cooling induced strain localization in carbonate mylonites within a large scale shear zone glarus thrust switzerland abstract the glarus thrust switzerland offers a great field example of strain localization under retrograde conditions along the thrust mylonitic microfabrics are characterized by a temperature stress controlled balance of grain size reducing mechanisms and grain growth consequently mean grain sizes decrease along the thrust with decreasing metamorphic conditions and towards the thrust contact in an opposite manner calcite twin densities increase towards the contact cpos are strongest between 0 5 and 15 m away from the thrust but become generally weaker in the last centimeters where also cataclasites occur the cpo weakening and grain size reduction towards the shear zone point to a change from predominant dislocation creep to granular flow finally ending in cataclastic deformation the microfabric changes correlate with a decrease in 13c and 18o towards the thrust contact indicating the presence of fluids during cycles of brittle and plastic deformation based on the microfabric changes variations in stable isotopes and cross cut relationships a subdivision into a low and high temperature shear zone can be made the microfabric modifications resulted from changes in deformation conditions due to ongoing thrusting and exhumation induced cooling promoting further strain localization of an existing high strain shear zone

dehydration induced damage and deformation in gypsum and implications for subduction zone processes 1 experimental heating tests were performed on volterra gypsum to study the micromechanical consequences of the dehydration reaction the experimental conditions were drained at 5 mpa fluid pressure and confining pressures ranging from 15 to 55 mpa one test was performed with a constantly applied differential stress of 30 mpa the reaction is marked by 1 a porosity increase and homogeneous compaction 2 a swarm of acoustic emissions 3 a large decrease in p and s wave velocities and 4 a decrease in vp vs ratio wave velocity data are interpreted in terms of crack density and pore aspect ratio which modeling pores as spheroids is estimated at around 0 05 crack like spheroid complementary tests performed in an environmental scanning electron microscope indicate that cracks first form inside the gypsum grains and are oriented preferentially along the crystal structure of gypsum most of the visible porosity appears at later stages when grains shrink and grain boundaries open extrapolation of our data to serpentinites in subduction zones suggest that the signature of dehydrating rocks in seismic tomography could be a low apparent poisson s ratio although this interpretation may be masked by anisotropy development due to preexisting crystal preferred orientation and or deformation induced cracking the large compaction and the absence of strain localization in the deformation test suggests that dehydrating rocks maybe seen as soft inclusions and could thus induce ruptures in the surrounding nonreacting rocks

downstream changes in the channel geometry of a large gravel bed river 1 field data obtained on a nearly contiguous segment of the colorado river in western colorado and eastern utah are used to examine the mechanisms driving downstream changes in channel geometry measurements characterizing the bank full hydraulic geometry bed material grain size and average channel gradient were made at closely spaced intervals in 10 alluvial and quasi alluvial reaches covering 260 km of the river these data indicate that changes in surface and subsurface grain sizes are small in relation to the change in channel slope over the full length of the study area the median grain size of the surface sediment decreases by a factor of a little more than 2 whereas the average channel slope decreases by a factor of about 5 the decreases in slope and median grain size are offset by a large increase in bank full depth relative to width such that the bank full shields stress b is constant downstream for the reach as a whole b averages 0 049 which is roughly 50 higher than the threshold for bed load transport

frictional strengths of talc serpentine and talc quartz mixtures 1 talc is a constituent of faults in a variety of settings and it may be an effective weakening agent depending on its abundance and distribution within a fault we conducted frictional strength experiments under hydrothermal conditions to determine the effect of talc on the strengths of synthetic gouges of lizardite and antigorite serpentinites and of quartz small amounts of talc weaken serpentinite gouges substantially more than predicted by simple weight averaging in comparison mixtures of quartz and talc show a linear trend of strength reduction at talc concentrations 15 wt and enhanced weakening at higher concentrations all of the strength data are fit by a modified version of the reuss mixing law that allows for the dominance of one mineral over the other the difference in the behavior of serpentinite talc and quartz talc mixtures at low talc concentrations is a reflection of their different textures lizardite antigorite and talc all have platy habits and displacement within gouges composed of these minerals is localized to narrow shears along which the platy grains have rotated into alignment with the shear surfaces the shears in the mixed phyllosilicate gouges maximize the proportion of the weaker mineral within them when mixed with a strong rounded mineral such as quartz some minimum concentration of talc is needed to form connected pathways that enhance strength reductions the typical development of talc by the reaction of si rich fluids with serpentinite or dolomite would tend to localize its occurrence in a natural fault and result in enhanced weakening

rock and mineral transformations in a fault zone leading to permanent creep interactions between brittle and viscous mechanisms in the san andreas fault creep processes may relax part of the tectonic stresses in active faults either by continuous or episodic processes the aim of this study is to obtain a better understanding of these creep mechanisms and the manner in which they change in time and space results are presented from microstructural studies of natural samples collected from san andreas fault observatory at depth borehole drilled through the san andreas fault which reveal the chronology of the deformation within three domain types i a relatively undeformed zone of the host rock reflects the first step of the deformation process with fracturing and grain indentations showing the coupling between fracturing and pressure solution ii shear deformation development that associates fracturing and solution cleavage processes leads to profound changes in rock composition and behavior with two types of development depending on the ratio between the amount of dissolution and deposition abundant mineral precipitation strengthens some zones while pervasive dissolution weakens some others iii zones with mainly dissolution trended toward the present day creeping zones thanks to both the passive concentration of phyllosilicates and their metamorphic transformation into soft minerals such as saponite this study shows how interactions between brittle and viscous mechanisms lead to widespread transformation of the rocks and how a shear zone may evolve from a zone prone to earthquakes and postseismic creep to a zone of steady state creep in parallel the authors discuss how the creeping mechanism mainly controlled by the very low friction of the saponite in the first 3 4 km depth may evolve with depth

fluid assisted healing processes in gouge bearing faults insights from experiments on a rock analogue system on the basis of both experimental and field studies solution transfer processes are expected to be important in contributing to fault strength recovery healing under the hydrothermal conditions prevailing in large parts of the seismogenic zone however most experimental work on healing effects in faults has been done using quartzo feldspathic sliding surfaces or fault gouges under conditions in which solution transfer processes are very slow mechanisms of fluid assisted fault healing are accordingly rather poorly understood we have performed slide hold slide fault healing experiments on simulated faults containing brine saturated granular halite as a fault rock analogue halite was used because solution transfer processes are known to be rapid in this system under easily accessible conditions we studied the effects of hold time duration shear stress during hold pre hold sliding velocity and pore fluid composition on strength evolution the results show rapid fluid assisted compaction and stress relaxation during hold periods and a significant hold time dependent strengthening upon re shear the data reveal that healing resulted from both a packing density increase and a contact area strength increase during hold periods the general type of behaviour observed is similar to that observed in quartzo feldspathic gouges at room temperature although important differences were observed as well notably the time dependence of healing deviates from the log linear trend observed in gouges where solution transfer processes are absent this means that if pressure solution is an important healing mechanism in natural faults applying a log linear trend will underestimate natural fault healing rates

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

deformation mechanisms and rheology why marble is weaker than quartzite when deformed together in nature calcite rocks invariably appear weaker and more ductile than quartz rocks this can be reconciled with experimental flow data only by taking into account grain size sensitive gss flow in calcite rocks which is predicted to dominate even at grain sizes on the order of 1mm at middle metamorphic grades using new experimental data that demonstrate the transition between intracrystalline plastic and gss flow of quartz rocks we predict that unnaturally small grain sizes at temperatures of 700 c or higher are required for gss flow of quartz in nature thus natural flow of quartzite is expected to occur by intracrystalline plastic processes even after recrystallization to a fine grain size

reduction of friction on geological faults by weak phase smearing most common crustal rock types display friction coefficients of 0 6 or higher but some faults must be frictionally weak as they slip when the stress state is unfavourably oriented i e the resolved shear stress is low for a given normal stress across the fault surface a role for low friction minerals and high pore fluid pressures either separately or in combination is frequently invoked to explain such slip but volume fractions of dispersed weak phases often seem to be present in fault gouge in amounts too small to produce significant mechanical weakening by means of mechanical tests on synthetic fault gouge and microstructural study of run products we show that the effective area of an embedded weak phase graphite on a slip plane can be substantially increased by mechanical smearing and that the enlarged area of the weak phase on the slip plane follows a linear mixing law this allows a relatively small volume fraction of the initially dispersed weak phase to have a disproportionately large effect provided the smearing is concentrated into a narrow planar slip zone or into an interconnected network of them

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

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

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

high pressure high temperature seismic velocity structure of the midcrustal and lower crustal rocks of the ivrea verbano zone and serie dei laghi nw italy the ivrea verbano zone and the adjacent serie dei laghi in the inner arc of the western alps nw italy display an upended cross section through much of the thickness of the continental crust as it existed in that region in triassic jurassic time we collected a suite of oriented rock samples that represent most of the volume of the rocks of the region p and s wave velocity measurements were made in three orthogonal directions related to the mesoscopic fabric of the rocks at room temperature and up to 550 mpa confining pressure combined high temperature and high pressure measurements were made up to 700 c on a subset of the samples the lithologic units were divided into 23 different principal rock types and we present velocity data averaged in terms of these groups vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section these show that vp and vs both increase and anisotropy decreases systematically with depth of burial reflecting variations in rock type and metamorphic grade with depth vp and vs anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding with the slowest direction tending to be normal to the foliation or banding the vp velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust these correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution

on the structure of the ivrea verbano zone northern italy and its implications for present day lower continental crust geometry the ivrea verbano zone in northern italy represents a section through the lower continental crust which has been tilted and emplaced into its present position during the alpine orogeny recent and on going structurally oriented geological mapping in this region is providing new information about the geometry of the complex the central part of the zone is dominated by a large basic complex the mafic formation which is intrusive into the surrounding gneisses the foliation within the envelope of gneisses is deflected around the intrusive complex as if by ballooning but in the region south west of monte capio both units are folded together into a tight to isoclinal steeply plunging fold with an amplitude of c 10 km this fold locally inverts the stratigraphy of the layered basic group of the complex and is thought to be the result of gravitational collapse following intrusion and inflation of a large magma body into the lower crust several high temperature shear zones have now been traced within the country rock for distances up to 20 km the geometry of these and their relationship to the basic complex suggests that at least some of the extensional collapse of the mafic body is related to uplift caused by intrusion of this body close parallels can be drawn between the observed structure in the ivrea verbano zone after removing the effects of late low temperature faulting and folding related to emplacement of the rocks into their present position and those inferred from deep seismic reflection profiling in areas of current extension such as parts of the us basin and range province

flow of synthetic wet partially molten granite under undrained conditions an experimental study 1 major problems in experimental studies of the flow of partially molten granitic rocks are 1 maintaining control of the grain size of the matrix of solid grains 2 controlling the melt fraction and 3 controlling the melt viscosity closely linked to water content to overcome these problems we used a synthetic granitoid comprising a solid matrix of quartz grains 50 m grain size mixed with an albite quartz melt prepared from oxides with added water 2 5 wt dissolved in the melt phase viscosity 9 104 pa s experiments were performed undrained with melt fractions of 0 1 0 2 0 25 and 0 3 constant strain rate creep and stress relaxation experiments were carried out in a gas medium apparatus at 1273 and 1173 k mostly at 300 mpa confining pressure up to 15 shortening strain strain rates ranged between 10 4 to less than 10 7 s 1 and fully ductile mechanical behavior was observed melt fraction has a more profound effect on strength than water content temperature or total confining pressure low strain rate 10 6 s 1 data were fitted to an empirical flow law of the form d dt a exp b m exp h rt n with the parameters loga 1 39 m 3 n 1 8 b 192 h 220 kjmol 1 where d dt is strain rate s 1 is flow stress mpa is melt fraction and t is temperature k strength is expected to fall rapidly above about 0 4 melt fraction to levels characteristic of flow of the melt containing suspended crystallites extrapolation to geological strain rates using this flow law shows that in nature migmatites bearing granitic melt will be extremely weak much weaker than silicate rocks deforming by intracrystalline plasticity microstructural study shows grains remain equant at all strains with no discernable formation of a grain flattening fabric nor a large strain contribution from microfracturing thus bulk intracrystalline plasticity is of minor importance and intergranular sliding is implied the nonlinear flow observed is tentatively attributed to a combination of sintering of grain contacts and sliding failure of such contacts with a substantial contribution from diffusive transfer processes

experimental intracrystalline plastic flow in hot pressed synthetic quartzite prepared from brazilian quartz crystals samples of synthetic ultrafine grained quartzite were prepared by hot pressing aggregates of crushed clear brazilian quartz of mean grain size 0 4 m at 300 mpa and 1373 and 1473 k the samples displayed rapid grain growth to ca 12 20 m with 2 porosity at 1473 k facilitated by the 0 6 wt of water adsorbed onto the grain boundaries during sample preparation this water could be driven off by preheating thereby preventing grain growth sufficient water was incorporated during growth into the coarsened samples to render them weak and ductile these were deformed experimentally in the quartz field using an argon gas medium apparatus at 300 mpa confining pressure at temperatures mainly between t 1273 and 1473 k ductile flow was described by the flow law 10 4 9 3 2 9 7 f h 2 o exp 242000 rt with stress a in mpa and grain size d in microns and strain rate in s 1 r is the gas constant and f h 2 o is water fugacity based on observation of grain flattening optical strain features shapes and densities of dislocation arrays and flow law parameters samples deformed dominantly by dislocation creep with some contribution from grain boundary sliding bearing in mind possible changes in the flow law parameters over the extrapolation interval and possible effects of the phase transition extrapolation to natural strain rates and temperatures predicts plastic flow at higher flow stresses at the same water fugacity at greenschist facies temperatures than previously published flow laws

a reappraisal of the sibson scholz fault zone model the nature of the frictional to viscous brittle ductile transition along a long lived crustal scale fault outer hebrides scotland the widely cited sibson scholz conceptual fault zone model suggests that seismically active upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10 15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep the crustal scale outer hebrides fault zone ohfz in nw scotland has been described as the type example of such a continental fault zone it cuts precambrian basement gneisses and is deeply exhumed allowing direct study of the deformation products and processes that occur across a wide range of crustal depths a number of fault rock assemblages are recognized to have formed during a long lived displacement history lasting in excess of 1000 myr during caledonian movements that are recognized along much of the 190 km onshore fault trace brittle cataclasite bearing faults in the west of the ohfz are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust related crustal thickening or shear heating microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids this led to retrogression with the widespread development of new finegrained phyllosilicate bearing fault rocks phyllonites and the onset of fluid assisted grain size sensitive diffusional creep in the most highly deformed and altered parts of the fault zone phyllonitic fault rocks also occur in older more deeply exhumed parts of the fault zone implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature depth range within crustal scale faults our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal scale faults containing interconnected networks of phyllosilicate bearing fault rocks will be characterized by long term relative weakness and shallow 5 km frictional viscous transition zones similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the san andreas fault

hydrothermal remobilization of rare earth elements and its effect on nd isotopes in rhyolite and granite abstract although several studies have shown that the sm nd radiometric system is not remobilized during the hydrothermal alteration of felsic rocks it is doubtful that the rare earth element ree mobility noticed under certain hydrothermal conditions always leaves the sm nd systematics unperturbed this contribution describes two cases of such isotopic disturbances of hydrothermalised felsic rocks from two different geological contexts in rhyolites from the esterel southeastern france and in the red granite of porto northwest corsica southeastern france although these two igneous bodies were initially very homogeneous as regards their nd isotopes some samples display very contrasting nd isotopic signatures i e comparatively low initial143nd 144nd ratios that are anomalous from a magmatic point of view moreover these isotopically perturbed rocks are invariably associated with peculiar ree patterns that are also difficult to explain in terms of differentiation or contamination at the magmatic stage it is thus inferred that the sm nd system in these samples was perturbed by hydrothermal alteration in the two examples studied the perturbation of the sm nd radiometric system cannot be solely explained by a modification of thesm nd ratio significantly after the emplacement of the igneous bodies although the net result of the hydrothermal alteration is leaching of the rees from the rocks i e mainly the middle and heavy ree for the granite and the light ree for the rhyolites the isotopic compositions of the samples were modified by a component introduced from a fluid having contrasting143nd 144nd ratios this inference together with structural observations indicates that rees were transported over distances exceeding several tens to hundreds of metres even in the case of the granite studied in this work where mineralised fractures are absent this is far greater than previous estimates and this conclusion is of great importance for environmental and nuclear waste disposal site studies where light ree are often taken as natural analogues for artificial radionuclides

dynamic weakening of serpentinite gouges and bare surfaces at seismic slip rates to investigate differences in the frictional behavior between initially bare rock surfaces of serpentinite and powdered serpentinite gouge at subseismic to seismic slip rates we conducted single velocity step and multiple velocity step friction experiments on an antigorite rich and lizardite rich serpentinite at slip rates v from 0 003 m s to 6 5 m s sliding displacements up to 1 6 m and normal stresses n up to 22 mpa for gouge and 97 mpa for bare surfaces nominal steady state friction values nss in gouge at v 1 m s are larger than in bare surfaces for all n tested and demonstrate a strong n dependence nss decreased from 0 51 at 4 0 mpa to 0 39 at 22 4 mpa conversely nss values for bare surfaces remained 0 1 with increasing n and v additionally the velocity at the onset of frictional weakening and the amount of slip prior to weakening were orders of magnitude larger in gouge than in bare surfaces extrapolation of the normal stress dependence for nss suggests that the behavior of antigorite gouge approaches that of bare surfaces at n 60 mpa x ray diffraction revealed dehydration reaction products in samples that frictionally weakened microstructural analysis revealed highly localized slip zones with melt like textures in some cases gouge experiments and in all bare surfaces experiments for v 1 m s one dimensional thermal modeling indicates that flash heating causes frictional weakening in both bare surfaces and gouge friction values for gouge decrease at higher velocities and after longer displacements than bare surfaces because strain is more distributed