shallow fault zone dilatancy recovery after the 2003 bam earthquake in iran earthquakes radiate from slip on discrete faults but also commonly involve distributed deformation within a broader fault zone especially near the surface variations in rock strain during an earthquake are caused by heterogeneity in the elastic stress before the earthquake by variable material properties and geometry of the fault zones and by dynamic processes during the rupture stress changes due to the earthquake slip both dynamic and static have long been thought to cause dilatancy in the fault zone that recovers after the earthquake decreases in the velocity of seismic waves passing through the fault zone due to coseismic dilatancy have been observed followed by postseismic seismic velocity increases during healing dilatancy and its recovery have not previously been observed geodetically here we use interferometric analysis of synthetic aperture radar images to measure postseismic surface deformation after the 2003 bam iran earthquake and show reversal of coseismic dilatancy in the shallow fault zone that causes subsidence of the surface this compaction of the fault zone is directly above the patch of greatest coseismic slip at depth the dilatancy and compaction probably reflects distributed shear and damage to the material during the earthquake that heals afterwards coseismic and postseismic deformation spread through a fault zone volume may resolve the paradox of shallow slip deficits for some strike slip fault ruptures

an assessment of dynamically recrystallized grainsize as a palaeopiezometer in quartz bearing mylonite zones abstract recrystallized grainsize has been measured in quartz and in some cases feldspar in about 200 samples from 22 mylonite zones from 3 discrete tectonic settings 1 1 supracrustal low temperature shallow dipping thrusts 2 2 deeper level major 100 km thrusts in high grade metamorphic rocks and 3 3 steeply dipping fault zones in granite within a low grade palaeozoic fold belt the concept of a steady state dynamically recrystallized grainsize largely unaffected by post creep processes e g static grain growth receives strong support especially from the microstructures and the independence of finite strain and grainsize in most mylonite zones however the usefulness of this grainsize for predicting stress magnitudes is limited by the apparently independent influence of metamorphic environment on grainsize the available theoretical and empirical stress grainsize relationships do not account for these effects the concept of a steady state grainsize and the ease of accurate measurement offer a potentially powerful method for predicting stress intensity during deformation predicted deviatoric stresses have been computed using twiss theoretical model for comparison with other crustal stress estimates and most samples fall in the range from 20 to 150 mpa in a few cases where discrete high strain mylonite zones occur within contemporaneously regionally deformed rocks the recrystallized grainsize indicates that stresses were 1 5 to 2 times higher within the zones there is a clear need for improved theoretical and or empirical stress grainsize relationships for quartz and other minerals before extensive further application of this technique

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

the influence of melting and melt drainage on crustal rheology during orogenesis partial melting significantly weakens crustal rocks by introducing a low viscosity liquid phase however near concomitant melt drainage can remove this weak phase potentially reversing the rheological effects such that the strength of a specific lithology depends on when the prograde pressure temperature path intersects a melting reaction how much melt is produced and how long this melt is retained before it is lost phase equilibria and mixed rheology modeling of typical metapelite and metagreywacke compositions indicate that these rocks undergo continuous but pulsed melt production during prograde metamorphism depending on whether melt removal is continuous or episodic and assuming geological strain rates the lithologies can retain a very low strength less than 1 mpa or transiently strengthen to 5 mpa following melt loss lithologies undergoing episodic melt loss can therefore cycle between being relatively weak and relatively strong components within a composite crustal section melt production retention and weakening in the middle to lower crust as a whole is more sustained during heating and melt production consistent with geodynamic inferences of weak melt bearing lower crust however the long term consequence of melting and melt loss is a 50 400 increase in the strength of residual lithologies the strengthening is more pronounced in metapelite than metagreywacke and is achieved through a combination of dehydration and the removal of the weak mica framework coupled to increased proportions of strong feldspars and garnet despite prolonged weakness melting and melt loss therefore ultimately result in a dry and elastic lower crust

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

transient creep of a composite lower crust 1 constitutive theory a composite model is proposed to describe the time dependent response of the earth s lower crust the motivation for such a model is twofold first new observations of widespread postseismic deformation indicate that the deep continental crust responds viscoelastically having both long and short term decay times second by any number of observationally based rationales the lower crust is compositionally and structurally heterogeneous over many length scales for heterogeneities that have much smaller characteristic lengths than the minimum deformation wavelength of interest the aggregate rheology can be described by composite media theory for wavelengths of the order of the thickness of the lower crust 25 40 km and larger composite theory may be applied to heterogeneities that are smaller than about several hundred meters or equivalent to the vertical extent of a thick lower crustal mylonitic shear zone the composite media theory developed here is constructed using both eshelby mori tanaka theory for aligned generalized spheroidal inclusions and a generalized self consistent method the inclusions and matrix are considered to be maxwellian viscoelastic a rheology that is consistent with past homogeneous models of postseismic stress relaxation the composite theory presented here introduces a transient response to a suddenly imposed stress field which does not appear in homogeneous maxwell models analytic expressions for the amplitude and duration of the transient and for the effective long and short term viscosities of the composite are given which describe the sensitivity to inclusion concentration to shape and to ratio of inclusion to matrix viscosity r

seismic and aseismic weakening effects in transtension field and microstructural observations on the mechanics and architecture of a large fault zone in se tibet

frictional viscous flow of phyllosilicate bearing fault rock microphysical model and implications for crustal strength profiles it is widely believed that around the brittle ductile transition crustal faults can be significantly weaker than predicted by conventional two mechanism brittle ductile strength envelopes factors contributing to this weakness include the polyphase nature of natural rocks foliation development and the action of fluid assisted processes such as pressure solution recently ring shear experiments using halite kaolinite mixtures as an analogue for phyllosilicaterich rocks for the first time showed frictional viscous behavior i e both normal stress and strain rate sensitive behavior involving the combined effects of pressure solution and phyllosilicates this behavior was accompanied by the development of a mylonitic microstructure a quantitative assessment of the implications of this for the strength of natural faults has hitherto been hampered by the absence of a microphysical model in this paper a microphysical model for shear deformation of foliated phyllosilicate bearing fault rock by pressure solution accommodated sliding along phyllosilicate foliae is developed the model predicts purely frictional behavior at low and high shear strain rates and frictional viscous behavior at intermediate shear strain rates the mechanical data on wet halite kaolinite gouge compare favorably with the model when applied to crustal materials the model predicts major weakening with respect to conventional brittle ductile strength envelopes in particular around the brittle ductile transition the predicted strength profiles suggest that in numerical models of crustal deformation the strength of high strain regions could be approximated by an apparent friction coefficient of 0 25 0 35 down to depths of 15 20 km

rheological behavior of high temperature garnet bearing migmatites the khondalite belt example north china craton the contribution presents microstructural and petrological results obtained on garnet bearing felsic migmatites from the khondalite belt north china craton the study focused on low melt fraction felsic migmatites in which peritectic garnet are strongly elongated and show plastic deformation these peritectic garnet grains show different typology depending on their petrographic location within the layered migmatite an elongated shaped garnet with sillimanite and biotite inclusion lies within al rich layer whereas poecilithic garnets are located within s rich layers forming a garnet quartz aggregate the study presents petrological microstructural observation and ebsd analyses of the garnet grains and garnet quartz aggregates phase diagram sections modeling allows to constrain the p t conditions of the deformation at 850 1000 c and 0 6 1 1 gpa at such high to ultra high temperature conditions a low melt fraction was produced between 1 and 12 generally under 7 forescattered diffraction and misorientation maps with orientation pole figure argue for dislocation creep with development of subgrains in garnets our results suggest that at high to ultra high temperature the quartz feldspar garnet solid framework of felsic migmatites strongly accommodate the deformation and contribute to weakening of migmatite even at low melt fraction

grain size effects on the closure temperature of white mica in a crustal scale extensional shear zone implications of in situ 40ar 39ar laser ablation of white mica for dating shearing and cooling tauern window eastern alps abstract in situ 40ar 39ar laser ablation dating of white mica grains was performed on samples from the footwall of a crustal scale extensional fault katschberg normal fault knf that accommodated eastward orogen parallel displacement of alpine orogenic crust in the eastern part of the tauern window this dating yields predominantly cooling ages ranging from 31 to 13 myr with most ages clustering between 21 and 17 myr folded white micas that predate the main katschberg foliation yield within error the same ages as white mica grains that overgrow this foliation however the absolute ages of both generations are older at the base 20 myr where their grain size is larger 300 500 m than at the top and adjacent to the hangingwall 17 myr of this shear zone where grain size is smaller

changing patterns of exhumation and denudation in front of an advancing crustal indenter tauern window eastern alps the changing shape of indenting crustal blocks during northward motion of the adriatic microplate induced migration of miocene doming and orogen parallel extension of orogenic crust in the tauern window new structural and kinematic data indicate that initial shortening of the penninic nappe pile in the tauern window by upright folding and strike slip faulting was transitional to coeval north south shortening and east west extension the latter was accommodated by normal faulting at the eastern and western margins of the window retrodeforming these post nappe structures in map view yields a map view reconstruction of the orogenic crust back to 30 ma including the onset of pronounced indentation at 21 ma this model supports the notion that indentation involved approximately equal amounts of north south shortening and orogen parallel stretching and extrusion toward the pannonian basin as measured from the indenter tip to the european foreland in the north and austroalpine units in the east comparison of areal denudation of the orogenic crust before and after indentation indicate that erosion associated with upright folding was the primary agent of denudation whereas extensional unroofing and limited erosion along normal faults at the eastern and western ends of the tauern window accounted for only about a third of the total denudation

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

fluid infiltration and volume change during mid crustal mylonitization of proterozoic granite king island tasmania the development of 10 30 m wide mylonite zones at mid crustal depths in late proterozoic granitoids on king island tasmania was associated with pervasive infiltration of low 18o fluids 5 to 7 on the scale of the shear zones syndeformational fluid rock interaction produced substantial differences in mineral composition and bulk rock chemistry among several adjacent shear zones which are hosted by the same granite in a shear zone at cape wickham with a normal slip component changes in whole rock chemistry between granite and mylonites indicate a gain of ca and losses of k and na during deformation which was nearly isovolumetric notable losses of k rb and si occurred during partial retrograde alteration of mylonites near the western margin of this shear zone the alteration suggests a component of up temperature fluid flow in contrast 3 km to the south east in a strike slip shear zone at disappointment bay complete albitization of plagioclase was associated with na gain and ca loss deformation also involved losses of mg and fe up to 60 volume gain occurred during the formation of closely spaced mesoscopic to microscopic quartz veins during mylonitization the substantial silica gain in this as well as in two mylonite zones further south east is interpreted to have been associated with upward flow of aqueous fluids along these shear zones on the basis of a gradient reaction model minimum time integrated fluid fluxes of 106 m3 m2 are estimated for the disappointment bay west shear zone

the brittle plastic transition and the depth of seismic faulting a simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow the two are separated by an abrupt brittle plastic transition which is assumed to be indicated by the lower limit of seismicity experimental studies however as well as the deformation structures of mylonites indicate that a broad transitional field of semi brittle behavior lies between these extremes this is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws for quartzofeldspathic rocks the semi brittle field lies between t1 the onset of quartz plasticity at about 300 c and t2 feldspar plasticity at about 450 c a model is presented in which the transition t1 does not correspond to a transition to bulk flow but to a change from unstable velocity weakening friction to stable velocity strengthening friction t1 thus marks the depth limit of earthquake nucleation but large earthquakes can propagate to a greater depth t3 t3 t2 which corresponds to the lower limit of dynamic frictional behavior in the semi brittle field and approximately to the peak in strength the zone between t1 and t3 is one of alternating behavior with flow occurring m the interseismic period and with co seismic dynamic slip occurring during large earthquakes this zone is characterized by mylonites interlaced by pseudotachylytes and other signs of dynamic faulting the transition t1 is also marked by a change in the generation mechanism of fault rocks from abrasive wear above which produces cataclastites to adhesive wear below which is proposed as an important generation mechanism of mylonites in the upper part of the semi brittle field

nonlinear dynamic failure process of tunnel fault system in response to strong seismic event strong earthquakes and faults have significant effect on the stability capability of underground tunnel structures this study used a 3 dimensional discrete element model and the real records of ground motion in the wenchuan earthquake to investigate the dynamic response of tunnel fault system the typical tunnel fault system was composed of one planned railway tunnel and one seismically active fault the discrete numerical model was prudentially calibrated by means of the comparison between the field survey and numerical results of ground motion it was then used to examine the detailed quantitative information on the dynamic response characteristics of tunnel fault system including stress distribution strain vibration velocity and tunnel failure process the intensive tunnel fault interaction during seismic loading induces the dramatic stress redistribution and stress concentration in the intersection of tunnel and fault the tunnel fault system behavior is characterized by the complicated nonlinear dynamic failure process in response to a real strong seismic event it can be qualitatively divided into 5 main stages in terms of its stress strain and rupturing behaviors 1 strain localization 2 rupture initiation 3 rupture acceleration 4 spontaneous rupture growth and 5 stabilization this study provides the insight into the further stability estimation of underground tunnel structures under the combined effect of strong earthquakes and faults

evidence of nonlinear elasticity of the crust from the mw7 6 manyi tibet earthquake satellite synthetic aperture radar sar interferometry shows that the magnitude 7 6 manyi earthquake of 8 november 1997 produced a 170 kilometer long surface break with up to 7 meters of left lateral slip reactivating a n76 e quaternary fault in western tibet the radar interferometric map reveals asymmetric along strike displacement profiles between the two sides of the surface rupture a pattern that cannot be explained with linear elastic theory this observation suggests that the elastic moduli of the crust in tension and in compression are different because of the presence of cracks in the crust at shallow depth a model indicates that a ratio of 2 between compressive and tensile elastic moduli can account for the observed asymmetry a ratio that is consistent with laboratory and borehole measurements

corrigendum to lower crustal flow and the role of shear in basin subsidence an example from the dead sea basin we regret that errors in the numerical evaluation of equations 11 and 12 resulted in an incorrect estimation of the eiective viscosity of the lower crust under the dead sea basin for the range of the observed heat ow in the region 45 53 mw m 3 the temperature range is between 280 and 343 c at a depth of 20 km and between 387 and 483 c at a depth of 30 km the values depend on the radiogenic heat production and its distribution within the crust with another commonly used value for the surface value of radiogenic heat production a0 1 2 mw m 3 and the logarithmic decrement of heat production with depth h 10 km e g 1 the temperature at a depth of 30 km under the dead sea basin can reach as high as 538 c

the displacement field of the landers earthquake mapped by radar interferometry geodetic data obtained by ground or space based techniques can be used to infer the distribution of slip on a fault that has ruptured in an earthquake although most geodetic techniques require a surveyed network to be in place before the earthquake1 3 satellite images when collected at regular intervals can capture co seismic displacements without advance knowledge of the earthquake s location synthetic aperture radar sar interferometry first introduced4 in 1974 for topographic mapping5 8 can also be used to detect changes in the ground surface by removing the signal from the topography9 10 here we use sar interferometry to capture the movements produced by the 1992 earthquake in landers california11 we construct an interferogram by combining topographic information with sar images obtained by the ers 1 satellite before and after the earthquake the observed changes in range from the ground surface to the satellite agree well with the slip measured in the field with the displacements measured by surveying and with the results of an elastic dislocation model as a geodetic tool the sar interferogram provides a denser spatial sampling 100 m per pixel than surveying methods1 3 and a better precision 3 cm than previous space imaging techniques12 13

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

rheological evolution of a mediterranean subduction complex abstract we use field and microstructural observations coupled to previously published p t time histories to track the rheological evolution of an intracontinental subduction complex exposed in the betic cordillera in the western mediterranean region the body of rock we focus on known as the nevado filabride complex nfc was originally part of the upper crust of the iberian margin it was subducted into hot asthenospheric mantle then exhumed back toward the surface in two stages an early stage of fast exhumation along the top of the subducting slab in a subduction channel and a late stage of slower exhumation resulting from capture by a low angle detachment fault rooted at the brittle ductile transition each stage of deformation in the nfc was punctuated by changes in the dominant deformation mechanism deformation during initial subduction of the complex was accommodated by pressure solution creep in the presence of a fluid phase the grain sizes stress magnitudes and estimated strain rates for this stage are most consistent with a thin film model for pressure solution in which the diffusion length scale is controlled by the grain size during the early stages of exhumation within the subduction channel deformation transitioned from pressure solution to dislocation creep due to increases in temperature which resulted in increases in both water fugacity and grain size each of which favor the dislocation creep mechanism differential stress magnitudes for this stage were 10 mpa and are consistent with simple models of buoyancy driven channel flow with continuing subduction channel exhumation deformation remained within the dislocation creep field because sequestration of free water into hydrous retrogressive minerals suppressed the pressure solution mechanism differential stresses progressively increased to 100 mpa near the mouth of the channel during cooling as the rocks moved into mid crustal levels during the final core complex stage of exhumation deformation was progressively concentrated into a narrow zone of highly localized strain beneath a mid crustal detachment fault localization was promoted by a transition from dislocation creep to dislocation creep accommodated grain boundary sliding at temperatures of 350 380 c grain sizes of 4 m and differential stress magnitudes of 200 mpa peak differential stress magnitudes of 200 mpa recorded just below the brittle ductile transition are consistent with byerlee s law in the upper crust assuming a vertical maximum principal stress and near hydrostatic pore fluid pressures overall the distribution of stress with temperature coupled to independent constraints on strain rate from field observations and geochronology indicate that the naturally calibrated hirth et al 2001 flow law for wet quartzite accurately predicts the rheological behavior of mid crustal rocks deforming by dislocation creep

regolith modeling and its relation to earthquake induced building damage a remote sensing approach regolith thickness is known as a major factor in influencing the intensity of earthquake induced ground shaking and consequently building damages it is however often simplified or ignored due to its variable and complex nature to evaluate the role of regolith thickness on earthquake induced building damage a remote sensing based methodology is developed to model the spatial variation of regolith thickness based on dem derived topographic attributes and geology regolith thickness samples were evenly collected in geological formations at representative sites of topographic attributes topographic attributes elevation slope twi distance from stream computed from the aster derived dem and a geology map were used to explore their role in spatial variation of regolith thickness stepwise regression was used to model the spatial variation of regolith thickness in erosional landscape of the study area topographic attributes and geology explain 60 of regolith thickness variation in the study area to test if the modeled regolith can be used for prediction of seismic induced building damages it is compared with the 2005 kashmir earthquake induced building damages derived from high resolution remote sensing images and field data the comparison shows that the structural damages increase with increasing regolith thickness the predicted regolith thickness can be used for demarcating site prone to amplified seismic response

flow stresses from microstructures in mylonitic quartzites of the moine thrust zone assynt area scotland recrystallized grain sizes subgrain sizes and dislocation densities of quartz grains in quartzose mylonitic rocks have been examined using optical and transmission electron microscopy tem the samples come from the moine thrust zone in the assynt district scotland they had been studied previously and described in detail with respect to their structural position in relation to the various thrusts in the region and to their preferred orientation stresses were derived from these samples using empirical and theoretical equations relating flow stress to the scale of the microstructures the stresses determined 43 244 mpa from recrystallized grain size 9 13 mpa from etched subgrain size 50 95 mpa from tem scale subgrain size and 75 147 mpa from dislocation density are not constant in individual samples simultaneous formation of the quartz fabrics and of the dynamically recrystallized grains during the period of mylonitization may have occurred under flow stresses ranging from 43 to 244 mpa a later dynamic recovery event reset subgrain sizes and dislocation densities to a constant value for each microstructure throughout the area there is insufficient empirical information available on the flow stress subgrain size relationship and on the effects of annealing recovery and annealing recrystallization to allow for a more detailed interpretation recrystallized grain size is still the most easily measured microstructural feature and the relationship of recrystallized grain size to flow stress has a sounder experimental basis than subgrain size or dislocation density the effect of chemical environment on the behaviour of all the microstructures is still unknown

numerical models of crustal scale convection and partial melting beneath the altiplano puna plateau we employ 2 d thermo mechanical modelling to study possible mechanisms for generating large scale crustal magmas in the altiplano puna region of the central andes the peak of ignimbrite activity in the late miocene and pliocene is associated in space and time with tectonic shortening and plateau uplift a seismic low velocity zone and other geophysical observations indicate that partial melting in the mid crust is still present under the ignimbrite province today we show that neither radiogenic heat production in a thickening crust nor shear heating due to tectonic shortening nor heat brought by intrusions of arc magmas into the mid crust can heat the mid crust to the degree and within the time frame suggested by the geologic petrologic and geophysical observations a viable mechanism to achieve high temperatures within the time constraints is convective heat and mass transfer by partially molten lower crust which itself was heated by enhanced mantle heat flow possibly associated with delamination of the mantle lithosphere during tectonic shortening and intensified magmatic arc activity the thermo mechanical models explain the mid crustal low velocity zone and the high and strongly variable surface heat flow observed in the altiplano convection by bulk flow of the crust appears when the middle and lower crust are mechanically weak quartz dominated rheology the basal heat flow from the mantle is high s 60 mw m 2 and tectonic shortening is active we argue that these conditions are satisfied in the central andes orogen 6 2002 elsevier science b v all rights reserved

formation of dome and basin structures results from scaled experiments using non linear rock analogues abstract dome and basin folds are structures with circular or slightly elongate outcrop patterns which can form during single and polyphase deformation in various tectonic settings we used power law viscous rock analogues to simulate single phase dome and basin folding of rocks undergoing dislocation creep the viscosity ratio between a single competent layer and incompetent matrix was 5 and the stress exponent of both materials was 7 the samples underwent layer parallel shortening under bulk pure constriction increasing initial layer thickness resulted in a decrease in the number of domes and basins and an increase in amplitude a arc length l wavelength and layer thickness hf samples deformed incrementally show progressive development of domes and basins until a strain of ey z 30 is attained during the dome and basin formation the layer thickened permanently while a l and increased a dominant wavelength was not attained the normalized amplitude a increased almost linearly reaching a maximum of 0 12 at ey z 30 during the last increment of shortening ey z 30 to 40 the domes and basins did not further grow but were overprinted by a second generation of non cylindrical folds most of the geometrical parameters of the previously formed domes and basins behaved stable or decreased during this phase the normalized arc length l hf of domes and basins is significantly higher than that of 2d cylindrical folds for this reason the normalized arc length can probably be used to identify domes and basins in the field even if these structures are not fully exposed in 3d

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

flow laws of multiphase rocks calculated from experimental data on the constituent phases abstract an attempt has been made to develop expressions for the flow laws of polyphase rocks in terms of the volume fractions and flow laws of their constituent phases the flow strengths predicted by the proposed model for granite diorite diabase and lherzolite are in good agreement with previously determined experimental values the study allows one to calculate to the first approximation the flow laws of a large number of rocks at geologic conditions based on the experimentally determined flow laws of a relatively small number of monomineralic aggregates

fabric development in experimentally sheared quartzites abstract a fine grained quartzite was deformed under a combination of simple shear and axial compression to varying amounts of shear strain 1 3 to 2 9 the experiments were conducted at 800 c 1500 mpa and an axial strain rate of 10 6 s in the dislocation creep regime shear zone boundaries were oriented at 45 or 30 to the compressior axis the progressive development of deformation microstructures including planar fabrics and c axis preferred orientations was studied as a function of increasing shear strain elongate original grains define a foliation s which rotates from 45 to 0 to the shear zone boundary with increasing strain producing a sigmoidal foliation in a single shear zone a second foliation invariant in orientation is defined by elongate recrystallized grains which lie at about 30 40 to the shear zone boundaries no c surfaces parallel to the shear zone boundaries were observed however extensional crenulation cleavages c defined by parallel zones of recrystallized grains as well as impurity and melt segregations were observed even at low strain lying at approximately 30 to the shear zone boundaries and these could easily be mistaken for c surfaces c axis preferred orientations of original grains define symmetric type i crossed circle girdles at low strain 2 with the greater concentration of c axes in the direction of shear these features are very similar to those observed in natural ductile shear zones

rheological implications of heterogeneous deformation at multiple scales in the late cretaceous sierra nevada california late cretaceous deformation in the east central sierra nevada arc of california was heterogeneous at multiple scales we quantify this heterogeneous deformation at centimeter meter and kilometer scales in the vicinity of the gem lake shear zone and infer variations in effective viscosity from our data at the centimeter scale variations in strain of different clast types in conglomerate suggest that effective viscosities varied by less than an order of magnitude lithology controlled the magnitude and nature of deformation recorded by clasts at the meter scale cleavage refraction between stratigraphic layers records variations in finite strain comparison of our observations with cleavage refraction models suggests a maximum effective viscosity contrast of 10 between layers bulk composition controls variations in deformation of the different layers at the kilometer scale variation in finite strain magnitude and orientation in similar rock types both within and outside the shear zone demonstrates that deformation inside the zone was relatively intense comparing these results to numerical models of heterogeneous regional deformation we estimate that the angle of oblique convergence inside the zone was 15 10 while outside it was greater than 60 these kilometer scale results imply regional deformation was moderately strike slip partitioned during the late cretaceous and suggest regional effective viscosity varied by a factor between 6 and 17 at each scale of observation the apparent range of effective viscosity varies by an order of magnitude or less consequently we infer that relatively modest strength variations produced the structures observed at hand sample to tectonic scales

brittle to ductile transition due to large strains along the white rock thrust wind river mountains wyoming abstract the deformed foreland of the rocky mountain cordilleras is characterized by major reverse faults that bring precambrian basement rocks to the surface the white rock thrust in the wind river mountains of wyoming is one such fault that developed at fairly shallow levels in the crust detailed study of grain size variations and textures within the fault zone provide a framework for discussion of the mechanics of movement along the fault textural criteria are suggested for distinguishing between dominantly brittle and dominantly ductile deformation a transition from brittle fracturing to ductile deformation is seen with increasing strain and resultant decrease in grain size along the fault zone diffusional creep in the final stages of deformation leads to strain softening and may allow large strains to take place along narrow zones at low deviatoric stresses this may be one way of obtaining large displacements along major faults at shallow levels in the crust

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

the internal deformation of the peridotite nappe of new caledonia a structural study of serpentine bearing faults and shear zones in the koniambo massif we present a structural analysis of serpentine bearing faults and shear zones in the koniambo massif one of the klippes of the peridotite nappe of new caledonia three structural levels are recognized the upper level is characterized by a dense network of fractures antigorite and polygonal serpentine form slickenfibers along fault planes with distinct kinematics as a result the upper level keeps the record of at least two deformation events the first associated with the growth of antigorite wnw ese extension the second with the growth of polygonal serpentine nw se compression the lower level coincides with the serpentine sole of the nappe which consists of massive tectonic breccias overlying a layer of mylonitic serpentinites the sole records pervasive tangential shear with top to sw kinematics and represents a decollement at the base of the nappe the intermediate level is characterized by the presence of several meters thick conjugate shear zones accommodating ne sw shortening like the sole these shear zones involve polygonal serpentine and magnesite as the main syn kinematic mineral phases the shear zones likely root into the basal decollement either along its roof or occasionally around its base compared to top to sw shearing along the sole the two deformation events recorded in the upper level are older

large strain deformation and strain partitioning in polyphase rocks dislocation creep of olivine magnesiowustite aggregates aggregates composed of olivine and magnesiowustite have been deformed to large strains at high pressure and temperature to investigate stress and strain partitioning phase segregation and possible localization of deformation in a polyphase material samples with 20 vol of natural olivine and 80 vol of mg0 7fe0 3 o were synthesized and deformed in a gas medium torsion apparatus at temperatures of 1127 c and 1250 c a confining pressure of 300 mpa and constant angular displacement rates equivalent to constant shear strain rates of 1 3 3 10 4 s 1 the samples deformed homogeneously to total shear strains of up to 15 during constant strain rate measurements the flow stress remained approximately stable at 1250 c while it progressively decreased after the initial yield stress at the lower temperature mechanical data microstructures and textures indicate that both phases were deforming in the dislocation creep regime the weaker component magnesiowustite controlled the rheological behavior of the bulk material and accommodated most of the strain deformation and dynamic recrystallization lead to grain refinement and to textures that were not previously observed in pure magnesiowustite and may have developed due to the presence of the second phase at 1127 c olivine grains behaved as semi rigid inclusions rotating in a viscous matrix at 1250 c some olivine grains remained largely undeformed while deformation and recrystallization of other grains oriented for a slip on 010 resulted in a weak foliation and a texture typical for pure dry olivine aggregates both a slip and c slip on 010 were activated in olivine even though the nominal stresses were up to 2 orders of magnitude lower than those needed to activate these slip systems in pure olivine at the same conditions

mode of crustal extension determined by rheological layering the results of numerical modelling show that the mechanical stratification of the crust provides the fundamental control on fault spacing and ultimately the mode of extension macroscopic pre existing structures and weaknesses are often thought to govern the behaviour of continental crust under extension and many prior studies have focussed on the effect of heterogeneities in triggering faulting and exhumation of lower crustal material the role played by such features is in fact subordinate to that exerted by the rheological contrast from upper to lower crust in our numerical model the temperature gradient dictates the transition from a strong brittle upper crust to a weaker ductile lower crust we see two distinct extension modes that depend on this vertical rheological contrast the distributed faulting mode and the metamorphic core complex mode the ratio of the integrated strength of the upper to lower crust is an indicator of the resulting mode of extension when this strength ratio is small i e the lower crust is relatively strong the result is distributed densely spaced faulting with limited slip on each fault and no exposure of lower crustal rocks an example could be faulting in the north sea a large strength ratio hence a weak lower crust that flows easily leads to stretching being strongly localised onto relatively few normal fault zones each fault accommodates large displacements eventually dissecting the upper crust and resulting in exhumation of the lower crust this is representative of the metamorphic core complexes of the western u s a and the aegean the actual critical strength ratio for the transition between modes will depend upon secondary factors such as the relative thickness of the lower crust with respect to the upper crust and the degree of fault weakening d 2005 elsevier b v all rights reserved

the significance of grain scale stresses in the kinetics of metamorphism during metamorphism phases grow and dissolve dominantly along grain boundaries bulk diffusion models for the development of metamorphic textures often assume that diffusion is homogeneous in regions of each phase rather than being localised along phase boundaries under isotropic stress chemical potentials of components involved in reactions are likely to be buffered along grain boundaries between phases thus inhibiting diffusion and growth motive forces must exist which overcome this and it is proposed here that local heterogeneous stresses will be important a mathematical theory for stress controlled growth is derived this theory predicts that the growth rates of two phases along their mutual interface will be linked in a quantifiable way it forms a basis for considering the important effects of stresses induced by local volumetric changes during metamorphism and it provides a link between these effects and those related to pressure solution in deforming rocks

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

the effect of bimineralic composition on extensional processes at lithospheric scale we investigate how lithospheric scale compositional heterogeneities affect kilometric scale deformation processes to this end we perform numerical experiments of lithospheric extension in which we vary the moho temperature and the mineralic composition of the mantle and the crust in both the crust and the mantle we use an explicit bimineralic composition by randomly distributing two mineral phases in the materials comparison of our models to simulations using an implicit bimineralic composite one average viscous flow laws for a two phase aggregate crust and mantle demonstrates that an explicit bimineralic composition assimilated to heterogeneities succeeds in explaining observations related to the formation of rifted margins such a 1 the absence of a sharp deformation zone at the brittle ductile transition bdt 2 the initiation of the rifting process as a wide delocalized rift system with multiple normal faults dipping in both directions 3 the development of anastomosing shear zones in the middle lower crust and the upper lithospheric mantle similar to the crustal scale anastomosing patterns observed in the field or in seismic data 4 the preservation of undeformed lenses of material leading to lithospheric scale boudinage structure and resulting in the formation of continental ribbons as observed along the iberian newfoundland margin

continental transforms a view from the alpine fault abstract continental transform faults are dominantly highly localized strike slip shear zones hundreds of kilometers long that accumulate tens to hundreds of kilometers of displacement from work on the alpine fault we pose the questions what is the deep structure of a continental transform and how does the displacement become localized we review research on the alpine fault and propose a model in which the fault partitions at depth into a steep zone extending into the mantle with largely fault parallel motion and a flat ductile decollement in the lower crust the fault localizes around two thirds of the plate motion within a 100 km wide zone of distributed deformation a review of other active continental fault systems suggests that variation between them may reflect their tectonic origins the nature of the crust in which they develop the presence of a significant oblique component of motion and the displacement rate all however have evidence for the development of a single principal fault zone that carries 50 of the total displacement and extends as a localized zone of shear into the upper mantle we review mechanisms of strain weakening and suggest that localization of a principal fault may be initiated in the seismogenic crust and through a series of positive feedbacks eventually extend through the lower crust into the upper mantle

slow and fast deformation in the dora maira massif italian alps pseudotachylytes and inferences on exhumation history abstract the dora maira massif dm western alps is a type example of an ultra high pressure uhp metamorphic terrain the occurrence of pseudotachylytes within the val gilba area of dm led to speculation regarding the role of seismic slip in the exhumation of the massif in this study new field microstructural and geochemical data on the pseudotachylyte veins and their mylonitic gneiss host are presented the formation of pseudotachylyte veins occurred late in the exhumation history at 20 1 0 5 ma and at shallow depths

on factors controlling the depth of interseismic coupling on the hikurangi subduction interface new zealand abstract we apply a finite difference numerical thermal model to the shallow region of the hikurangi subduction margin north island new zealand this subduction zone is characterised by subduction of thick and buoyant crust of the hikurangi plateau and along strike variations in subduction obliquity onshore surface heat flow low in the south high in the north inferred pore fluid pressure and seismic style locked in the south aseismic in the north the model reproduces low onshore surface heat flow observed in the southern hikurangi margin we propose that greater surface heat flow in the north is caused by shear heating and convective heat flow by fluids escaping from the interface the model predicts that the hikurangi megathrust is comparatively cold so that if the lower limit of the interface seismogenic zone is thermally controlled it should occur near the intersection with the hangingwall moho this agrees with observation in the southern locked segment but thermal control alone cannot explain the shallow interseismic locking depth in the central and northern margin in crustal strength curves changing fluid pressure is capable of moving the brittle viscous transition vertically by tens of kilometres we propose that subduction zone seismic style depends on the fluid pressure regime both in the hangingwall and along the interface and define four end member types of megathrust fault segments contrary to many previous studies we suggest that the high fluid pressure low shear strength end member is fully coupled with potential for large interplate earthquakes in the hydrostatic case the fault is stronger but the brittle viscous transition occurs significantly shallower than in the near lithostatic case inferred to promote aseismic creep at relatively shallow levels along strike variation in seismic style on the hikurangi megathrust can therefore be explained by variation in fluid pressure state along the margin

weaker than weakest on the strength of shear zones thin laterally continuous ultramylonites within kilometer scale ductile shear zones may control mid lower crustal strength where deformation is localised interconnected phyllosilicate networks are commonly suggested to be the weakest geometry a shear zone can reach yet fine grained polyphase mixtures are commonly found in the cores of high strain zones we study a continental strike slip shear zone which deformed granulite facies quartzofeldspathic migmatitic gneisses at retrograde amphibolite to greenschist facies conditions a brittle feldspar framework and interconnected phyllosilicate networks control the strength of the lower strain protomylonites and mylonites respectively whereas the ultramylonites comprise a fine grained mixture of the host rock minerals the localisation of strain in ultramylonites demonstrates how fine grained polyphase mixtures can be weaker than and supersede interconnected phyllosilicate networks with increasing shear strain this contradicts the common assumption that interconnected layers of phyllosilicates is the weakest state a shear zone can reach

present day lithospheric strength of the eastern alps and its relationship to neotectonics 1 we calculate the present day lithospheric strength of the eastern alps along the new reflection seismic profile transalp to examine vertical and lateral strength variations and their implications on neotectonic activity of the eastern alps the large scale geometry of the eastern alps and the spatial distribution of upper and lower crustal layers and the lithospheric mantle is constrained by the deep seismic line two rheological models coupled to a kinematic thermal model that accounts for the thermal evolution of the eastern alps for the last 30 myr are investigated for the present day lithospheric configuration in the eastern alps models with strong model a and weak model b crustal rheologies predict the european and the adriatic plates to be stronger than the central zone of the orogen comprising the region between the inntal fault and the periadriatic fault model a is characterized by a brittle ductile boundary between 14 and 9 km depth and strong coupling of the mechanically strong lower crust to the upper mantle whereas model b suggests the presence of a thick decoupling zone between the upper crust and the upper mantle and a shallower brittle ductile boundary 7 10 km of these end member scenarios model a is in better agreement with neotectonic data including seismicity down to the upper lower crust boundary within the adriatic plate uplift of the central zone of the eastern alps and the southern alps and eastward escape of fault bound blocks such deformation pattern is best explained by lateral extrusion upon north south compression supporting a strong weak strong configuration of tectonic units along the transalp line

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

extracting damages caused by the 2008 ms 8 0 wenchuan earthquake from sar remote sensing data abstract bad weather conditions usually limit the acquisition of optical remote sensing images while all day and all weather synthetic aperture radar sar shows the ability of providing timely remote sensing data for emergency response and rescue works after earthquake because sar is sensitive to the surface changes caused by earthquake the modified electromagnetic behaviour by geological disasters and the collapse of buildings can be recorded in sar images as backscattering intensity changes absolute radiometric calibration was performed to sar products to derive backscattering coefficient sigma nought from image digital number dn based on change detection methods advanced land observing satellite alos phased array type l band synthetic aperture radar palsar data and terrasar x data acquired for the ms 8 0 wenchuan earthquake were used to extract earthquake damage information this study revealed that landslides showed stronger backscattering and barrier lakes showed lower backscattering in post earthquake 10 m alos palsar images comparing to pre earthquake and collapsed buildings showed lower backscattering compared to un collapsed buildings in 1 m terrasar x image results showed that sar data with different spatial resolutions are useful for different earthquake damage information extraction medium spatial resolution sar data e g 10 m alos palsar data were efficient for secondary geological disaster extraction high resolution sar data e g 1 m terrasar x data with the help of ancillary gis data or high resolution optical data could be used to extract building collapse information in urban areas this study indicates that sar remote sensing data can provide earthquake damage information at early emergency stage and assist the field surveying further damage assessment and post earthquake reconstruction

tectono metamorphic evolution of a high to medium grade ductile deformed metagabbro metadiorite from the arenosa creek shear zone western sierras pampeanas argentina abstract textural and compositional changes affecting a quartzose metagabbro metadiorite in a ductile shear zone which is part of the early palaeozoic famatinian orogenic belt of the western sierras pampeanas argentina allow reconstruction of its tectono metamorphic evolution as well as the metamorphic conditions achieved on the basis of paragenetic associations and microfabrics three overprinted deformation events are differentiated a a relict ductile event developed within the granulite facies at temperatures exceeding 800 c and pressures 5 5 5 kbar b a two stage mylonitic deformation event under high to medium grade metamorphic conditions at temperatures 500 700 c and pressures between 6 and 7 kbar and c a brittle deformation event developed at low greenschist facies below 400 c evidence of the first event is preserved in the protolith although the subsequent deformation obliterated it partially slight modal and chemical changes were detected between the protolith and its mylonitic products deformation ages of the mylonites that vary from 441 9 1 9 ma to 438 7 1 9 ma are internally consistent and compatible with existing geological and geochronological data for the region suggesting that during the famatinian orogeny the western margin of gondwana was characterized by several episodes of ductile deformation that varied in time and space

frictional viscous flow in mylonite with varied bimineralic composition and its effect on lithospheric strength abstract a theoretical composite flow law is presented for mylonite containing interconnected layers of a weak mineral undergoing power law creep and porphyroclasts of a stronger mineral undergoing fracture and frictional sliding such mylonite termed clastomylonite is said to undergo frictional viscous fv mylonitic flow its bulk strength is expressed as a function of bimineralic composition temperature effective pressure and shear strain rate as well as the material parameters for the constituent minerals the fv flow law predicts that the rheology of clastomylonite is predominantly non linear viscous and only slightly pressure sensitive for most bimineralic compositions fv mylonitic flow is shown to occupy a depth interval between cataclastic flow involving adhesive wear in the upper crust and fully viscous mylonitic flow at greater depths the transition from adhesive wear to fv mylonitic flow is related to the onset of dislocation creep glide plus climb in the weakest phase and is inferred to coincide with a crustal strength maximum a peak strength of about 80 mpa is calculated with a combination of byerlee s constants for frictional sliding of granite and the fv flow law for granitic clastomylonite 30 vol quartz at elevated fluid pressures this value falls within the range of flow stresses independently obtained from quartz palaeopiezometry in many greenschist facies granitic mylonite zones

equatorially trapped rossby waves in the presence of meridionally sheared baroclinic flow in the pacific ocean topex poseidon altimeter data are analyzed for the 8 5 year period november 1992 to may 2001 to investigate the sea surface height ssh and geostrophic velocity signatures of quasi annual equatorially trapped rossby waves in the pacific the latitudinal structures of ssh and both components of geostrophic velocity are found to be asymmetric about the equator across the entire pacific with larger amplitude north of the equator the westward phase speeds are estimated by several different methods to be in the range 0 5 0 6 m s 1 these observed characteristics are inconsistent with the classical theory for first vertical first meridional mode equatorially trapped rossby waves which predicts a phase speed of about 0 9 m s 1 with latitudinally symmetric structures of ssh and zonal velocity and antisymmetric structure of meridional velocity the observations are even less consistent with the latitudinal structures of ssh and geostrophic velocity components for other modes of the classical theory the latitudinal asymmetries deduced here have also been consistently observed in past analyses of subsurface thermal data and altimeter data and have been variously attributed to sampling errors in the observational data a superposition of multiple meridional rossby wave modes asymmetric forcing by the wind and forcing by cross equatorial southerly winds in the eastern pacific we propose a different mechanism to account for the observed asymmetric latitudinal structure of low frequency equatorial rossby waves from the free wave solutions of a simple 1 5 layer model it is shown that meridional shears in the mean equatorial current system significantly alter the potential vorticity gradient in the central and eastern tropical pacific the observed asymmetric structures of sea surface height and geostrophic velocity components are found to be a natural consequence of the shear modification of the potential vorticity gradient the mean currents also reduce the predicted westward phase speed of first meridional mode rossby waves improving consistency with the observations

magnetic fabric in amphibolized eclogites and serpentinized ultramafites in the marianske lazně complex bohemian massif czech republic product of exhumation driven retrogression abstract the largest body of metabasic rocks in the bohemian massif the marianske lazn complex mlc represents an early paleozoic ophiolite complex which experienced poly metamorphic evolution with an early high pressure amphibolite followed by eclogite and subsequent amphibolite facies overprint this study aims to investigate the fabric relationships among the rock bodies and to analyze the deformation recrystallization history of the mlc using the anisotropy of magnetic susceptibility ams magnetic susceptibility of the mlc rocks varies according to the presence of para and ferromagnetic minerals as well as according to the primary lithologies in weakly magnetic rocks km the results of ams indicate that the orientation of magnetic minerals in most rocks is related to the last exhumation and retrogression event that occurred in amphibolite facies conditions however there are some well preserved eclogites possessing the magnetic fabrics that are at least partially related to prograde or peak pt conditions during metamorphism in addition massive metagabbros that suffered only weak deformation partially preserve the intrusive magnetic fabrics

earthquakes and rock deformation in crustal fault zones the physical origin of earthquakes lies ultimately in the geological structure of fault zones and the deformation processes that occur therein in response to tectonic stress although the possibility now exists that the nucleation regions for large earthquakes at depths of 10 km may shortly become directly accessible by deep drilling our present knowledge of fault structure and the shallow earthquake source is derived largely from a variety of indirect sources these include seismological studies surface studies of fault zones and earthquake ruptures geodetic information on modes of fault slip geophysical constraints on fault zone structure and rheology and information garnered from materials science and experi mental rock deformation studies of fault zone structure and the rock products of faulting provide complementary information on deformation processes at depth in fault zones although descriptions of fault rocks are widespread in the geological literature e g spry 1969 higgins 1971 it is only in the past decade that they have begun to be interpreted in the context of the physical conditions and processes prevalent in seismically active fault zones at different crustal depths sibs on 1977 watts williams 1979 anderson et a11983 wise et ai 1984 such interpretations are still at an early stage but the deformation textures and structural associations of fault rocks have already been shown to have the potential to yield information on such diverse topics as shear stress levels power dissipation and seismic efficiency during earthquake faulting on fluid pressure levels and episodic fluid flow accompanying

synthetic aperture radar interferogram of the 1995 kobe earthquake and its geodetic inversion crustal deformation from the mw 6 9 january 17 jst 1995 kobe japan earthquake was detected by synthetic aperture radar interferometry sar more than four fringes with ellipsoidal shape were observed surrounding the nojima fault on northern awaji island suggesting a shortening in the range of more than 11 75 4 cm on the east side of this fault the sar sensor shows ellipsoidal fringes with an uplift zone in the rokko mountain range and sharp subsidence around the kobe area in addition to sar interferometry leveling and gps surveys were conducted by the geographical survey institute of japan gsi the permanent gps observation sites of the gsi monitored ground displacements from this earthquake with daily updates based on these geodetic data we estimated the faulting mechanism of the earthquake the analytical procedure was an iterative linearized least squares method a multi fault system consisting of six subfaults was constructed the locations of the estimated model faults were close to those of the active faults in this area running from the nojima fault on awaji island to the rokko faults in the kobe area with a change in strike angle near the ohtsuki fault in the kobe area the slip vectors of the nojima fault on awaji island had a large component of reverse dip slip of about 2 m and strike slip of about 2 m in the kobe area the amount of slip ranged from 0 3 to 1 2 m and was small compared with the slip on the nojima fault the results of these slip vectors agreed well with the results of other seismic geodetic and field studies

creep events at the brittle ductile transition 1 we present an analytic formulation to model creep events at the transition between brittle behavior in the crust and viscous behavior in ductile shear zones we assume that creep events at the brittle ductile transition bdt are triggered by slip on optimally oriented fractures or network of fractures filled with weak ductile material these events are expressed as transient flow in ductile shear zones likely aided by the release of crustal fluids we show that the creep in the shear zone can be modeled as the motion of a forced damped oscillator composed of a brittle viscoelastic crust a ductile shear zone and a creeping zone of fractures at the bdt the time scale of the events varies between seconds to thousands of years depending on the viscous elastic and brittle plastic properties of the fractured bdt the shear zone and the crust the nature of the events depends on the aspect ratio of the shear zone thickness hw to the length of the fractured zone w we find that thick shear zones with small fractures at the bdt are stiff and generate creep oscillations thin shear zones with well connected fractures over a large width have very small stiffness and are well lubricated they generate slow creep events or steady creep event the former are similar to transient slip events and the latter to creep at the far field tectonic rates the viscosity of the shear zone w enhances lubrication if it is small and stiffness if it is large

strain distribution within a km scale mid crustal shear zone the kuckaus mylonite zone namibia the subvertical kuckaus mylonite zone kmz is a km wide crustal scale proterozoic dextral strike slip shear zone in the aus granulite terrain sw namibia the kmz was active under retrograde amphibolite to greenschist facies conditions and deformed felsic and minor mafic gneisses which had previously experienced granulite facies metamorphism during the namaqua orogeny lenses of pre to syn tectonic leucogranite bodies are also deformed in the shear zone pre kmz deformation d1 is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering shear strain within the kmz is heterogeneous and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges strain is localized along the edge of leucogranites and between gneissic lozenges preserving d1 migmatitic foliations strain localization appears controlled by pre existing foliations grain size and compositional anisotropy between leucogranite and granulite the local presence of retrograde minerals indicate that fluid infiltration occurred in places but most ultramylonite in the kmz is free of retrograde minerals in particular rock composition and d1 fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space with deformation localization along planes of rheological contrast and along pre existing foliations therefore the spatial distribution of strain in crustal scale ductile shear zones may be highly dependent on lithology and the orientation of pre existing fabric elements in addition foliation development and grain size reduction in high strain zones further localizes strain during progressive shear maintaining the anastomosing shear zone network established by the pre existing heterogeneity

mapping active fault associated with the 2003 mw 6 6 bam se iran earthquake with aster 3d images abstract satellite images reveal the geomorphology and geometry of an active fault associated with an earthquake that caused over 40 000 deaths in southeast iran this earthquake of moment magnitude mw 6 6 occurred at 01 56 52 utc on december 26 2003 near the towns of bam and baravat an active fault that ruptured during the earthquake can be seen on three dimensional 3d pre and post earthquake images generated from advanced spaceborne thermal emission and reflection radiometer aster visible and near infrared vnir data the interpretation and analysis of satellite images suggest that a dextral fault extends for 65 km through the bam baravat region it strikes north northwest and steps leftward at a segment boundary between two towns transpressional stress at this step over bend probably triggered the 2003 earthquake rupture as suggested also by source mechanism solution of the bam earthquake and by landforms observed in the field in the past 2000 years bam has had no earthquake as destructive as the one in 2003 the potential for this catastrophe could have been identified through a combination of satellite remote sensing technique and field reconnaissance

weakening of deforming granitic rocks with layer development at middle crust abstract in the middle crust deformation of granitic rocks is characterized by crystal plastic deformation of quartz and cataclasis of feldspar for common granitic rocks with textures of framework feldspar and interstice filling quartz brittle strength of feldspar is a close approximation of the bulk rock strength because feldspar forms the load bearing framework with deformation these rocks tend to develop localized high strain zones the high strain zones commonly consist of quartz rich layers and feldspar rich layers we have examined how the strength of granitic rocks changes with the volume fraction of these layers by combining the flow laws developed by handy et al handy m r wissing s b streit l e 1999 frictional viscous flow in mylonite with varied bimineralic composition and its effect on lithospheric strength tectonophysics 303 175 191 our results suggest a dramatic weakening of the bulk rock strength at an early stage of the layer development we also found that formation of muscovite in the layer structures by breakdown of feldspar can result in further weakening in the layer structure the bulk rock strength with mica bearing layers is reduced by a significant amount compared with the mica absent layers our analysis suggests that layer development with syntectonic formation of mica can be a major source of weakening for granitic rocks

modelling of porphyroclasts in simple shear and the role of stress variations at grain boundaries abstract grain scale numerical experiments involving simple shear of a two phase non linear viscous material are described and compared with mineral fish or lozenge shaped porphyroclasts such as muscovite two types of 2d models are considered either a single elongate grain or two parallel elongate grains in both cases supported by a weaker polygonal grain matrix the relative viscosities of the contrasting grain structures were systematically varied allowing us to observe the effects of non linear viscous rheology on the resulting microstructure and flow patterns the results show that the finite rotation of the hard elongate grain was similar within any one experiment but was largely influenced by viscosity contrast the geometry of the model and the imposed shear strain models involving single elongate hard grains show increasing instability at their ends and less strain compatibility with the deforming matrix grains as the viscosity contrast is increased in the paired grain models the greatest variation in the matrix grain microstructure is seen in the region where the two hard grains are oriented at a high angle to the direction of shear finally we consider the changes in intragranular stress by comparing microstructural observations using different viscosities with the distribution of stress in space and during progressive shear in the plane approximately parallel to the maximum principal stress direction 1 a localised change of stress occurs across and along the interface between the hard and soft grains variations in the mean stress at these boundaries are directly attributable to changes in the minimum principal stress we propose that with shear strains greater than 2 it is the minimum principal stress that can control diffusion processes at the grain boundary rather than mean stress in conclusion we suggest that our models have the potential for providing useful insights into why metamorphic reactions can occur at the interface between a porphyroclast and matrix at high shear strains and how stress distribution can control the initiations of such reactions

strength of two phase rocks a model based on fiber loading theory abstract in order to predict the strength of two phase rocks in terms of the component volume fraction the aspect ratio of the strong phase and the strength contrast between the strong and weak phases a simple model is developed according to a fiber loading theory modified for short fiber composites the analytical solution derived from elastic theory is found to hold in good approximation for rocks consisting of power law viscous minerals there is a good agreement between the predicted results and the experiments for two phase aggregates anhydrite halite calcite halite fe ag and clinopyroxene plagioclase the model is compared with several previous different models

strain rate influence on fracture development in experimental ductile multilayers abstract the far field strain rate is a crucial parameter that controls the transition between brittle and ductile deformation we have used analogue experiments to study the strain rate influence on the development of brittle fractures in a ductile composite material plasticine multilayer models were deformed under coaxial boundary conditions at three different strain rates to analyse the transition from non localised deformation to the development of a brittle fracture network that accommodates part of the deformation the results show that tension cracks and voids are the first macroscopic structures that nucleate after an early stage of ductile deformation coalescence and collapse of these structures lead to the development of brittle shear fractures the evolution of fracture orientations lengths and displacements was systematically analysed the ratio of the accumulated fracture displacement vs fracture length dmax l depends not only on the total deformation but also on the strain rate at which the system is deformed the accumulated displacement with respect to fracture length increases with strain rate

magnitude of weakening during crustal scale shear zone development we describe and apply a field based approach for calculating the bulk strength of a heterogeneous material to a crustal scale shear zone defining the margin of the ma parry sound domain in the grenville province of southeastern ontario using a numerical method we calculate bulk strength defined as effective viscosity as the ratio between the surface traction needed to deform a square block in simple shear and the velocity gradient across that block we use natural shear zone geometries to define the internal block structure and assign internal relative viscosities based primarily on textural criteria the margin of the parry sound domain developed into the km scale twelve mile bay shear zone accommodating several tens of km of transport while the domain interior remained rigid fracturing and fluid infiltration drove development of an amphibolite facies meter scale shear zone network that evolved into the twelve mile bay structure we analyzed three sites across the w5 km wide strain gradient from near the granulitic domain to the large scale shear zone the rocks at the shear zone margin weakened by approximately 30 those in the core weakened by at least 77 and probably by an order of magnitude these values lie between but differ substantively from the isostress and isostrain rate bounds indicating that a numerical approach such as presented here markedly improves the accuracy of bulk strength calculations

finite element modelling of the propagation of a pressure solution cleavage seam abstract the propagation of a cleavage seam is modelled by a two dimensional finite element technique which extends fletcher and pollard s elastic anticrack theory to a composition dependent viscous rheology the viscous solution is obtained by repeated solutions of the force equilibrium equations at successive time steps with the viscosity of each element varying as a function of its strain history the rheology assumed follows previous modelling by robin the rock is modelled as a quartz mica mixture in which deformation proceeds by diffusion of silica the viscosity of an element depends on its proportion of quartz varying from a minimum for intermediate quartz mica mixtures to higher values for either pure quartz or pure micas as a result of its incremental strain an element either loses or gains quartz i e volume and therefore changes its viscosity for the next strain increment the system can be either open silica escapes the system all individual elements lose volume or closed individual elements may gain or lose volume but the total volume of the system is preserved at the start of a run a nucleus of a cleavage seam is introduced as a thin layer of elements with a lower viscosity corresponding to a higher mica content than the rest of the rock the stress concentration at the tip of the seam leads to a weakening of the elements in front of it through loss of quartz this results in crack propagation for both open and closed systems all other parameters being equal the speed of propagation is greater for open systems than for closed ones

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

deformation of granitoids at low metamorphic grade ii granular flow in albite rich mylonites abstract the high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite rich polyphase aggregates these aggregates formed by retrograde breakdown reactions from intermediate plagioclase and k feldspar the predominant deformation mechanism changes in granitoids as the grain size is reduced coarse grained low strain examples are deformed by a combination of intracrystalline plasticity quartz and fracturing feldspar in the mylonites intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non cataclastic granular flow of polyphase aggregates consisting largely of albite and quartz deformation appears to be stable probably because grain growth in albite quartz mixtures is inhibited these fine grained aggregates are mechanically weaker than pure quartz aggregates thus the change in deformation mechanism mainly due to feldspar breakdown reactions appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar the rheological behaviour of albite dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar dominated lithologies 1 feldspar in the presence of aqueous fluids can be an important mineral controlling the rheology at low metamorphic grades that is below amphibolite facies p t conditions 2 the occurrence of granular flow of fine grained polyphase aggregates in low grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz

subduction interface processes recorded by eclogite facies shear zones monviso w alps abstract the monviso ophiolite lago superiore unit constitutes a well preserved almost continuous upper fragment of oceanic lithosphere subducted at c 80 km depth thereby providing a unique opportunity to study mechanical coupling processes and meter scale fluid rock interactions occurring at such depths in present day subduction zones it is made of i a variably thick 50 500 m section of eclogitized basaltic crust associated with minor calcschist lenses overlying a 100 400 m thick metagabbroic body and of ii a c 1 km thick serpentinite sole we herein focus on the three major eclogite facies shear zones found at the top of the unit at the boundary between basalts and gabbros and between gabbros and serpentinites respectively strain localization occurred at lithological interfaces irrespective of material strength while ductile deformation dominates along the shear zones local brittle behavior is demonstrated by the existence of numerous eclogite breccias of fe ti metagabbros and widespread garnet fractures possibly linked with intermediate depth eclogite facies micro seismicity these m to hm sized fragments of fe ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro serpentinite boundary lower shear zone lsz pervasive and focused fluid flow is attested in the lsz by significant alteration of bulk rock compositions weakening of the rocks and widespread crystallization of hydrous parageneses by contrast the intermediate shear zone isz shows evidence for more restricted short range fluid flow the activity of both the isz and lsz ceased during early lawsonite eclogite facies exhumation when deformation localized deeper within the serpentinite sole allowing for the detachment and preservation of this large ophiolitic fragment

analysis of continental midcrustal strain localization induced by microfracturing and reaction softening 1 low angle extensional shear zones which often characterize the brittle ductile transition of the continental crust are seen here to result from strain localization the potentially destabilizing deformation mechanism is assumed to be the progressive transformation of fractured coarse feldspar grains into white mica as observed in the east tenda shear zone alpine corsica the coupling between microfracturing and feldspar to mica reaction is coeval with strain localization that occurred in that field case at a depth close to 15 km this reaction is proposed as the main destabilizing factor responsible for the onset of localization with feldspar having a stationary dislocation creep flow stress larger than mica to test this hypothesis a rheological model is constructed based on the field observations for a mixture of three phases mica quartz and feldspar deforming at a common strain rate the phase concentrations change with time according to the feldspar to mica reaction which takes place only if feldspar grains are fractured a condition detected with the mohr coulomb criterion the tendency for the strain to localize is assessed by numerical means for the structure composed of an upper crust gliding rigidly over the lower crust which sustains an overall simple shear the onset of strain localization is defined by an increase of at least two orders of magnitude in strain rate over part of the lower crust the upper crust gliding velocity has to be increased by at least a factor of 5 for localization to occur the time lapse for this velocity change determines the depth of the shear zone 15 17 km the kinetics of the metamorphic reaction and the final amount of white mica control its width 1 4 km the time of the shear zone formation is less than half a million years