strength and anisotropy of foliated rocks with varied mica contents abstract we have shortened 15 schists and gneisses with varying compositions 15 75 mica by volume at angles of 45 and 90 to foliation s to investigate the influence of micas on the strength and anisotropy of foliated rocks at the conditions tested t 25 c p c 200 mpa and e 10 5 s 1 compressive strengths vary by a factor 4 in the 45 and 90 orientations and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic trends of decreasing strength decreasing anisotropy and increasing ductility are observed with increasing mica content strains in all samples were localized within inclined shear zones accommodated by dislocation slip and grain scale microkinking in micas and microcracking in all other silicates stress strain response and grain scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial arrangement of micas shear zone formation is associated either with discrete brittle fracture transitional strain softening or steady strength ductile shear in both brittle and transitional samples extensive microcracking of strong quartzo feldspathic bridges occurs and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent critically oriented micas steady strength ductile shear zones develop in samples containing domains of interconnected micas and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions

patterns of strain localization in heterogeneous polycrystalline rocks a numerical perspective the spatial and temporal patterns of strain localization in materials with pre existing heterogeneities are investigated via a series of two dimensional numerical models models include i a dynamic feedback process to simulate rheological weakening in response to the transition from non linear flow dislocation creep to linear flow diffusion creep grain boundary sliding and ii a time dependent strengthening process counteracting the weakening process different load bearing framework geometries with 20 weak component are used to evaluate the impact of geometry on the strength of the material and its ability to localize strain into an interconnected weak layer iwl our results highlight that during simple shear if dynamic weakening with or without strengthening feedbacks is present strain is quickly localized into an iwl where an increasing proportion of weak material increases the interconnections between the iwls thereby increasing the anastomosing character of the shear zones we establish that not only bulk strain localization patterns but also their temporal patterns are sensitive to the dominance of the weakening or strengthening process consequently shear zones are dynamic in time and space within a single deformation event hence the pattern of finite strain can be an incomplete representation of the evolution of a shear zone network

flow laws for rocks containing two non linear viscous phases a phenomenological approach abstract mylonitic rocks and rock analogue materials reveal two basic types of structure 1 a load bearing framework lbf of strong phase contains isolated pockets of weak phase 2 interconnected layers of weak phase iwl separate boudins and clasts of strong phase aggregates with the lbf microstructure are characterized by nearly uniform strain rate stress is concentrated in the load bearing framework in aggregates with an iwl microstructure strain rate and sometimes also stress are higher in the interconnected weak phase than in the boudins and clasts of strong phase the degree of stress and strain partitioning depends strongly on the viscous strength contrast and on the relative amounts of the constituent mineral phases based on these observations the rheology of two phase rock is modelled with separate functions for lbf and iwl microstructures a new flow law is derived for rock with iwl structure in which two phases undergo dislocation creep the flow law expresses composite creep strength in terms of temperature bulk strain rate and the volume proportions and creep parameters of the minerals in the rock strain rate and stress are averaged in the constituent phases and slip along phase boundaries maintains strain compatibility within the aggregate composite strengths predicted with the iwl flow law fall well within the uniform stress and uniform strain rate bounds and are generally consistent with the viscous strengths of experimentally deformed bimineralic aggregates a hypothesis of viscous strain energy minimization is used to determine the relative stability of the lbf and iwl microstructures during steady state creep the iwl microstructure is predicted to be stable over a broad range of two phase compositions and mineral strength contrasts whereas the lbf microstructure is stable only in rocks with low volume proportions of weak phase and low to moderate mineral strength contrasts the iwl flow law indicates that rheological stratification in the lithosphere depends strongly on rock composition especially in rocks with low volume proportions of a weak phase and high mineral strength contrasts

mechanisms of weak phase interconnection and the effects of phase strength contrast on fabric development abstract to determine how deformation mechanisms and fabrics evolve during the strain weakening and localization that accompany the transition from a load bearing framework to an interconnected weak phase shear experiments were conducted on a fine grained gneiss 58 quartz interconnected 28 plagioclase dispersed 13 biotite aligned but dispersed at 745 and 800 c 1500 mpa and two strain rates 2 10 5 and 2 10 6 s 1 in experiments with a high phase strength contrast hpsc 25 and 45 between dispersed biotite and framework quartz grains biotite grains become interconnected due to stress concentration at their tips that allow local semi brittle flow of intervening quartz and cataclasis of intervening plagioclase in the higher temperature hpsc samples the biotite partially dehydrated in high strain zones a single narrow shear zone formed because the psc between the quartz and biotite reaction product layers remained high in the lower temperature hpsc samples there was no reaction and a penetrative s c fabric formed the fabric is defined by the many multiply interconnected biotite strands that formed because the psc between the quartz and biotite decreased as the biotites kinked in slower strain rate experiments with a much lower psc 10 biotite interconnection occurs by shearing into quartz quartz boundaries where new weak strain free recrystallized grains form at low strain a weak s c fabric forms but it evolves to an s c fabric as the psc decreases with strain thus the magnitude of the strength contrast between a weak phase and its matrix influences the mechanism of weak phase interconnection the degree of strain localization and the fabric

influence of microscale weak zones on bulk strength shear zones have different rheological properties than the surrounding rocks indicating that the bulk strength of regions containing shear zone networks cannot be determined by considering the the host rock rheology alone we demonstrate the value of this concept at the microscale we first consider the phase arrangements in naturally deformed rocks and document that weak phases exhibit little interconnection within a microstructure rather three dimensional weak zones analogous to viscous shear zones can interconnect or bridge weak phases these zones typically form at high stress sites comprise multiple minerals and deform by mechanisms independent of those in the surrounding minerals the presence of weak zones strongly affects the bulk strength of the rock disproportionate to the mode of the weak zones for example the development of 1 mode of a weak zone at a high stress site can reduce the bulk strength of the rock nearly an order of magnitude calculation of the bulk strength of the rock by some averaging algorithm of the deformation mechanisms operating outside the weak zones will overestimate strength instead accurate calculations and predictions of bulk strength require accounting for the presence and geometry of weak zones for this reason we advocate use of the scale independent conceptual rheological model of interconnected weak zones or layers rather than that of interconnected weak phases more generally the way forward in improving quantification of the mechanical properties of the lithosphere requires recognizing and explicitly accounting for the spatial and temporal distribution of deformation mechanisms operating throughout a rock this article is protected by copyright all rights reserved

strain geometry microstructure and mineral chemistry in metagabbro shear zones a study of softening mechanisms during progressive mylonitization abstract the strain geometry microstructure and metamorphism is described from two minor shear zones from the chatelaudren metagabbro n brittany a serially slabbed shear zone reveals a strain geometry consistent with simple shear deformation strain calculations based on x trajectory angles coincide with those obtained from elliptical mineral clusters at high values of strain only strain profiles typically show a broad low strain region with a narrow high strain peak at the centre of the zones microstructures typically show distinct grain size reduction in both amphibole and feldspar towards the high strain region of shear zones and this is discussed in terms of deformation mechanisms related to strain softening a palaeostress estimate based on recrystallized feldspar grain sizes gives a differential stress of 32 mpa for the low strain region and 119 mpa in the shear zone centre electron probe analyses reveal chemical and mineralogical changes accompanying metamorphism within the shear zones this suggests local conditions favourable for ionic diffusion and the activity of fluids is implied

the role of transiently fine grained reaction products in syntectonic metamorphism natural and experimental examples it is proposed that one of the most important mechanisms of interrelationship between deformation and metamorphism is the facilitation of one of several grain size sensitive deformation mechanisms resulting from the formation of fine grained products of a metamorphic reaction during prograde metamorphism such effects are likely to be transient because grain coarsening and textural equilibration are likely in response to rising temperature conditions thus deformation mechanisms are often difficult to infer from such naturally deformed rocks in localized shear zones exhibiting retrogressive metamorphism evidence of enhanced deformability by such mechanisms is most likely to be preserved because cooling conditions inhibit grain growth and both deformed and undeformed rocks are likely to be preserved an experimental study has been made of the effects of deformation on serpentinite under conditions of progressive dehydration but with controlled pore pressure a marked weakening near linear viscous rheol

a new type of numerical experiment on the spatial and temporal patterns of localization of deformation in a material with a coupling of grain size and rheology abstract the patterns of localization of deformation in recrystallising materials are investigated via a series of two dimensional grain scale numerical simulations these simulations couple a grain size and strain dependant viscous rheology with grain size reduction and grain growth processes the simulations are able to predict the mechanical microstructural and strain evolution of the polycrystals to moderate strain and allow us to examine the nature of the time dependent feedback between mechanical and microstructural behavior it was found that significant strain localization occurred only when the grain size dependence of the viscosity was non linear and was greatly enhanced by the activity of the grain size modifying processes the intensity and location of the zone of strain localization varied spatially and temporally and displayed two length and time scales with the result that the finite strain state showed a much broader and hence less intense zone of localized deformation than the instantaneous state

sediments as tracers for transport and deposition processes in peri alpine lakes a case study the benthic sediment fingerprint is analysed in the small peri alpine lake levico trentino italy to identify the causes of recurrent phenomena of turbidity peaks particularly evident in a littoral region of the water body in order to study the sediment transport processes we exploit the fact that the sediment supply from the major tributary has a specific chemical composition which differs from that of the nearby lake basin three elements fe al k have been used as tracers to identify the source and the deposition patterns of tributary sediments and another typical element si has been critically analysed because of its dual allochthonous and autochthonous origin several samples of the benthic material have been analysed using sem eds and the results of the sedimentological characterisation have been compared with the patterns of sediment accumulation at the bed of the lake obtained using a three dimensional numerical model in response to the tributary supply under different external forcing and stratification conditions the coupled use of field measurements and numerical results suggests that the turbidity phenomena are strongly related to the deposition of the sediments supplied by the tributary stream and shows that it is possible to reconstruct the process of local transport when the tributary inflow is chemically specific

crustal deformation in the india eurasia collision zone from 25 years of gps measurements the india eurasia collision zone is the largest deforming region on the planet direct measurements of present day deformation from global positioning system gps have the potential to discriminate between competing models of continental tectonics but the increasing spatial resolution and accuracy of observations have only led to increasingly complex realizations of competing models here we present the most complete accurate and up to date velocity field for india eurasia available comprising 2576 velocities measured during 1991 2015 the core of our velocity field is from the crustal movement observation network of china i ii 27 continuous stations observed since 1999 56 campaign stations observed annually during 1998 2007 1000 campaign stations observed in 1999 2001 2004 and 2007 260 continuous stations operating since late 2010 and 2000 campaign stations observed in 2009 2011 2013 and 2015 we process these data and combine the solutions in a consistent reference frame with stations from the global strain rate model compilation then invert for continuous velocity and strain rate fields we update geodetic slip rates for the major faults some vary along strike and find that those along the major tibetan strike slip faults are in good agreement with recent geological estimates the velocity field shows several large undeforming areas strain focused around some major faults areas of diffuse strain and dilation of the high plateau we suggest that a new generation of dynamic models incorporating strength variations and strain weakening mechanisms is required to explain the key observations seismic hazard in much of the region is elevated not just near the major faults

textural and mechanical evolution with progressive strain in experimentally deformed aplite determining meaningful flow laws for the polyphase aggregates that characterize the crust is difficult because the strength of such aggregates depends on the strengths volume proportions and geometrical arrangement of their constituent phases and all of those factors may change with progressive strain in order to investigate some of these factors we have performed a series of axial compression experiments on a fine grained granite aplite over a range of shortening strains 20 85 and temperatures 700 900 c at 1 5 10 6 s 1 and 1400 mpa we have also done comparison experiments on pure quartz and feldspar aggregates the aplite is composed of 1 3 quartz and 2 3 oligoclase and microcline with minor biotite and magnetite the quartz grains are dispersed in a continuous matrix of feldspar optical and tem scale microstructures indicate that the deformation mechanisms of the quartz and feldspar grains in the aplites are similar to those of pure quartz and feldspar aggregates deformed at the same conditions the feldspars deform by semi brittle flow at 700 and 800 c and by recrystallization accommodated dislocation creep at 900 c while quartz deforms by recrystallization accommodated dislocation creep at 700 c and by climb accommodated dislocation creep at 800 and 900 c pure albite aggregates are stronger than quartz aggregates over this range of conditions however in the aplite an apparent strength reversal is inferred from grain strain measurements and changes in phase distribution with increasing temperature at 700 c the dispersed quartz grains in the aplite remain less deformed than the feldspar grains because the scarcity of quartz quartz grain boundaries limits recovery by grain boundary migration recrystallization causing the quartz strength to rise above that of feldspar however the aplite is weaker than a pure feldspar aggregate because cracking is enhanced by stress concentrations at the quartz grains at 800 c the two phases undergo approximately equal strain because easy dislocation climb in quartz allows individual grains to strain homogeneously the aplite strength is close to that of a pure feldspar aggregate at 900 c the initially continuous feldspar grains deform inhomogeneously and undergo microboudinage while the quartz grains deform homogeneously and gradually become more interconnected and undergo higher strain than the feldspars the aplite strength is initially closer to that of a feldspar aggregate but with increasing strain it becomes closer to that of a quartz aggregate the aplite undergoes strain softening over the whole range of conditions at 700 and 800 c the weakening is due to grain boundary migration recrystallization of feldspar in the fine grained crush products along grain scale faults at 900 c the weakening is due to a combination of grain boundary migration recrystallization of the feldspars plus increasing interconnection of the weaker quartz grains these results demonstrate that the textural evolution of a polyphase aggregate with increasing strain is determined by both the phase distribution and the deformation mechanism of each phase and that very high strains may be required to approach microstructural and mechanical steady state therefore measuring or calculating a flow law appropriate for granite at mid crustal conditions is not simple

creep cavitation bands control porosity and fluid flow in lower crustal shear zones shear zones channelize fluid flow in earth s crust however little is known about deep crustal fluid migration and how fluids are channelized and distributed in a deforming lower crustal shear zone this study investigates the deformation mechanisms fluid rock interaction and development of porosity in a monzonite ultramylonite from lofoten northern norway the rock was deformed and transformed into an ultramylonite under lower crustal conditions temperature 700 730 c pressure 0 65 0 8 gpa the ultramylonite consists of feldspathic layers and domains of amphibole quartz calcite which result from hydration reactions of magmatic clinopyroxene the average grain size in both domains is 25 mm microstructural observations and electron backscatter diffraction analysis are consistent with diffusion creep as the dominant deformation mechanism in both domains festoons of isolated quartz grains define c type bands in feldspathic layers these quartz grains do not show a crystallographic preferred orientation the alignment of quartz grains is parallel to the preferred elongation of pores in the ultramylonites as evidenced from synchrotron x ray microtomography such c type bands are interpreted as creep cavitation bands resulting from diffusion creep deformation associated with grain boundary sliding mass balance calculation indicates a 2 volume increase during the protolith ultramylonite transformation which is consistentmore with synkinematic formation of creep cavities producing dilatancy thus this study presents evidence that creep cavitation bands may control deep crustal porosity and fluid flow nucleation of new phases in creep cavitation bands inhibits grain growth and enhances the activity of grain size sensitive creep thereby stabilizing strain localization in the polymineralic ultramylonites less

a phenomenological numerical approach for investigating grain size evolution in ductiley deforming rocks abstract the sizes of recrystallised grains in exhumed ductile shear zones are often used to infer conditions of deformation i e stress strain rate and temperature here we present a simple numerical method of calculating the dynamic evolution of grain size during ductile deformation our phenomenological method is based on the fact that the dynamic competition between grain growth and recrystallisation will drive grains towards a steady state size at each time increment grain growth and reduction contributions are calculated with magnitudes which depend on the difference between the current grain size and a desired steady state grain size in our models we use a recrystallised grain size piezometer to calculate the steady state grain size for a given stress our numerical routine is incorporated into the sulec finite element package allowing us to explore spatial and temporal changes in grain size as a test we compare model results to measured grain sizes in quartz layers thinned and recrystallised around rigid garnet porphyroclasts under simple shear dominated deformation in the alpine fault zone of new zealand numerical models are able to replicate observed grain size variations with boundary conditions consistent with those constrained for the central alpine fault zone

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

on the rheologically critical melt fraction abstract with increasing melt fraction the strength of partially molten granite decreases from a value characteristic of solid competent rock to a value nearly equal to that of the melt previously published mechanical and microstructural data indicate that deformation in partially molten rocks often involves brittle processes thus the pressure in the melt is expected to be important in determining strength when the volume changes during deformation strength and fluid flow will be coupled by such parameters as permeability k storage capacity per unit volume or storativity s melt compressibility f grain size d fluid viscosity and strain rate experiments on brittle rock at low temperatures show that the strain rate at which the internal fluid pressure can be maintained constant is approximately proportional to k s an evaluation of published experimental data suggests that this relation also holds for partially molten granites indicating that the strength of these rocks depends on their transport properties since the permeability is related to below a critical melt fraction rcmf the melt pressure will change if deformation is not iso volumetric by assuming a power law relationship between k and we estimate that rcmf f d m 1 n 1 where m and n relate permeability to grain size and respectively

cretaceous high p granulites at milford sound new zealand metamorphic history and emplacement in a convergent margin setting granulite facies orthogneiss of the arthur river complex arc at milford sound western fiordland records a complex early cretaceous magmatic and orogenic history for the pacific gondwana margin that culminated in the emplacement and burial of a dioritic batholith the western fiordland orthogneiss wfo enstatite bearing mafic to intermediate protoliths of the arc and wfo intruded the middle to upper crust the early deformation history of the arc is preserved in the pembroke granulite where two pyroxene s1 assemblages that reflect p 750 c were only patchily recrystallized during later deformation s1 is cut by garnet bearing leucogabbroic to dioritic veins which are cut by distinctive d2 fractures involving anorthositic veins and garnet diopside plagioclase bearing reaction zones these zones are widespread in the arc and wfo and record conditions of p 14 kbar and t 750 c garnet clinopyroxene bearing corona reaction textures that mantle enstatite in both the arc and wfo reflect early cretaceous burial by approximately 25 km of continental crust most of the arc is formed from the milford and harrison gneisses which contain steeply dipping s4 assemblages that envelop the pembroke granulite and involve garnet hornblende diopside clinozoisite rutile and plagio clase with or without kyanite the p t history of rocks in western fiordland reflects pronounced early cretaceous convergence related tectonism and burial possibly related to the collision and accretion of island arc material onto the pacific gondwana margin

strain localisation in bimineralic rocks experimental deformation of synthetic calcite anhydrite aggregates abstract deformation of synthetic calcite anhydrite aggregates to large shear strains up to 12 4 at 600 c 300 mpa confining pressure and a constant angular displacement rate corresponding to a shear strain rate of 10 3 s 1 resulted in the first experimental observation of strain localisation from initially homogeneous rocks in contrast to experiments on pure calcite and anhydrite which deformed homogeneously to large strains 5 all experiments on calcite anhydrite mixtures resulted in heterogeneous deformation at 1 and the formation of narrow localised bands in the microstructures at 4 in these bands the amount of strain is at least twice as large as in the rest of the sample and individual grains of the same phase cluster and align thereby forming microstructural layering similar to planar fabrics in natural mylonites a switch in deformation mechanism in anhydrite from dislocation creep to diffusion creep and or grain boundary sliding occurs simultaneously with strain localisation it is concluded that deformation induced heterogeneous phase distributions cause local strength differences initiating strain localisation in the calcite anhydrite mixtures the study suggests that the presence of two phases in combination with a change in deformation mechanism may be responsible for strain localisation in natural poly mineralic mylonites

the ephemeral development of c shear bands a numerical modelling approach abstract c shear bands are ubiquitous structures in shear zones but their development is poorly understood previous research has determined they mostly occur in rocks with a high mechanical strength contrast using numerical models of viscoplastic deformation we studied the effect of the proportion of weak phase and the phase strength contrast on c shear band development during simple shearing to a finite strain of 18 we found that c shear bands form in models with 5 weak phase when there is a moderate or high phase strength contrast and they occur in all models with weak phase proportions 15 contrary to previous research we find that c shear bands form when layers of weak phase parallel to the shear zone boundary rotate forwards this occurs due to mechanical instabilities that are a result of heterogeneous distributions of stress and strain rate c shear bands form on planes of low strain rate and stress and not in sites of maximum strain rate as has previously been suggested c shear bands are ephemeral and they either rotate backwards to the c plane once they are inactive or rotate into the field of shortening and thicken to form x and triangle shaped structures

rheology and deformation mechanisms of an isotropic mica schist we have investigated the transitional semibrittle deformation of a mica schist 75 biotite by shortening cylinders cored at 0 45 and 90 to foliation to varying strains at confining pressures pc to 500 mpa constant strain rates e from 1 5 10 7 to 1 6 10 4 s 1 and temperatures t from 25 to 400 c deformation is concentrated within one or more throughgoing millimeter wide shear zones at all conditions these localize at low strains e 2 through the nucleation and coalescence of dense sets of intragranular microkink bands mkbs despite distinct differences in the relative number of mica grains oriented favorably for slip and kinking in different loading directions the differential stresses required for shear zone development vary little with fabric orientation biotite schist undergoes a transition from strain softening to steady strength mechanical response at confining pressures in the range 75 to 150 mpa the pressure sensitivity of strength characterized by the slope of the mohr envelope decreases from 0 5 at pc 100 mpa to 0 1 at pressures greater than 200 mpa reflecting the increasing contribution of glide and kinking in biotite at higher pressures however dilatancy associated with microcracking and void formation along mkb boundaries persists to at least 500 mpa within the pressure insensitive regime 200 pc 500 mpa temperature and strain rate dependencies of strength determined in stepping tests reveal a strong history dependence to flow that cannot strictly be described by a steady state constitutive law samples deformed in steps from low to high temperatures or fast to slow strain rates consistently exhibit stronger temperature and strain rate sensitivities than those deformed along t decreasing or e increasing paths path dependent effects may reflect differences in the degree to which inherited dislocation substructures are utilized or overprinted during later deformation increments by assuming an exponential relationship between differential stress d and strain rate e of the form e c exp d exp q rt we fit the data with two end member flow laws with a single activation energy q 89 kj mol and exponential constants ss 0 15 0 01 mpa 1 and ws 0 55 0 04 mpa 1 that account for the different responses observed along stepping paths that are strongly sensitive or weakly sensitive to t and e respectively application of the results to crustal deformation suggests that mica rich aggregates are weaker than other common rock types throughout a broad midcrustal depth range supporting the inference that retrograde reaction to phyllosilicates may be important in localizing crustal deformations within large faults and shear zones

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

finite element modelling of shear zone development in viscoelastic materials and its implications for localisation of partial melting the development of shear zones initiating on random weaker initial perturbations is modelled numerically for low deborah number viscoelastic materials considering the influence of effective viscosity contrast power law rheology strain softening and different imposed bulk deformation geometries ranging from pure to simple shear conjugate shear zones initiate at 90 to one another and rotate with increasing bulk deformation the basic pattern not being markedly influenced by the vorticity of imposed deformation the rate of propagation of individual conjugate shear zones is little affected by increased effective viscosity contrast between matrix and inclusion but is promoted by power law rheology however the most marked effect is observed for strain softening behaviour where rapid propagation produces straighter and narrower shear zones the localisation of strain is reflected in a correspondingly heterogeneous stress distribution in particular mean stress or pressure is higher in the extending near planar weaker zones of localised shear melting of gneissic or pelitic compositions is pressure dependent with free water present increased pressure promotes melting whereas the opposite is true for water absent melting for water present conditions a positive feedback could develop between localised shearing increased pressure and partial melting this is potentially more effective in concentrating melt in shear zones than shear heating where melt related softening has a negative feedback effect

mid crustal shear zone formation in granitic rocks constraints from quantitative textural and crystallographic preferred orientations analyses abstract this paper presents quantitative microstructural and crystallographic preferred orientation cpo analyses of an alpine amphibolite facies shear zone developed in the fibbia metagranite gotthard massif central alps the weakly deformed metagranite and orthogneiss at the margins of the shear zone are characterized by a bulk strain partitioning between harder coarse grained monomineralic aggregates derived from quartz and k feldspar porphyroclasts and softer fine grained plagioclase bearing shear bands a characteristic feature is a dilatant fracturing of strong quartz and feldspar aggregates cpos and microtextures suggest that quartz and k feldspar aggregates are dynamically recrystallized via dislocation creep while plagioclases show evidences of fluid assisted diffusive mass transfer and grain boundary sliding in the mylonite and ultramylonite shear zone core the porphyroclasts derived quartz and k feldspar layers are broken down to produce a polyphased matrix that is characterized by a homogeneous micron scale grain size and regular random distribution here the deformation of the whole aggregate occurs via a fluid assisted dissolution precipitation creep and grain boundary sliding referred as a fluid assisted granular flow we propose a model of shear zone formation associated with the nucleation of shear zone followed by lateral widening of the sheared domain the lateral broadening of the shear zone is driven by 1 the increase in fluid pressure in permeable albite oligoclase shear bands that results in expulsion of fluids to the shear zone margins and hydraulic fracturing of strong aggregates and 2 the thermodynamic re equilibration via metasomatic reactions of the shear zone walls

a tale of two viscosities abstract we describe a series of 12 large strain two dimensional simple shear numerical experiments on two phase linear and non linear viscous materials the modal percentage of the two phases their relative viscosities and the stress exponent of the dependence of viscosity on stress were systematically varied allowing us to observe the effects of these parameters on the flow behaviour of the system all experiments exhibited localisation but the extent and type of localisation varied considerably and the range of parameters allows both classical sc as well as sc structures to form the inter granular finite strain geometries also varied systematically although there is a tradeoff between non linearity of viscosity and the viscosity contrast that results in similar geometries for quite different mechanical behaviour the finite rotation of the harder phases was similar within any one experiment originally parallel passive marker lines thus remain sub parallel after deformation and rotation which may be mistaken for non rotation the internal deformability and interaction between grains allows meta stable grain elongation orientations to form without any additional consideration of external slip surfaces the slight variability of initial grain geometry and subsequent grain interactions leads to a wide variability in final grain shapes for the same initial viscosities

strain weakening enables continental plate tectonics much debate exists concerning the strength distribution of the continental lithosphere how it controls lithosphere scale strain localization and hence enables plate tectonics no rheological model proposed to date is comprehensive enough to describe both the weakness of plate boundary and rigid like behaviour of plate interiors here we show that the duality of strength of the lithosphere corresponds to different stages of microstructural evolution geological constraints on lithospheric strength and large strain numerical experiments reveal that the development of layers containing weak minerals and the onset of grain boundary sliding upon grain size reduction in olivine cause strain localisation and reduce strength in the crust and subcontinental mantle respectively the positive feedback between weakening and strain localization leads to the progressive development of weak plate boundaries while plate interiors remain

quantitative genetic modelling of the turkel anorthosite eastern ghats belt orissa india abstract principal components analysis of the anorthositic and adjoining felsic suite of rocks of the turkel anorthosite complex suggests that these are unlikely to be comagmatic this has been supported by 18 o values of these rocks in synoptic principal component space the compositions of anorthosite leuconorite and ferrodiorite of turkel are compared with the chemistry of coexisting phases based on experimental studies on proterozoic massif type anorthosite and naturally occurring high al gabbros hag such studies in congruence with the experimental observations strongly indicate that the turkel ferrodiorite represents the residual melt after separation of anorthosite and leuconorite major and minor element mass balance computations suggest that an ensemble of 15 32 wt anorthosite 34 74 wt leuconorite and 29 71 wt ferrodiorite may have originated from a melt which was derived from a hag magma through fractional crystallisation of 8 86 wt olivine and 11 37 wt ca poor clinopyroxene consistent with petrological observations a small amount of zircon and apatite assimilation from the granitoid country rocks has been invoked to best explain the observed high concentration of zr and rees in the anorthositic suite of turkel

regional slaty cleavage formation and fold axis rotation by re use and reactivation of pre existing foliations the fiery creek slate belt north queensland abstract a deformation history comprised of six separate deformation events of differing intensity has affected the rocks of the south palmer river region of the the hodgkinson province north queensland within this region a zone of pervasive slaty cleavage herein termed the fiery creek slate belt has developed as a result of the superposition of fabrics formed during several of these events the most important processes in the formation of this composite cleavage were the re use and reactivation of the favourably oriented steep n s trending s2 foliation by the intense fourth deformation event d4 this produced micro meso and macroscopic folds in an originally shallow s3 foliation produced during the intervening d3 deformation with an axial planar s2 s4 foliation the d4 stretching lineation l44 plunges subvertically to steeply north and indicates that shear during d4 was oriented steeply north south in the fiery creek slate belt d2 fold axes are interpreted to have formed in much shallower orientations than their present moderately n s plunging to subvertical orientations we consider this to be a result of d4 shear which caused variable degrees of rotation of d2 fold axes toward the d4 stretching lineation due to subparallelism of the bulk shortening directions of the d2 and d4 events near total destruction of the pre d4 foliations during slaty cleavage formation has produced a misleading impression of a simple deformation history there is no relationship between metamorphic grade and intensity of slaty cleavage development

rheological weakening of high grade mylonites during low temperature retrogression the exhumed continental ailao shan red river fault zone se asia abstract we present a detailed case study of an exhumed continental strike slip fault zone the ailao shan red river asrr strike slip fault zone to investigate how deformation promotes strain localization and how the weak second phases and fluids trigger rheological weakening during retrogression near the ductile to brittle transition during exhumation along the asrr strike slip fault zone in the diancang shan dcs metamorphic massif high temperature ductile deformation d 1 pervasively occurred during shearing and exhumation since late oligocene the high temperature microstructures and textures are in part or entirely altered by subsequent low temperature shearing d 2 since late miocene which is under the conditions of frictional viscous transition of k feldspar ca 450 c during further exhumation to the upper crustal levels the formation of d 2 microstructures and shear bands overprinted high temperature intracrystalline plasticity phases d 1 in mylonitic rocks depending on the main rock forming minerals the results also demonstrate that the brittle ductile transition involves a combination of different deformation mechanisms and possible rheological paths in quartz rich rocks quartz was deformed in the dislocation creep regime and records transition of microfabrics and slip systems during decreasing temperature which lasted until retrogression related to exhumation as a result grain size reduction associated with fluids circulating within the asrr strike slip fault zone at brittle ductile transition leads to reaction and texture weakening rheological weakening is the consequence of the syntectonic deformation fluid flow reaction softening reaction creep and textural softening the hydrous fluids resulted in hydration of silicates decompression occurred during shearing and as a result of tectonic exhumation all these results demonstrate that the exhumation through the ductile to ductile brittle transition involves a combination of different deformation mechanisms rheological transition features and feedbacks between deformation decreasing temperature and fluids in the asrr fault zone

high strain zones a unified model abstract where a shear zone is bounded by undeformed wall rocks and there is no volume change the deformation path within the zone on the bulk scale approximates a simple shear where the zone boundaries are stretched and the shear direction between the two boundaries is parallel to one of the principal stretching directions of the boundaries the deformation path within the zone is not a simple shear but still has monoclinic symmetry both simple shear and general monoclinic deformation paths should be regarded as special cases in nature because generally not only are the zone boundaries stretched but also the shear is oblique to the principal stretching directions of the boundaries in this general case the deformation path is triclinic we establish a model which includes this general case also taking into consideration volume change five independent parameters are required to characterize the rate of deformation of the flow four are sufficient to characterize the flow kinematics the model is generally triclinic and orthorhombic and monoclinic deformations are subgroups by varying the values of the characterizing parameters the various types of high strain zones can be represented the kinematics of flow within such zones and the accumulated finite deformation geometry are investigated many field observations such as for example lineations in transcurrent shear zones that vary between vertical and horizontal cleavage transected folds and the absence of sheath folds in some shear zones where folds have been significantly rotated can sometimes be better interpreted in the context of our general model

numerical modelling of strain localisation during extension of the continental lithosphere dipping reflectors in the upper mantle are observed on reflection seismic data and in some cases have been interpreted to represent mantle shear zones it is not clear however how these shear zones develop and under what circumstances previous attempts to model strain localisation by strain softening in the ductile regime have failed to show the development of mantle shear zones we present here results from a numerical model of lithospheric extension the model uses an elasto visco plastic rheology dependent on temperature pressure deviatoric stress and composition strain softening is incorporated into the mechanical model via a simple ad hoc strain viscosity relationship the results demonstrate that strain localisation can dominate lithospheric deformation in an extensional regime we illustrate the effects of varying the parameters which define the strain viscosity relationship on the geometry of lithospheric scale shear zones and the rift basin we also investigate the effects on shear zone development of introducing a variety of pre extension weak zones in the crust and or mantle our results suggest that if dipping reflectors observed on deep seismic data around the world are evidence of the existence of mantle shear zones there is a need to review our conceptual models of strain strength evolution during the development of continental rifts

structure and development of an anastomosing network of ductile shear zones a detailed structural analysis of an anastomosing shear zone network in metagabbros from the archean rainy lake zone canada revealed the existence of prevalent dextral and minor sinistral conjugate shear zones with the obtuse angle 130 facing the main shortening direction a typology of shear zone intersections confluences and other features shows that all shears formed during a single deformation event with dextral and sinistral shears being active together or in an alternating fashion in spite of the difficulty of establishing a complete kinematic sequence early and late shears can be distinguished the final angular pattern between dextral and sinistral shears is not an original feature dextral and sinistral shears formed at nearly right angles and the angles progressively opened towards the extension direction as a result of increasing strain the obtuse angles were achieved by the combined effects of continued shearing on newly forming shears and internal deformation of the lozenge shaped domains of lesser deformed rock bounded by the shears through time there was an increasing prevalence of dextral shears over sinistral ones the studied pattern and sequential analysis indicate that the bulk deformation was noncoaxial with a deformation regime evolving from a pure shear dominated dextral transpression to a higher vorticity dextral transpression

pre collisional anastomosing shear zones in the kohistan arc nw pakistan ductile strain localisation commonly forms a pattern of shear zones anastomosing around lenses of less deformed rock initiation and development history of anastomosing shear zones which are little documented are studied through the description of the structures and the deformation history of plutonic rocks that form the lower crust of the kohistan complex structures and textures developed in these rocks result from primary magmatic to solid state regional strain overprinted by anastomosing shear zones the primary strain was mainly acquired during magmatic emplacement at 100 90 ma strain localisation took place continuously from magmatic emplacement to solid state deformation during cooling of the plutons and formed 3 successive sets of shear zones set 1 is composed of associated discrete riedel and thrust shear zones developed above solidus conditions during sw ward thrusting continuous deformation from solidus to amphibolite facies conditions between 100 and 83 ma formed the second set of shear zones the lower amphibolite facies set 3 shear zones are differentiated by larger strain recorded in the thicker mylonitic zones and enlargement of the spacing between shear zones during cooling the anastomosing pattern of shear zones described here probably represents arc related deformation during subduction of the tethys oceanic lithosphere below the kohistan arc complex

experimental deformation of muscovite shear zones at high temperatures under hydrothermal conditions and the strength of phyllosilicate bearing faults in nature abstract layers of fine grained muscovite were hot pressed then sheared between alumina sliders to shear strains up to 2 at temperatures between 300 and 700 c confining pressures of 206 mpa and various pore water pressures high pore water pressures helped suppress dehydroxylation of the mica and permitted testing at temperatures higher than previously used shear strain rates between 10 3 s 1 and 10 7 s 1 were accessed using constant shear strain rate and stress relaxation testing except for strain rates 5 s 1 at 700 c deformation was strain rate and temperature insensitive but effective normal stress sensitive with a friction coefficient at yield of 0 3 rising with strain to 0 5 steady state sliding was not attained from the mechanical data and microstructural study deformation was inferred to have occurred by a mixture of brittle frictional and crystal plastic processes at 700 c and low strain rates the shear strength falls rapidly with a linear viscous characteristic in a way not previously reported this is tentatively attributed to rate control by viscous glide of basal dislocations extrapolating these results to geological strain rates we expect mica rich fault zones will exhibit frictional behaviour with a low friction coefficient between 0 25 and 0 5 giving way at mid crustal conditions to a rapid strength drop as viscous creep supervenes thus mid to lower crustal mica dominated faults of any orientation and terrains of schistose metapelitic rocks in the cores of orogeneic belts are expected to be very weak supporting shear stresses in the range 1 10 mpa the frictional behaviour of mica rich faults in upper crustal regions is however expected to be too strong to account alone for the proposed weakness of some major fault zones