shear zone the orthoclase mole fraction of the recrystallized alkali feldspar is not less than 0 94 this composition suggests that deformation occurred below 500 c selected area diffraction by transmission electron microscopy tem for an electron transparent foil and an x ray texture goniometer xtg were used to determine the lpo of plagioclase burgers vectors of dislocations were determined by invisibility criteria the 100 010 and 001 pole figures reveal strong preferred orientations the xtg results are consistent with the lpo determined by tem the tem observations confirmed the presence of at least six burgers vectors these burgers vectors or the net slip directions of multiple systems are almost parallel to the lineation thus the present study indicates ductility by dislocation creep for fine grained plagioclase in ultramylonite deformed at relatively low temperature the lpo pattern and the burgers vectors determined in this study are different from those of deformation at much higher temperature thus the present study also suggests a switching of the slip systems for plagioclase depending on the temperature

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

granulite microfabrics and deformation mechanisms in southern madagascar optical microstructures and crystallographic preferred orientations were studied in naturally deformed granulite to high amphibolite facies quartzo feldspathic rocks in southern madagascar the microstructures of coarse grained granulite suggest that feldspar and quartz accommodated deformation by both dislocation and diffusion creep in the absence of melt the extreme ductility of feldspar in dynamically recrystallized granulite is explained by activity of dislocation creep in conjunction with stress controlled intracrystalline diffusion in the studied rocks the considerable volume of quartz is not interconnected even at high strain the lack of its interconnectivity in coarse grained granulites and in amphibolite facies granoblastic platy quartz rocks is explained by an extreme stability of the load bearing framework structure at high temperatures in dynamically recrystallized granulite the feldspar viscosity decreases so that quartz becomes enveloped by a weak feldspar matrix which inhibits its coalescence and interconnectivity we predict an important decrease in strength of quartzo feldspathic granulites due to activity of diffusional creep and convergence of viscosity of recrystallized feldspar and quartz

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

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

correlation between oh concentration and oxygen isotope diffusion rate in diopsides from the adirondack mountains new york the concentration of structural oh in diopside was determined for four granulite facies siliceous marble samples from the adirondack mountains new york using ftir spectroscopy single crystal polarized ir spectra were measured on 100 and 010 sections of diopside the relative intensities of four oh bands in the 3700 3200 cm 1 region vary among the samples with the 3645 cm 1 band dominating the spectra of diopside from a xenolith at cascade slide total oh content in the diopsides ranges from 55 to 138 ppm h 2 o by weight the oh concentration in diopside increases monotonically with increasing f h2o for the sample as estimated using oxygen isotope systematics for diffusion rates in these samples from edwards and valley 1998 there is no significant variation in oh content within a single diopside grain or among diopside grains from the same hand sample charge coupled substitution with m 3 and ti 4 in the crystal structure may have allowed retention of oh in the diopside structure during and after peak metamorphism 750 c 7 8 kbar the cascade slide diopsides have an fe 3 fe 2 of 0 98 compared to fe 3 fe 2 0 to 0 05 for the other samples implying that some loss of hydrogen through oxidation of fe was possible in this sample this is the first study we know of which shows that the oh content in anhydrous minerals from natural samples correlates to the rate of oxygen isotope diffusion

analog modeling of one way gravitational spreading of hot orogens a case study from the svecofennian orogen fennoscandian shield abstract geodynamic models offer insight into deformation styles of syn and post collisional extensional processes such models often ignore the effect of lateral viscosity contrast between accreted terranes and inherited tectonic boundaries this paper presents a set of crustal scale analog experiments of one way post collisional gravitational spreading testing the effect of crustal heterogeneity by using two mechanically contrasting blocks pre existing cuts with varying dips and three layers with contrasting rheologies two blocks represent strong mature and weak juvenile crustal parts both blocks have three horizontal crustal layers brittle upper weak partially molten middle and dry strong lower layer cuts represent pre existing crustal scale shear zones and tectonic boundaries the experiments indicated that tectonic boundaries may facilitate exhumation and increase extensional strain when the boundaries dip opposite to the flow direction the tectonic boundaries may also split and divide the crust into separate blocks with independent deformation signatures shortening elongation or rotation the tectonic boundaries rotated along vertical axes toward the flow direction the modeling results suggest that in areas with inherited tectonic boundaries and compositional differences the amount of extension is unevenly distributed between the different parts during the widespread unilateral gravitational spreading and that the flow has different effects on the contrasting crustal units in both lateral and vertical directions to validate the modeling the results are compared to geophysical and geological data from the paleoproterozoic svecofennian orogen in the fennoscandian shield the comparison suggests that the orogen underwent 50 extension and was thinned by maximum of 20 via westward gravitational spreading this spreading rotated the crustal scale blocks shear zones and tectonic contacts it also brought exhumed high grade middle crustal blocks into contact with subsided low grade upper crustal blocks we suggest that results of the analog models can be used as proxies in both ancient and modern hot accreted orogens which have undergone post convergence continental mid crustal weakening and have comprised of juxtaposed terranes with varying rheological compositions

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

towards an improved understanding of the mechanical properties and rheology of the lithosphere an introductory article to rock deformation from field experiments and theory a volume in honour of ernie rutter ernie rutter s influential contribution to our understanding of rock deformation spans the whole continental crust from deformation just under the grass e g rutter green 2011 to melt migration in the lower crust e g rutter neumann 1995 ernie has also worked on many aspects of the deformation of oceanic crust rutter brodie 1988 however here we reflect on the wide range of deformation conditions ernie has studied by presenting key aspects of the strength and rheology of the continental lithosphere as we understand it today with the aim of providing the context for the contributions included within this volume the continental crust forms just over a third of the earth s surface and it is different from the crust of any other planet in our solar system it is rich in those elements that partition in a silicate melt and its formation and evolution determined the composition of the mantle and that of the atmosphere hawkesworth 2006 thanks to its physical and chemical fingerprint the earth s crust has sustained life uninterrupted for 3 8 ga and it is the source of major natural resources such as hydrocarbons and mineral deposits and others such as geothermal energy the continental crust is less dense than its oceanic counterpart and is therefore more buoyant it forms a substantial part of the cold thermal layer of the earth and its rheology from greek r1v rhe flow and logia logia study the study of the flow of matter is a key factor controlling plate tectonic processes at the earth s surface and in its interior davies 2011 beneath the continental crust lies the lithospheric mantle and together they form the continental lithosphere considerable variations in structure thickness and composition of the continental lithosphere mean that despite its importance we currently have limited understanding of the rheology of this complex system thatcher pollitz 2008 significant advances in our knowledge of continental rheology have been possible through

himalayan metamorphic sequence as an orogenic channel insight from bhutan the bhutan himalayas differ from the rest of the himalayas in two major ways i low grade sedimentary rocks lie above the greater himalayan sequence as klippen i e erosional remains of the south tibetan detachment and ii an out of sequence thrust the kakhtang thrust lies structurally above the klippen and it doubles the exposed thickness of the greater himalayan sequence our field observations and geochronological data constrain the main kinematic events in the bhutan himalayas crystallisation ages of leucogranite dykes deformed by the main central thrust and the south tibetan detachment indicate that these two structures operated together between 16 and 22 ma the out of sequence kakhtang thrust was active at 10 14 ma and was concurrent with reactivation of the south tibetan detachment restoration of the bhutan himalayas prior to the out of sequence thrusting shows that the greater himalayan sequence was the core of a long low viscosity crustal channel extending under the tibetan plateau we propose that the gravity driven southward extrusion of the channel material from underneath the tibetan plateau caused the inverted metamorphic sequence in the lesser himalayan sequence and in the greater himalayan sequence this process also led to occurrences of present day surface rocks that were derived from variable distances down dip but from similar crustal depths such an exhumation pattern can explain the similar peak pressures for the greater himalayan sequence along the length of the himalayas

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

prolonged post seismic deformation of the 1960 great chile earthquake and implications for mantle rheology 1 contemporary crustal deformation of the southern andean margin shows an interesting feature while nearly all coastal gps sites move landward consistent with inter seismic deformation near a locked subduction fault sites 300 400 km landward of the rupture region of the mw9 5 1960 chile earthquake are moving in the opposite direction we attribute the seaward motion of these inland sites to a prolonged crustal deformation due to mantle stress relaxation following the 1960 great earthquake in order to reproduce the observed seaward motion using a three dimensional finite element model we need to incorporate a mantle viscosity of about 3 1019 pa s the possibility that the seaward motion is caused by a silent slip event on the plate interface at large depths cannot be completely excluded and our analysis provides a working model for future field tests

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

earthquake induced transformation of the lower crust the structural and metamorphic evolution of the lower crust has direct effects on the lithospheric response to plate tectonic processes involved in orogeny including subsidence of sedimentary basins stability of deep mountain roots and extension of high topography regions recent research shows that before orogeny most of the lower crust is dry impermeable and mechanically strong 1 during an orogenic event the evolution of the lower crust is controlled by infiltration of fluids along localized shear or fracture zones in the bergen arcs of western norway shear zones initiate as faults generated by lower crustal earthquakes seismic slip in the dry lower crust requires stresses at a level that can only be sustained over short timescales or local weakening mechanisms however normal earthquake activity in the seismogenic zone produces stress pulses that drive aftershocks in the lower crust 2 here we show that the volume of lower crust affected by such aftershocks is substantial and that fluid driven associated metamorphic and structural transformations of the lower crust follow these earthquakes this provides a top down effect on crustal geodynamics and connects processes operating at very different timescales during continent collision and associated mountain building a surprisingly large volume of the lower crust is shown to be affected by earthquake aftershocks producing a top down effect on crustal geodynamics

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

high temperature microstructures and rheology of deformed granite erzgebirge bohemian massif abstract three stages of granite deformation were recognized in porphyric granitoids sheared under amphibolite facies conditions in the erzgebirge mts 1 weakly deformed granite in lozenge shaped pods 2 s c orthogneiss and 3 thin zones of banded mylonites weakly deformed granite forms a load bearing framework lbf structure in which quartz and feldspar aggregates exhibit similar strain intensities and shapes k feldspar deforms by fracturing and onset of dynamic recrystallization along clast margins plagioclase recrystallizes completely and quartz shows effects of grain boundary migration recrystallization and activity of basal a slip s c orthogneiss is interpreted as an interconnected weak layer iwl structure with high viscosity contrast between quartz and both feldspars concentration of deformation into thin quartz layers is indicated by intense strain of quartz grain size variations of recrystallized quartz and dominant prism a slip feldspars show well annealed structures and slip on 010 001 banded mylonite is marked by similar strain intensity of quartz and feldspar aggregates and low viscosity contrast between constituent phases quartz shows evidence for grain size increase and rhomb a c slip fully recrystallized feldspars show evidence for 010 100 slip the whole microstructural sequence and the transitions from lbf structure to iwl structures show the dependence of microstructural evolution of different materials on the bulk strain intensity

feldspar fabrics in a greenschist facies albite rich mylonite from electron backscatter diffraction abstract the current database of plagioclase cpos in naturally deformed rocks is small and is not supported by cpos from experimentally deformed rocks or by numerical modelling so that the current understanding of plagioclase cpos is limited electron backscatter diffraction ebsd has the potential of measuring the full crystallographic orientation of small domains of any crystalline material including plagioclase we generate and test a file of the relative electron diffraction intensities for lattice planes in the albite structure this enables rapid and accurate individual orientation measurements of albite using ebsd ebsd is used to collect 360 data points from 7 areas across a 2 cm by 1 mm pure albite band in a greenschist facies albite mylonite from a gabbro protolith the cpos are unlike any published the a b and c axes of albite are not symmetrically distributed with respect to the kinematic axes of deformation and do not possess an internal monoclinic or orthorhombic symmetry there is some microstructural evidence for the operation of dislocation creep although this interpretation conflicts with the generally held view that crystal plastic deformation does not occur in plagioclase below about 500 c an alternative mechanism is one of granular flow accommodated by solution transfer however this mechanism would be expected to give a random cpo for either mechanism we need to explain the triclinic nature of the cpo it is possible that the cpo represents the modification by deformation of a pre existing cpo probably defined by a small number of original plagioclase grains

the geologic record of deep episodic tremor and slip transient and episodic slow slip accommodates a great deal of tectonic strain and may be mechanically linked with locked regions of seismically hazardous faults best documented in subduction zones and associated with nonvolcanic tremor most proposed mechanisms for slow slip revolve around the transition between stable and unstable frictional sliding the dilemma is that slow slip is generated at a wide range of crustal depths including at pressure temperature conditions where frictional deformation mechanisms give way to predominantly viscous ones we present a model for how fracture and viscous flow within mid crustal shear zones can produce episodic creep transients our model for such transients stems from geological examples of shear zones that formed at temperatures and pressures of 500 c and 0 6 gpa during early orogenesis following late cretaceous subduction of a backarc ocean basin within these shear zones relatively strong lenses of metabasalt localized fluid filled fractures that were subsequently deformed by viscous flow in surrounding quartzofeldspathic gneiss the spatial and temporal characteristics of the modeled creep events are similar to those of slow slip events observed in modern subduction zones we therefore suggest that some episodic tremor and slip can originate through combined fracture and viscous flow across shear zones comprising mixtures of strong and weak materials

evidence for dominant grain boundary sliding deformation in greenschist and amphibolite grade polymineralic ultramylonites from the redbank deformed zone central australia abstract microstructural and textural investigations by scanning sem and transmission electron microscopy tem techniques have been performed on samples taken across two quartzo feldspathic mylonite zones from the redbank deformed zone central australia one has been deformed at greenschist facies gs the second at amphibolite facies am conditions with increasing strain the rock type changes from protomylonite to mylonite to ultramylonite the protomylonites and mylonites consist of alternating quartz and polymineralic quartz feldspar bands at the highest strains a homogeneous fine grained polymineralic ultramylonite occurs shear zone geometry and microscale structures indicate that these ultramylonites experienced higher strains and were weaker than the encapsulating protomylonites and mylonites tem and sem studies of the ultramylonites reveal a rectangular to square grain shape a continuous alignment of grain and interphase boundaries across several grain diameters a grain size gs 0 5 m am 5 11 m less than the equilibrium subgrain size and open and void containing grain and interphase boundaries analysis of local textures by electron back scatter diffraction ebsd in the sem showed a very weak crystallographic preferred orientation cpo for the quartz the grain misorientation relationships are not consistent with dislocation creep being the dominant deformation mechanism all structures are of the type expected if grain boundary sliding processes had contributed significantly to the deformation consequently the deformation of such quartzo feldspathic rocks and by implication the rheology of the redbank deformed zone must have been controlled by the mechanical properties of these fine grained polymineralic ultramylonites deforming by grain boundary sliding processes this is in contrast to the pure quartz bands which deformed by dislocation creep mechanisms and were less important in the rheology of the redbank deformed zone

grain size evolution and the rheology of ductile shear zones from laboratory experiments to postseismic creep abstract geologic experimental and geodetic studies indicate that ductile shear zones do not deform in steady state we present a theoretical model for an elastically loaded ductile shear zone deforming by combined dislocation and diffusion creep in which grain size is allowed to evolve toward its equilibrium recrystallized value several grain size evolution laws are considered simulations of laboratory experiments show that grain size evolution can lead to changes in the dominant deformation mechanism that are accompanied by transient strain weakening or strain strengthening episodes reminiscent of laboratory observations ductile shear zones can also produce postseismic creep as observed in geodetic data following large earthquakes non steady state ductile creep is difficult to differentiate from frictional sliding with velocity strengthening friction hence ductile creep may play a role in the transition from velocity weakening to velocity strengthening friction with increasing temperature