mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

population wide garnet growth zoning revealed by la icp ms mapping implications for trace element equilibration and syn kinematic deformation during crystallisation the strong partitioning of many trace elements into garnet and their slow diffusivities in both garnet and the rock matrix means that their distribution may record valuable petrogenetic information not documented by major elements in metamorphic rocks complex trace element growth zoning in garnet porphyroblasts from a garnet grade metapelite from the barrovian sequence of the sikkim himalaya is assessed using quantified la icp ms raster mapping coupled with x ray micro computed tomography the data document systematic changes in the zoning patterns from early to late nucleated crystals and also suggest that the ree y chemistry incorporated into garnet is dependent on persistent disequilibrium in the rock volume there is evidence for hree y diffusion haloes surrounding growing garnets and a heterogeneous hree y distribution in the rock matrix annuli superimposed on oscillatory zoning are not consistent with formation during some rock wide event but are dependent on the spatial disposition of the garnet annuli are interpreted to reflect an integrated history of varying growth rates and the incorporation of pre existing heterogeneities due to relatively slow matrix diffusivities conversely smooth zoning of many transition metals indicate that their distribution in garnet may be controlled by equilibrium partitioning between garnet and the rock matrix significant rotation of garnet porphyroblasts during growth is revealed due to immobility of cr over the duration of the crystallisation interval and overprinting of the heterogenous precursor cr distribution strain rate estimates derived from this zoning are on the order of 10 11 10 12 hbox s 1

one kilometre thick ultramylonite sierra de quilmes sierras pampeanas nw argentina abstract we describe a 1 km thick ultramylonite forming the high strain base of the 3 5 km thick el pichao shear zone in the sierra de quilmes this shear zone thrusted granulite facies migmatites onto amphibolite facies rocks during the 470 ma famatinian orogeny strain grades upwards from ultramylonites to weakly sheared migmatites across the 3 5 km zone and the mylonitic rocks define a geochemical field narrower than the protolith suggesting they underwent mixing and homogenization through shearing ultramylonites this thick are uncommon the width of a shear zone in the absence of significant compositional rheological contrasts controlling strain localization is controlled by the balance between shear heat generation and diffusion under typical crustal conditions a strain rate of 10 12 s 1 is required to form a 1 km thick ultramylonite and this is achieved when large movement velocities are imposed across the shear zone we postulate that the el pichao shear zone and its thick ultramylonite accommodated a significant fraction of convergence velocities driving the orogeny and that the wide mylonitic shear zones characteristic of the cambrian ordovician deformation of the sierras pampeanas result from the convergent movement being taken up by only a few active major shear zones

the effect of microstructural and rheological heterogeneity on porphyroblast kinematics and bulk strength in porphyroblastic schists abstract the kinematic record and bulk viscous strength of polyphase rocks depend in part upon the relative strengths and distributions of rheologically distinct fabric elements here we explore the effects of microstructural and rheological heterogeneity in porphyroblastic schists electron backscatter diffraction and petrographic analyses reveal asymmetric microboudinage of staurolite indicating relative rotation of staurolite porphyroblasts synchronous with bulk non coaxial strain boudinage and relative rotation both require porphyroblast matrix shear coupling based on 2d optical observations the extent of the coupling appears related to the initial and boudinaged staurolite grain shape and orientation as well as the geometry of heterogeneities such as mica domains or shear bands we designed 2d finite element numerical models to assess the role of microstructural variation and rheological heterogeneity on the degree of porphyroblast matrix shear coupling and bulk viscous strength model results indicate that the bulk strength of a three phase system comprising inclusion weak domain and matrix is sensitive to the relative proximity of weak and strong domains particularly at high viscosity contrasts i e matrix weak 10 the threshold for bulk weakening below the matrix strength occurs over a narrow range of weak domain viscosities matrix weak 2 6 5 5 regardless of the relative abundance and spatial distribution of weak domains kinematic decoupling of porphyroblasts occurs at low viscosity contrasts when weak domains are proximal matrix weak 2 5 but for all other spatial distributions and modal abundances investigated kinematic decoupling occurs at viscosity contrasts of matrix weak 15 20 these data indicate that bulk weakening due to rheological heterogeneity is not necessarily coincident with kinematic decoupling

porphyroblast rotation and strain localization debate settled the topic of whether or not porphyroblasts rotate relative to a fixed external reference frame during deformation has engendered much debate mainly owing to the fact that pre deformational orientations of porphyroblasts and their inclusion trails can rarely be reconstructed in 20 years of

mantled porphyroclast gauges we investigate the behavior of the isolated mantled porphyroclast in a shear zone the method employed is a finite element model three distinct phases clast mantle and matrix are present the rheologies are power law with exponents ranging from 1 to 5 and the far field boundary condition is simple shear the effective viscosity of the mantle is assumed to be less than those of the clast and the matrix we show for which parameter sets mantled porphyroclasts reach super horizontal stabilization with respect to the shear plane and sense clasts in natural mylonites frequently exhibit similar orientations which are interpreted as stable inclinations the systematic examination of the matrix mantle clast system allows for the construction of attractor maps that can be directly used as gauges for i the effective viscosity contrast between matrix and mantle ii the production rate of mantle material around the clast as a function of the bulk shear strain and iii for the total shear strain the necessary data required to use the attractor maps are simple geometrical parameters that can be measured in the field i e clast aspect ratio clast inclination versus the shear plane mantle thickness and mantle and clast area this new method successfully reproduces the characteristics of natural porphyroclasts and is in good agreement with data from natural shear zones q 2005 elsevier ltd all rights reserved

emplacement of the giant kunene amcg complex into a contractional ductile shear zone and implications for the mesoproterozoic tectonic evolution of sw angola massif type anorthosites are voluminous plagioclase dominated batholiths that occur worldwide but only during the proterozoic eon granitoids are closely associated with the anorthosites and they collectively may form an amcg suite i e anorthosite mangerite charnockite granite many fundamental questions regarding the origin of the amcg complexes remain unresolved including their tectonic setting and mode of emplacement here we present new data from reconnaissance structural mapping u pb la icp ms zircon dating and 40ar 39ar amphibole and mica dating from a 500 km2 area straddling the western margin of one of the world s largest 15 000 km2 but also least studied amcg suites the kunene complex kc in angola the oldest recognized pre kunene deformation event d1 produced a steep wnw ese striking s1 fabric in supracrustal rocks migmatite and paleoproterozoic 1800 1780 ma granodiorite before 1680 ma this was folded by the d2 event resulting in steep n s striking s2 leucosomes and gneissosity in the rocks surrounding the kc concordant magmatic layering and lamination and high to low temperature gneissic foliation in the marginal kunene anorthosite and coeval megacrystic granite are parallel with s2 fabrics in the paleoproterozoic country rocks mineral stretching lineations l2 in all rock types across intrusive contacts are steep and associated with west side up thrust shearing overlapping u pb zircon baddeleyite ages from the kunene megacrystic granite and kunene anorthosite with 40ar 39ar mica ages from the mantling paleoproterozoic country rocks attest to progressive contraction thrusting and exhumation of the paleoproterozoic country rocks onto the kunene anorthosite in the interval 1410 1380 ma we suggest that thermal softening of the country rocks caused by heat dissipation from the earliest phase 1500 1410 ma of ponding of hot anorthositic magma in the deep crust localized d2 deformation at the margin of the kc our data point to a tectonic model whereby anorthosite sheets were emplaced at a high angle to the shortening direction and along host rock s2 anisotropy deep in the crust and cooled during progressive ductile thrusting and exhumation of the country rocks these results question the current view that proterozoic amcg suites are emplaced in extensional environments as large diapiric bodies and favor formation in a continental arc setting

kinematic vorticity analysis and evolving strength of mylonitic shear zones new data and numerical results the kinematic vorticity number is an important quantity in structural geology and tecton ics giving a nonlinear ratio of simple shear to pure shear deformation we use natural obser vations and numerical models to show how rigid clast methods for determining the kinematic vorticity number w k are compromised where strain localization occurs at the matrix clast interface our numerical results show that the critical shape factor cutoff between perma nently rotating and stable clasts used to determine w k is highly sensitive to coupling between the clast and the matrix this fi nding provides an elegant explanation for the fact that rigid clast methods tend to underestimate w k relative to other methods we present numerically determined envelopes for clast behavior across a range of kinematic vorticity numbers clast shape factors and matrix clast coupling our numerical models show that the shape pre ferred orientations of feldspar clasts trend toward the positions of mica fi sh with increasing localization at the clast boundary suggesting that mica fi sh behave as highly lubricated clasts our data and numerical results show that the clast matrix interface may be several orders of magnitude weaker than the surrounding matrix and that weak interfaces can lead to a marked drop in the bulk shear strength of faults and shear zones

distinguishing and correlating multiple phases of metamorphism across a multiply deformed region using the axes of spiral staircase and sigmoidal inclusion trails in garnet schists from the appalachian orogen in south east vermont have undergone multiple phases of garnet growth these phases can be distinguished by the trend and relative timing of foliation inflexion or intersection axes fias of foliations preserved as inclusion trails in garnet porphyroblasts the relative timing of different generations of fias is determined from samples containing porphyroblasts with two or three differently trending fias developed outwards from core to rim multi fia porphyroblasts schists from south east vermont show a consistent pattern of relative clockwise rotation of fia trends from oldest to youngest four populations or sets of fias can be distinguished on the basis of their relative timings and trends from oldest to youngest the four sets have modal peaks trending sw ne w e nnw sse and ssw nne these peaks show that each of the four fia sets has a statistically consistent trend at all scales across a 35 125 km area containing numerous mesoscopic and macroscopic folds the fias of set 4 are defined by inclusion trails that are continuous with matrix foliations have trends subparallel to most folds and are inferred to have developed contemporaneously with these structures conversely sets 1 to 3 are oblique to and pre date most matrix foliations and folds all four fia sets occur in siluro devonian rocks and must have formed in the acadian orogeny the lack of statistically significant differences in the distribution of fia trends across the study area and their consistent relative timings in multi fia porphyroblasts despite a complex regional deformation history involving numerous phases of folding at all scales suggest the porphyroblasts have not rotated relative to one another the change in fia trend with time resulted from rotation of the kinematic reference frame of bulk flow possibly as a consequence of the reorganization of lithospheric plates responsible for acadian orogenesis recognition of distinct generations of fias provides a means of distinguishing different phases of porphyroblast growth four periods of garnet porphyroblast growth occurred in the schists of south east vermont this growth was heterogeneously distributed on the cm2 m2 scale no single porphyroblast records all stages of growth and adjacent samples from the same or dissimilar rock types commonly contain porphyroblasts that preserve different sequences of growth factors that may have been responsible for switching porphyroblast growth on and off at this scale include i subtle differences in bulk chemical composition ii oscillating levels of heat owing to the buffering effect of endothermic garnet forming reactions iii channelized infiltration of fluids with localized fluid buffering of bulk composition and iv cyclic controls on the rates of diffusion and material transport of reactants either by channelized fluid flow or by a changing pattern of microfracturing during foliation development consistency in fia trend and relative timing provide a new method for potentially distinguishing and correlating successive metamorphic events or even phases of metamorphism within a progressive tectonothermal event along and across orogens using a consistent pattern of core to rim changes in fia trend multiple phases of growth of a single porphyroblastic mineral can be quantitatively distinguished allowing correlation of different phases of growth around and across macroscopic folds the relative timing of growth of different porphyroblastic minerals can also be quantitatively determined using fia data and correlated around and across macroscopic folds conceptually the paragenetic history preserved in each generation of porphyroblast growth in the form of chemical zoning and the minerals in inclusion trails could be combined to produce a more detailed p t t deformation path than previously determined

viscous inclusions in anisotropic materials theoretical development and perspective applications abstract theories and numerical solutions for a viscous ellipsoid in an infinite anisotropic viscous medium subjected to far field homogeneous deformation lie at the heart of self consistent homogenization models and multiscale simulations of texture and fabric development in earth s lithosphere there is considerable literature on ellipsoid inclusions focused on anisotropic elastic materials published in multi disciplinary fields to make this body of work more accessible as well as to advance viscous inclusion studies an effort is made here to summarize recent advances and to further develop formally more explicit and where possible analytic solutions for incompressible viscous materials the point force concept and equivalent inclusion method of eshelby are used together with the green function approach this leads to generalized equations for ellipsoid inclusion behaviors in anisotropic materials in the particular case of isotropic materials the new mathematical development here enables the use of existing methods for elastic materials to get solutions for corresponding viscous inclusion problems efficiently and accurately a 2d formulation is also presented for elliptic cylinders in plane straining flows of anisotropic materials using the same green function method that is adopted for 3d inclusions the 2d formulation is benchmarked with existing analytic solutions a reconnaissance investigation to compare the behaviors of 2d elliptic inclusions and triaxial ellipsoids in a matrix of planar anisotropy undergoing far field plane straining flows suggests that conclusions drawn from 2d cannot be applied to 3d in anisotropic cases the application of the viscous inclusion theory to the rheologically heterogeneous and non linear lithosphere is discussed by regarding a heterogeneous element as an ellipsoidal inclusion embedded in a hypothetical homogeneous equivalent matrix whose effective rheology is obtained through micromechanical homogenization and assuming that the conditions of scale separation are satisfied the viscous inclusion theory forms the backbone of multiscale models for natural deformation and the accompanying texture and fabric development

basin seismic stratigraphy and aspects of prospectivity in the ne baffin bay northwest greenland abstract subsurface mapping in the northeastern part of baffin bay offshore northwest greenland indicates the presence of large basement structures and sedimentary basins we identify eight seismic stratigraphic units that can be genetically related to the regional geology and tectonic development the seismic interpretation includes the top of paleocene to eocene volcanic rocks which cover most parts of the southern and western study area using this horizon as a time marker the seismic units are divided into pre cenozoic and palaeogene to neogene strata based on comparison to equivalent units exposed onshore west greenland and canada and sampled by exploration wells farther south in the disko west offshore nuuk region our interpretation of the faults and tectonic elements suggest that the main extensional faults and basin formation took place mainly during the cretaceous and into the paleocene subsequent compressional tectonics and inversion were accompanied by strike slip faulting and local transtensional faults and flexures this last phase of deformation corresponds to the well documented eurekan orogeny in ellesmere island and northern greenland that developed as a result of northward motion and possible counterclockwise rotation of greenland relative to the north american plate we surmise that these tectonic phases exerted a strong control on basin infill by activation of hinterland sediment sources during cretaceous and palaeogene times the deep basins may include cretaceous and palaeogene organic rich sediments with hydrocarbon source potential we estimate that maturation could have started during the eocene

fabric transitions in quartz via viscoplastic self consistent modeling part i axial compression and simple shear under constant strain quartz is a common crustal mineral that deforms plastically in a wide range of temperatures and pressures leading to the development of different types of crystallographic preferred orientation cpo patterns in this contribution we present the results of an extensive modeling of quartz fabric transitions via a viscoplastic self consistent vpsc approach for that we have performed systematic simulations using different sets of relative critical resolved shear stress of the main quartz slip systems we have performed these simulations in axial compression and simple shear regimes under constant von mises equivalent strain of 100 1 73 assuming that the aggregates deformed exclusively by dislocation glide some of the predicted cpos patterns are similar to those observed in naturally and experimentally deformed quartz nevertheless some classical cpo patterns usually interpreted as result from dislocation glide e g y maxima due to prism slip are clearly not developed in the simulated conditions in addition we reported new potential preferred orientation patterns that might happen in high temperature conditions both in axial compression and simple shear we have demonstrated that cpos generated under axial compression are usually stronger that those predicted under simple shear due to the continuous rotation observed in the later simulations the fabric strength depends essentially on the dominant active slip system and normally the stronger cpos result from dominant basal slip in followed by rhomb and prism c slip whereas prism slip does not produce strong fabrics the opening angle of quartz 0001 fabric used as a proxy of temperature seems to be reliable for deformation temperatures of 400 c when the main slip systems have similar behaviors

the dilemma of asymmetric porphyroclast systems and sense of shear abstract asymmetric porphyroclast systems have long been used as independent shear sense indicators based on their clast tail asymmetry and or stair stepping tail geometry over the last two decades advances in exploring the mechanical behavior of rigid clasts in a viscous flow has shed new light on understanding the formation of porphyroclast systems embedded in noncoaxial viscous flow and subsequently quantifying many aspects of flow kinematics using examples from naturally deformed rocks combined with results from recent advances our study indicates that 1 in general only porphyroclast systems composed of equant shaped clasts can be treated as independent shear sense indicators 2 special attention should be given to type porphyroclast systems composed of high angle intermediate aspect ratio clasts which may contain meaningful information on bulk flow kinematics 3 bookshelf structures cannot serve as independent shear sense indicators multi criteria and thorough three dimensional shape analyses are suggested for a proper interpretation of flow kinematics in ductile shear zones

high strain deformation of conglomerates numerical modelling strain analysis and an example from the wutai mountains north china craton abstract conglomerates have been widely used to investigate deformation history and rheology strain vorticity and viscosity previous studies reveal that several factors such as pebble shapes and concentrations as well as material properties affect conglomerate deformation however how pebble concentration and interaction between pebbles affect deformation is not understood very well we use the 2d numerical modelling platform elle coupled to the full field crystal visco plasticity code vpfft to simulate the deformation of conglomerates with various viscosity contrasts between pebbles and matrix and different pebble concentrations with both linear stress exponent n 1 and power law n 3 viscous rheologies under simple shear conditions up to a shear strain of ten pebbles can behave as effectively passive deformable or effectively rigid an increase in pebble concentrations viscosity contrasts enhances pebble deformation but reduces their rotation a mean aspect ratio rf orientation plot is proposed to gain an estimate of pebble deformation behaviour and the amount of bulk strain closely spaced rigid or deformable pebbles can form clusters that mechanically act as single inclusions rigid clusters rotate and survive for only short strain increments whereas the more stable deformable ones keep on elongating with minor rotation we provide a natural example of deformed conglomerates from the wutai mountains north china craton these consist of banded iron formation bif pebbles embedded in a schistose matrix using the mean rf plot a finite strain of 6 under simple shear could be determined the viscosity of the pebbles is estimated at about 5 8 times that of the matrix for a linear rheology n 1 or 2 to 5 times if a power law rheology with n 3 is assumed

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

the rotation rate of cylindrical objects during simple shear the rotational behaviour of circular objects is modelled using a two dimensional finite element simulation we present results for linear and non linear viscous rheologies and explore parameter space for the rheology contrast between object and matrix and for object size relative to the shear zone width l for high our results confirm analytical descriptions for l 0 in that the rotation rate of the object is half of the bulk shear strain rate however we show that for 0 1 l 0 9 the rotation rate as a function of l can have a minimum a maximum or it can decrease or increase depending on the stress exponent and in fact for some rheologies the rotation rate may decrease to 0 3 of the shear strain rate at intermediate l 0 5 20 area of porphyroblast in thin section these counter intuitive results have important consequences for the interpretation of the rotational behaviour of minerals in deforming rocks for example in rocks where garnet constitutes a volumetrically significant proportion of the bulk rock the effective shear zone boundaries may be given by neighbouring crystals corresponding to intermediate l as described here thus the interpretation of the rotation rate of crystals in such rocks can only be performed if the distance to the effective system boundaries is known

porphyroblast rotation versus nonrotation conflict resolution data on foliation intersection and or inflection axes preserved in porphyroblasts fias indicate that no porphyroblast rotation occurs during ductile deformation relative to spatial coordinates this contrasts with 99 of investigations of rigid objects in non coaxially deforming media where the objects rotate when anastomosing shear zone formation around relatively strong objects in a weaker matrix is modeled no porphyroblast rotation occurs formation of these anastomosing zones controls the development of this phenomenon called gyrostasis if such zones are absent porphyroblasts rotate in weak materials the gyrostatic situation arises because the superposition of simple shearing deformation normal to initial coaxial shortening results in only small rotations of principal axes of stress since shear zones are controlled by the orientations of principal axes of stress initial anastomosing zones retain their orientations and positions during subsequent non coaxial deformation the porphyroblast is isolated from the embedding non coaxially deforming matrix but this material close to the porphyroblast continues to deform coaxially no local rotation occurs this has major tectonic significance because allowing for the effects of rotation due to brittle deformation porphyroblasts can now be routinely used to access lengthy structural and or metamorphic histories destroyed in the matrix by reactivation such as movement directions shear senses and extended pressure temperature time paths

multiscale modeling of ice deformation behavior understanding the flow of ice in glaciers and polar ice sheets is of increasing relevance in a time of potentially significant climate change the flow of ice has hitherto received relatively little attention from the structural geological community this paper aims to provide an overview of methods and results of ice deformation modeling from the single crystal to the polycrystal scale and beyond to the scale of polar ice sheets all through these scales various models have been developed to understand describe and predict the processes that operate during deformation of ice with the aim to correctly represent ice rheology and self induced anisotropy most of the modeling tools presented in this paper originate from the material science community and are currently used and further developed for other materials and environments we will show that this community has deeply integrated ice as a very useful model material to develop and validate approaches in conditions of a highly anisotropic behavior this review by no means exhaustive aims at providing an overview of methods at different scales and levels of complexity

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

time for anisotropy the significance of mechanical anisotropy for the development of deformation structures hr acknowledges financial support by the china scholarship council csc grant nr 201506400014 egr acknowledges the support of the beatriu de pinos programme of the government of catalonia s secretariat for universities and research of the department of economy and knowledge 2016 bp 00208 we thank bruce hobbs and an anonymous reviewer for their suggestions to improve this article

the preservation potential of microstructures during static grain growth grain growth simulations using the microstructure simulation system elle have been performed in materials with a pre existing grain shape foliation as might be expected the foliation is destroyed by the end of the experiment and grain areas have increased by a factor of seven the area of material swept by the migrating grain boundaries was monitored and it was found that at every stage virtually all of the grains which survived the grain growth process contain one and only one core of unswept material remarkably these remnant unswept cores preserve a useable record of the initial grain size and the orientation of the grain shape foliation this work suggests that even for samples where no equivalent protolith can be found it may be possible to see past a grain growth episode to estimate the original grain shape and grain size of the rock and perhaps even reconstruct the grain boundary kinematics in addition the identification of unswept cores has the potential to help unravel the evolution of grain boundary chemistry in rocks during metamorphism as an example of a natural system showing these microstructures we describe a peridotite from almklovdalen norway this peridotite was infiltrated by aqueous fluids at several stages during late caledonian exhumation and retrogressive metamorphism grain boundary migration associated with the last of these infiltration events swept off abundant intragranular fluid inclusions in the original chlorite peridotite at the grain scale microstructural mapping of the fluid inclusion rich areas shows that as with the numerical simulations many of the grains retain exactly one core of unswept material examples of other natural systems discussed include dislocation density distributions and trace element zoning

applications of inclusion behaviour models to a major shear zone system the nordfjord sogn detachment zone in western norway rigid inclusion models have reached a stage where one should be able to use them to obtain quantitative values from ductile shear zones we used natural data collected in three sites and combined analogue and theoretical modelling to assess vorticity strain nature of rigid inclusion matrix interface and confinement in the large scale ductile nordfjord sogn detachment zone nsdz of the caledonides of western norway our study shows that 1 the observed shape preferred orientation spo at higher structural levels of the nsdz at site 1 gjervika can be explained by simple shear pure shear simple shear ratio sr 0 associated with a slipping inclusion matrix interface 2 the observed spo at deeper structural levels of the nsdz at site 2 near sandane can be produced by simple shear associated with a significant amount of shortening across the shear zone sr z 1 acting upon rigid inclusions in slipping contact with the enclosing matrix 3 observed back rotated boudins deeper in the nsdz at site 3 biskjelneset can form in confined flow associated with a considerable amount of shortening across the shear zone sr 0 4 4 the observed tails of porphyroclasts indicate a minimum at least local strain of ca g z 20 5 the clasts in the studied shear zones strongly depart from jeffery s model jeffery g b 1922 the motion of ellipsoidal particles immersed in a viscous fluid proceedings of the royal society of london a102 161e179 the large scale extensional nsdz under investigation shows evidence of strain partitioning rocks vary from protomylonites to ultramylonites and the simple shear and pure shear components are heterogeneously distributed therefore we conclude that flow in the nsdz was very heterogeneous both at the kilometre and the metre scale however the present study suggests that the amount of shortening across the shear plane throughout the nsdz increases with depth and the flattening component contributes to exhumation of the eclogite facies rocks in its footwall 2007 elsevier ltd all rights reserved

cenozoic prograding sequences of the antarctic continental margin a record of glacio eustatic and tectonic events abstract sedimentary sections up to 6 14 km thick lie beneath many areas of the antarctic continental margin the upper parts of the sections contain up to 6 km of cenozoic glacial and possibly non glacial sequences that have prograded the continental shelf up to 85 km we describe the cenozoic sequences using two general categories based on their acoustic geometries type ia sequences which account for most prograding of the antarctic continental shelf have complex sigmoidal geometries and some acoustic characteristics atypical of low latitude margins such as troughs and mounds lying parallel and normal to the shelf edge and high velocities 2 0 2 6 km s for flat layers within 150 m of the seafloor type iia sequences which principally aggrade the paleoshelf lie beneath type ia sequences and have mostly simple geometries and gently dipping reflections the prograding sequences are commonly located near the seaward edges of major mesozoic and older margin structures relatively rapid cenozoic subsidence has occured due to the probable rifting in the ross sea thermal subsidence in the antarctic peninsula and isostatic crustal flexure in wilkes land in prydz bay and the weddell sea prograding sequences cover mesozoic basins that have undergone little apparent cenozoic tectonism grounded ice sheets are viewed by us and others as the principal mechanism for depositing the antarctic prograding sequences during the initial advance of grounded ice the continental shelf is flexurally overdeepened the inner shelf is heavily eroded and gently dipping glacial strata are deposited on the shelf i e type iia sequences the overdeepened shelf profile is preserved a during glacial times by grounded ice sheets episodically crossing the shelf eroding sediments from onshore and inner shelf areas and depositing sediments at the front of the ice sheet as outer shelf topset banks and continental slope foreset aprons i e type ia sequences and b during interglacial times like today by little or no clastic sedimentation on the continental shelf other than beneath retreated ice shelves lying far from the continental sheld edge ice streams carve broad depressions across the shelf and carry abundant basal sediments directly to the continental shelf edge thereby creating troughmouth fans and sheet like prograding sequences i e type ia sequences numerous acoustic unconformities and multiple overcompacted layers within the prograding sequences suggest major fluctuations of the antarctic ice sheet the available drilling and seismic interpretations provide the following history 1 cenozoic ice sheets have existed in places near the continental shelf since middle to late eocene time 2 a grounded antarctic ice sheet first expanded to the continental shelf edge with probable overdeepening of the outer shelf in late eucene to early oligocene time in prydz bay possibly in early miocene time in the ross sea and at least by middle miocene time in the weddell sea 3 the relative amounts of shelf prograding and inferred ice volume variations and related sea level changes have increased since middle to late miocene time in the eastern ross sea prydz bay and possibly weddell sea our analysis is preliminary further acoustic surveys and scientific drilling are needed to resolve the proximal antarctic record of glacio eustatic climatic and tectonic events recorded by the prograding sequences

numerical modelling of porphyroclast and porphyroblast rotation in anisotropic rocks abstract the rotational behaviour of rigid objects in a weaker rock matrix during deformation has been the subject of many field experimental and numerical modelling studies often centred on the question whether objects rotate or not in non coaxial deformation with numerical studies gaining increasing popularity and importance we here provide an overview of the results published so far and provide new simulations originally shape and orientation were investigated while the emphasis shifted to rheology and slip between object and matrix in the nineties of the last century due to improved numerical techniques anisotropic rheology has become the focus of most recent studies indicating that it is a primary factor in the rotation behaviour of objects we present new simulations investigating the role of anisotropy on different scales relative to the object and show how this influences the rotation rate as well as the inclusion patterns in case of syntectonically growing porphyroblasts these simulations show that a variety of factors play a role to determine the rate and sense of rotation of objects the variability of the inclusion patterns that can develop necessitates extreme caution in the kinematic interpretation of these structures when observed in the field

rotation of rigid elliptical cylinders in viscous simple shear flow analogue experiments we used experiments to investigate the behaviour of rigid elliptical cylinders embedded in a viscous fluid of finite thickness and for very small ratios s between shear zone width z and the shortest principal axis e2 of the ellipse that constitutes the base of the elliptical cylinder 3 5 s z e2 1 2 former theoretical and experimental work on the rotation of rigid inclusions immersed in a viscous matrix considered infinite shear zone thickness and or a very large ratio between shear zone thickness and inclusion size s 1 and concluded that inclusions rotate continuously and synthetically with the applied bulk simple shear flow except for inclusions with infinite axial ratio our results depart greatly from the analytical predictions experiments were carried out at constant shear zone thickness but variable s and degree of coupling at the inclusion matrix interface the results show that 1 confined inclusion rotation always deviates from theoretical predictions for infinite shear zone thickness even for synthetic rotation 2 the deviation in the angular velocity and or in the sense of rotation increases as s approaches 1 and antithetic rotation is possible from a position with ellipse longest axis parallel to the shear plane and 3 a slipping inclusion matrix interface greatly enhances deviation from theoretical predictions with antithetic rotations to as much as 20

rigid inclusions rotate in geologic materials as shown by torsion experiments jeffery showed 86 years ago that a sphere rotates synthetically with applied simple shear at a rate equal to half the shear strain rate however it can be argued that rocks do not behave like viscous fluids hence jeffery s model does not apply in order to test this argument we took a two phase synthetic aggregate rock made of 70 halite and 30 muscovite as anisotropic ductile matrix with embedded rigid bodies with contrasting shapes circular cylinder sphere and cube the sphere approximates the shape of a garnet and the cube the shape of pyrite crystals both so common in nature when subject to torsion at 250 mpa and 100 c the matrix deformed homogeneously and all rigid bodies rotated as theoretically predicted for a viscous fluid matrix therefore we conclude that rocks deforming in a ductile fashion behave macroscopically like a continuum hence fluid mechanics can be used to study rigid inclusion rotation behaviour

dynamic recrystallisation of ice aggregates during co axial viscoplastic deformation a numerical approach results of numerical simulations of co axial deformation of pure ice up to high strain combining full field modelling with recrystallisation are presented grain size and lattice preferred orientation analysis and comparisons between simulations at different strain rates show how recrystallisation has a major effect on the microstructure developing larger and equi dimensional grains but a relatively minor effect on the development of a preferred orientation of c axes although c axis distributions do not vary much recrystallisation appears to have a distinct effect on the relative activities of slip systems activating the pyramidal slip system and affecting the distribution of a axes the simulations reveal that the survival probability of individual grains is strongly related to the initial grain size but only weakly dependent on hard or soft orientations with respect to the flow field dynamic recrystallisation reduces initial hardening which is followed by a steady state characteristic of pure shear deformation

the influence of strain localisation on the rotation behaviour of rigid objects in experimental shear zones abstract mica fish and tourmaline fish from natural mylonites were analysed in thin section to determine their orientation distribution they are oriented with their long axes tilted with respect to the mylonitic foliation and fish with a small aspect ratio exhibit a slightly larger angle than fish with a large aspect ratio this orientation seems to be a stable orientation for the mica and tourmaline fish analogue experiments with two rheologically different matrix materials were performed to explain the data one material was pdms a linear viscous polymer the other was tapioca pearls a granular material with low cohesion and mohr coulomb type behaviour in contrast to a fairly homogeneous strain distribution in pdms distinct small scale shear bands developed in tapioca pearls during deformation experiments modelled different vorticity numbers and parallelogram shaped rigid objects with different aspect ratios were used rotation rates of objects in a viscous matrix are very similar to analytical solutions for ellipses in viscous flow but stable orientations differ from data of natural examples in all experiments with a mohr coulomb matrix elongated objects had a stable orientation due to small scale strain localisation we therefore suggest that small scale strain localisation mm that might be hidden by ongoing deformation and recrystallisation processes is an important characteristic of the rheology of mylonites

increase in melt fraction along a south north traverse below the tibetan plateau evidence from seismology abstract a mid crustal low velocity zone and crustal melt fraction contrast the lhasa block in the maxiang yangbajain zone with the tethyan himalayas in the south evidence from wide angle reflections is extended by vertical reflection and differential s to p wave teleseismic delays the temperature and rheology implied allow phase transitions which may change crustal buoyancy and elevation le pichon et al 1997 and may allow lateral mass flow in the uppermost mantle particular path geometries to a tight temporary teleseismic array allow detection of a decrease in velocity northward through the lhasa block a larger relative variation in late teleseismic s with respect to p indicates an increase in poisson s ratio hence of partial melt fraction this may be seen as evidence of a higher position of the mantle asthenosphere level tibet hence appears to be underlain by variation in the structure within the crust across the indus tsangpo suture and in the structure within the mantle further northward of the middle lhasa block these variations are most readily attributable to spatial variation in partial melt fraction and hence temperature which also induces phase transformations mode and amount of deformation and control of buoyancy on elevation may vary accordingly the dynamics of the system is evidenced by seismic anisotropy which we relate to ductile flow

clast based kinematic vorticity gauges the effects of slip at matrix clast interfaces clast based methods for estimating the mean kinematic vorticity number wm are compromised by strain localization at the clast margins localization increases with modal matrix mica content as determined with samples from the sandhill corner mylonite zone a crustal scale high strain strike slip shear zone in maine using these samples we estimate wm with the oblique quartz shape preferred orientation and rigid clast rotation methods the rigid clast rotation method yields much lower values for wm than the quartz method to investigate whether or not slip at the matrix clast interface can explain the discrepancy in calculated wm we conducted numerical modeling of rigid clasts enveloped by a low viscosity layer both embedded within a shearing viscous matrix within this dynamic framework we carried out numerical sensitivity analyses in which we varied the viscosity ratio between the lubricating layer and the surrounding matrix the thickness of the lubricating layer and the kinematic vorticity number of the bulk flow our data and numerical results succeed in explaining why wm estimates from clast based rotational methods are typically lower than estimates from other methods and this has implications for testing hypotheses related for example to vorticity partitioning in oblique convergent settings crustal scale extrusion or channel flow and exhumation of ultra high pressure rocks all of which rely on robust estimates of wm the relation between the shape preferred orientations of clasts and modal mica content lead to the hypothesis that mica is the cause of the lubrication at clast matrix interfaces if so then we surmise that mica fish should be self lubricating and would therefore form an end member shape preferred orientation regardless of matrix modal mica content the unique role of mica allows us to speculate about the bounds on viscosity contrast between the matrix and lubricated clast interfaces

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

analogue modelling of the influence of shape and particle matrix interface lubrication on the rotational behaviour of rigid particles in simple shear abstract the rotational behaviour of a rigid particle embedded in a linear viscous matrix undergoing cylindrical simple shear couette flow was studied in 2d rock analogue experiments the influence of particle shape elliptical vs monoclinic aspect ratio and the nature of the matrix particle interface lubricated vs unlubricated was investigated both matrix pdms and lubricant liquid soap were linear viscous with a viscosity ratio of ca 10 4 without lubricant the rotational behaviour of all particles closely approximates the jeffery theory lubricated monoclinic particles with the long diagonal initially parallel to the shear direction show back rotation and approach a stable position lubricated elliptical particles initially parallel to the shear direction also show back rotation but only transiently stabilize weak planar zones in the matrix adjacent to unlubricated elliptical particles do not induce backward rotation in general for elliptical particles rotation rate as a function of orientation depends on axial ratio and thickness of the lubricant mantle for thick mantles initially 10 of the volume of the particle rotation rates are faster than jeffery theory for very thin mantles they are markedly slower compared with thick mantles particularly when the long axis is nearly parallel to the shear direction rotation rates are never strictly zero so true stabilization does not occur however for more elongate particles axial ratio 6 rotation rates are so slow that a very strong shape preferred orientation would develop in a lubricated elliptical particle population in experiments the volume of lubricant is constant and the thickness adjacent to the long side of the particle progressively decreases with increasing strain in natural examples of porphyroclast systems the weak mantle continually develops by recrystallization and or cataclasis of the rigid clast core and a steady state between production and thinning could be attained potentially leading to true stabilization for particles with a high axial ratio

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

origin of the middle pleistocene transition by ice sheet erosion of regolith the transition in the middle pleistocene 0 9 ma seen in 18o deep sea core records from relatively low amplitude high frequency 41 kyr to high amplitude low frequency 100 kyr ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft bedded to a mixed hard soft bedded laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock a one dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant 23 and 41 kyr orbital forcing progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change 100 kyr reflects nonlinear deglaciation processes in model simulations over the last 3 ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records

when do folds unfold during progressive shear folded layers in rocks can be stretched again potentially unfolding the folds back to straight layers little is known however about how to recognize partly or even entirely unfolded layers when folded layers can unfold what determines their mechanical behavior and how can we recognize them in the field in order to address these questions we present a series of numerical simulations of the stretching of previously folded single layers and multilayers in simple shear layers do not completely unfold when they undergo softening before or during the stretching process or when adjacent competent layers prevent them from unfolding intrafolial folds and cusp like folds adjacent to straight layers as well as variations in fold amplitudes and limb lengths of irregular folds are indicative of stretching of a fold train

identification and preservation of landforms diagnostic of past ice sheet activity on continental shelves from three dimensional seismic evidence ice ages have occurred a number of times in earth9s history and are important in understanding changes in long term climate however it is difficult to demonstrate the presence of ice sheets in the ancient record because their sedimentary products can resemble those from nonglacial processes e g debris flows diagnostic large scale glacial landforms produced beneath ice sheets and preserved on continental shelves after deglaciation can establish a glacial origin three dimensional seismic evidence from the 2 7 ma naust formation norwegian margin illustrates several glacial landforms that are also commonly occurring features on modern high latitude shelves 1 streamlined mega scale lineations produced by fast flowing ice streams 2 ploughmarks formed by iceberg keels and 3 regularly spaced transverse ridges or push moraines formed during ice retreat we have found for example buried ice keel ploughmarks on a paleoshelf dating to ca 2 ma norwegian margin seismic stratigraphy data show that ice advanced and retreated many times and some paleoshelf surfaces were preserved such paleosurfaces must survive ocean basin destruction for glacial landforms to be useful in identification of ancient depositional environments

microstructural and rheological evolution of calcite mylonites during shear zone thinning constraints from the mount irene shear zone fiordland new zealand abstract layers of calc mylonite in the mount irene shear zone fiordland new zealand show substantial variations in thickness due to deflection of the shear zone boundaries around wall rock asperities in relatively thick parts c 2 6 m of the shear zone calcite porphyroclasts are internally strained contain abundant subgrain boundaries and have a strong shape preferred orientation spo and crystallographic preferred orientation cpo suggesting that deformation occurred mainly by dislocation creep involving subgrain rotation recrystallization in relatively thin parts c 1 5 m of the shear zone aggregates of fine grained recrystallized calcite surrounding flattened porphyroclasts have a weak spo and cpo and contain polygonal calcite grains with low degrees of internal misorientation the recrystallized aggregates also contain microstructures e g grain quadruple junctions randomized misorientation axes similar to those reported for neighbor switching processes during grain boundary sliding comparison of subgrain sizes in the porphyroclasts to published grain size differential stress relationships indicates that stresses and strain rates were substantially higher in relatively thin parts of the shear zone the primary microstructural response to higher stresses and strain rates was an increase in the amount of recrystallization to produce aggregates that deformed by grain boundary sliding however even after the development of interconnected networks of recrystallized grains dislocation creep by subgrain rotation recrystallization continued to occur within porphyroclasts this behavior suggests that the bulk rheology of shear zones undergoing thinning and thickening can be controlled by concomitant grain size insensitive and grain size sensitive mechanisms overall our observations show that shear zone thickness variations at constant p t can result in highly variable stresses and strain rates which in turn modifies microstructure deformation mechanism and shear zone rheology

the dynamics of ice sheet outlets a comparison of data from aircraft altimetry landsat imagery and radia echo sounding has shown characteristic surface topographies associated with sheet and stream flow the transition between the two is abrupt and occurs at a step in the subglacial topography this marks the onset of basal sliding and high velocities caused by subglacial water it results in crevassed amphitheatre like basins round the head of outlet glaciers it is also the zone of maximum driving stress beyond which values decline rapidly as velocities increase this abrupt transition appears to be topographically controlled since basal temperatures are at the pressure melting point well inland of the change in regime the marie byrd land ice streams exhibit qualitative differences from other ice sheet outlets however the change to lower driving stresses is much more gradual and occurs several hundred kilometres inland such ice streams have particularly low surface slopes and appear in form and flow regime to resemble confined ice shelves rather than grounded ice the repeated association of the transition to rapid sliding with a distinct subglacial feature implies a stabilizing effect on discharge through outlet glaciers acceleration of the ice is pinned to a subglacial step and propagation of high velocities inland of this feature seems improbable rapid ice flow through subglacial trenches may also ensure a relatively permanent trough through accentuation of the feature by erosion this is concentrated towards the heads of outlet glaciers up stream of the region where significant basal decoupling occurs this may be a mechanism for the overdeepening of fjords at their inland ends and the development of very steep fjord headwalls

the development of δ clasts in non linear viscous materials a numerical approach abstract winged porphyroclasts are often used as kinematic or strain indicators in shear zones in particular it has been suggested that the geometry of clasts in mylonites can give information about the rheological behaviour of the matrix around the clast passchier et al 1993 we tested this hypothesis with a numerical finite element simulation of the flow field of a viscous matrix around a relatively rigid clast power law viscous flow properties of the matrix with stress exponents of 1 linear viscous 3 and 5 were tested as well as three different sets of approximately simple shear boundary conditions which are relevant to experimental and or natural situations the simulations show that the flow field is primarily determined by the boundary conditions the stress exponent is of secondary importance in general the different boundary conditions produce two types of flow fields about the rigid clast an eye shaped flow field and a bow tie shaped flow field the former is produced by boundary conditions that approximate simple shear applied at an infinite distance from the rigid clast and the latter is produced by boundary conditions which are equivalent to a shear box or ring shear apparatus although both the shear box and ring shear boundary conditions produce bow tie flow patterns the wing geometries which result from the flow patterns for these two cases can be significantly different the geometry of the wings that develop on a clast is a direct function of the flow field around the clast for linear and non linear rheologies only the ring shear boundary conditions produce wings that show stair stepping i e wings that are parallel but not in the same plane in natural shear zones clasts both with and without stair stepping are observed in comparing geologic observations with experimental or numerical modelling it is essential to understand which boundary conditions are appropriate

eleven phases of greenland ice sheet shelf edge advance over the past 2 7 million years the reconstruction of former ice sheets is important for testing earth system models that can assess interactions between polar ice sheets and global climate but information retrieved from contemporary glaciated margins is sparse in particular we need to know when ice sheets began to form marine outlets and the mechanisms by which they advance and retreat over timescales from decades to millions of years here we use a dense grid of high quality two dimensional seismic reflection data to examine the stratigraphy and evolution of glacial outlets or palaeo ice streams that drained the northwest greenland ice sheet into baffin bay seismic horizons are partly age constrained by correlation with cores from drill sites progradational units separated by onlap surfaces record 11 major phases of shelf edge ice advance and subsequent transgression since the first ice sheet expansion 3 3 2 6 million years ago the glacial outlet system appears to have developed in four stages each potentially caused by tectonic and climatic changes we infer that an abrupt change in ice flow conditions occurred during the mid pleistocene transition about 1 million years ago when ice movement across the shelf margin changed from widespread to a more focused flow ice streams forming the present day glacial troughs eleven phases of advance of the greenland ice sheet following the first expansion between 3 3 and 2 6 million years ago are documented in analyses of a grid of seismic reflection data

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

structural geology meets micromechanics a self consistent model for the multiscale deformation and fabric development in earth s ductile lithosphere abstract earth s lithosphere is made of rheologically heterogeneous elements of a wide range of characteristic lengths a micromechanics based self consistent multiorder power law approach is presented to account for lithospheric deformations and the accompanying multiscale fabric development the approach is principally based on the extended eshelby theory for the motion of a power law viscous ellipsoid in a power law viscous matrix and the idea of embedding inhomogeneities within inhomogeneities the extended theory provides a general means for investigating deformation partitioning in heterogeneous rocks the inhomogeneities within inhomogeneities method allows multi hierarchical levels of flow field partitioning and hence multiscale deformations to be investigated partitioned flow fields are used to investigate fabric development being based fully on micromechanics the approach generates model predictions of both kinematic quantities strain strain rates and vorticity and stress histories the former can be directly compared with field and laboratory structural observations while the latter can help to understand the physics of natural deformations the self consistent and multiscale approach is applied to a natural example of the cascade lake shear zone in the east sierra nevada of california the modeling shows that the fabrics are most consistent with a steeply dipping transpression zone with a convergence angle of 20 and a strike slip displacement about 26 km further the strength evolution of the model zone confirms that a transpression zone is a weakening system with respect to the simple shearing component and a hardening one for the pure shearing component this is consistent with slip partitioning in obliquely convergent plate boundaries boundary normal convergence tends to spread over a broad area whereas boundary parallel shear tends to localize in major strike slip zones

palaeoglaciology of an ice sheet through a glacial cycle the european ice sheet through the weichselian abstract satellite images provide unique means of identifying large scale flow generated lineations produced by former ice sheets they can be interpreted to reconstruct the major elements which make up the integrated large scale structure of ice sheets ice divides ice streams interstream ridges ice shelves calving bays the evolving palaeoglaciological structure of the european ice sheet during its decay from the last glacial maximum lgm is reconstructed by reference to these components and in the context of a new map showing isochrons of retreat during the retreat phase in particular the time dependent dynamic evolution of the ice sheet and the pattern of ice stream development are reconstructed crossing lineations are widespread the older ones are suggested to have formed during molten bed phases of ice sheet growth and preserved by frozen bed conditions during the glacial maximum particularly in areas which lay during deglaciation beneath ice divides and inter ice stream ridges both areas of slow flow and possibly frozen bed conditions four phases of growth a1 to a4 and five phases of decay r1 to r5 are used to describe the major climatically and dynamically determined stages in the evolution of the ice sheet through the last glacial cycle the growth and decay patterns are quite different and associated with major shifts in the ice divide reflecting growth from the fennoscandian mountains and decay away from marine influenced margins these patterns were determined by the locations of nucleation areas spatial patterns of climate and calving at marine margins the prevalence of streaming within the retreating ice sheet suggests that the mean elevation of the ice sheet was lower than predicted from glaciological models which do not include streaming and that this might reconcile glaciological models and earth rheology models which infer paleao ice sheet thickness by inverting sea level data

porphyroblast inclusion trail orientation data eppure non son girate extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non coaxial ductile deformation of the matrix subsequent to their growth this has been demonstrated for all tectonic environments so far investigated the work also has provided new insights and data on metamorphic structural and tectonic processes including 1 the intimate control of deformation partitioning on metamorphic reactions 2 solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones and relative plate motion and 3 a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens

what happens to deformed rocks after deformation a refined model for recovery based on numerical simulations deformation in large parts of the middle crust results in strained rocks consisting of grains with variable dislocation densities and microstructures which are characterized by gradual distortion

strain localization and porphyroclast rotation it has been debated for decades whether rigid inclusions such as porphyroclasts and porphyroblasts do or do not rotate in a softer matrix during deformation experiments and numerical simulations with viscous matrix rheologies show ongoing rotation of circular inclusions whereas using mohr coulomb plasticity results in nonrotation because the rocks in which inclusions are found normally undergo deformation by dislocation creep we applied a full field crystal plasticity approach to investigate the rotation behavior of rigid circular inclusions we show that the inclusion9s rotation strongly depends on the anisotropy of the matrix minerals strongly anisotropic minerals will develop shear bands that reduce the rotation of inclusions inhibition of rotation can only occur after a significant amount of strain our results may help to explain why geologic rigid objects often show evidence for rotation but not necessarily in accordance with the viscous theory that is usually applied to these systems

porphyroblast rotation eppur si muove in a number of recent papers the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates and can be used to determine the original orientation of older foliations complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis involving several alternating phases of horizontal shortening and extension critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws millipede structures used as proof for flow partitioning can also form by other flow geometries evidence quoted to support irrotational behaviour of porphyroblasts is unsound porphyrobl asts do occur in sets with a preferred orientation of the internal foliation trace but these cannot be shown to represent original orientations microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis they can however also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation finally porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations

porphyroblast rotation and strain localization debate settled comment this comment addresses two inferences used by johnson 2009 to argue for porphyroblast rotation during bulk coaxial shortening firstly he interprets that porphyroblast inclusion trails that are inclined his figure 1 became inclined because the porphyroblast rotated during growth he used orientation data from millipede microstructures from one hand sample and concluded on the basis of the total spread of inclusion trail orientations and porphyroblast axial ratios that porphyroblasts had rotated relative to one another during ductile deformation secondly he presents a numerical model that indicates that asymmetrically shaped porphyroblasts rotated during coaxial deformation however i will show that the porphyroblasts did not rotate after they grew in spite of localized shearing along the developing s3 that rotated visible matrix s2 in johnson s figure 1

the influence of crenulation cleavage development on the bulk elastic and seismic properties of phyllosilicate rich rocks abstract the anisotropy of seismic wave propagation is strongly influenced by the mineralogy and microstructure of rocks phyllosilicates are elastically highly anisotropic and are therefore thought to be important contributors to seismic anisotropy in the continental crust crenulation cleavage is one of the most common microstructural fabrics found in multiply deformed phyllosilicate rich crustal rocks we calculated the bulk elastic properties and resulting wave velocities for rock samples that preserved three different stages of crenulation cleavage development an initial planar foliation a moderately developed crenulation cleavage and a well developed crenulation cleavage mineral orientation maps were obtained using electron backscatter diffraction and calculations were made using asymptotic expansion homogenization combined with the finite element method the difficulties involved with sample preparation and data acquisition of phyllosilicate rich rock samples are also discussed we compare our results to more conventional methods for calculating an aggregate stiffness matrix from a mineral orientation map namely voigt and reuss averages these averages do not account for grain scale interactions and therefore deviate from the results calculated using asymptotic expansion homogenization our results show that the rocks characterized by a planar foliation and a moderately developed crenulation cleavage are highly anisotropic with p wave anisotropies up to 30 9 and s wave anisotropies up to 34 2 whereas the rock characterized by a well developed crenulation cleavage is only mildly anisotropic with a p wave anisotropy of 15 5 and s wave anisotropy of 10 7 progressive development of the fabric also causes the orientations of p and s wave velocity maxima and s wave polarization directions to change markedly despite the high anisotropy imparted by a planar schistosity the variety of folds and fabrics typically found in phyllosilicate rich rocks within larger scale crustal volumes will tend to mute the anisotropy possibly to the point of appearing nearly isotropic

full field predictions of ice dynamic recrystallisation under simple shear conditions understanding the flow of ice on the microstructural scale is essential for improving our knowledge of large scale ice dynamics and thus our ability to predict future changes of ice sheets polar ice behaves anisotropically during flow which can lead to strain localisation in order to study how dynamic recrystallisation affects to strain localisation in deep levels of polar ice sheets we present a series of numerical simulations of ice polycrystals deformed under simple shear conditions the models explicitly simulate the evolution of microstructures using a full field approach based on the coupling of a viscoplastic deformation code vpfft with dynamic recrystallisation codes the simulations provide new insights into the distribution of stress strain rate and lattice orientation fields with progressive strain up to a shear strain of three our simulations show how the recrystallisation processes have a strong influence on the resulting microstructure grain size and shape while the development of lattice preferred orientations lpo appears to be less affected activation of non basal slip systems is enhanced by recrystallisation and induces a strain hardening behaviour up to the onset of strain localisation and strain weakening behaviour simulations demonstrate that the strong intrinsic anisotropy of ice crystals is transferred to the polycrystalline scale and results in the development of strain localisation bands than can be masked by grain boundary migration therefore the finite strain history is non directly reflected by the final microstructure masked strain localisation can be recognised in ice cores such as the edml from the presence of stepped boundaries microshear and grains with zig zag geometries

modeling of anisotropic grain growth in minerals the role of surface energy anisotropy during grain growth is investigated using both physical experiments on octachloropropane and numerical experiments using the elle microstructural modelling system in particular the effects of anisotropy on growth rates grain shapes and lattice preferred orientations are analysed anisotropic growth in thin polycrystalline sheets of octachloropropane is found to systematically remove certain c axis orientations from the sample without obviously modifying the grain shapes by comparison with equivalent numerical experiments we can explain these observations with a simple boundary layer model that treats each side of each grain boundary as an independent system the observed growth exponent in the physical experiment of 0 35 is significantly below the theoretical level of 1 and although the numerical experiments have growth rates as low as 0 68 apparently some other factor which suppresses the growth rate in the octachloropropane was also active the anisotropy of surface energies also leads to an increase in the grain size distribution and when it occurs in rocks with a strong pre existing crystallographic preferred orientation may even lead to the development of a shape preferred orientation which is completely unrelated to the shape of the finite strain ellipsoid corresponding author e mail address bons mail uni mainz de accepted for publication in the hans ramberg volume bulletin of the american geological society bons et al anisotropic grain growth 17 januar 2000 page 2