the exponential flow law applied to necking and folding of a ductile layer summary the uniaxial exponential law has been applied to experimental results for steady state creep including that of wet and dry olivine pyroxenite and carbonates at a deviatoric stress greater than 100 mpa such stress levels likely occur in the upper mantle lithosphere and middle crust in a layered rock in layer parallel extension or shortening high deviatoric stress can occur in the stiffest layers for example a dolomite layer in fine grained marble under assumptions of isotropy and incompressibility the uniaxial law yields where j2 sijsij 2 is an invariant of the deviatoric stress tensor sij linearization about a homogeneous basic state of flow yields equations identical in form to those obtained for the familiar power law this establishes expressions for an effective viscosity and stress exponent where is an invariant of the basic state deformation rate these results allow application of existing analytical folding and necking solutions for a rock layer of power law fluid to a rock layer with an exponential flow law the effective stress exponent for the exponential flow law increases with decreasing temperature through the dependence of c on the latter and weakly with increasing deformation rate for dry olivine effective stress exponents are between 10 and 30 for temperatures between 400 and 600 c with little dependence on deformation rate finite element simulations employing full non linear forms of the flow laws show that large strain necking is nearly identical for power law and exponential flow laws the results suggest that the instability in necking and folding in ductile rock layers can be considerably stronger than inferred from results based on flow laws representing diffusion and dislocation creep the large values of the effective stress exponent ne 15 that may be attained for exponential flow laws can account for observed outcrop scale ductile necking

structural reactivation in plate tectonics controlled by olivine crystal anisotropy structures formed during ancient tectonic events are commonly reactivated during subsequent tectonism numerical models point to mechanical anisotropy arising from the inherited orientation of crystals of the mineral olivine in the lithospheric mantle as the cause of this behaviour

age k ar phengite temperature structure relations a case study from the ishigaki high pressure schist belt southern ryukyu arc japan the ishigaki high pressure schist belt in the southern ryukyu arc is correlated with the suo high pressure schist belt in southwest japan the former metamorphic sequence is composed mainly of basic and pelitic schists and is subdivided into three zones the lower grade zone a the medium grade zone b and the high grade zone c based on the mineral assemblages of the basic schists the k ar phengite age gives 188 205 ma for zone a 196 206 ma for zone b and 208 220 ma for zone c while the apparent d 002 spacing of carbonaceous materials is 3 590 3 437 a 3 415 3 390 a and 3 387 3 364 a respectively the age d 002 relationships suggest that the ages become older with increasing metamorphic temperature this positive age temperature relationship in the ishigaki area contrasts with a negative relationship in the nishiki area in the suo belt the two areas also display a contrasting thermal structure with the former area having an inverted metamorphic gradient and the latter displaying a normal thermal structure these contrasting age temperature structure relationships in the metamorphic belt could be due to different tectonic styles relating to the exhumation of the metamorphic sequences we suggest that the ages obtained are related directly to the ductile deformation history of the matrix phengite below the closure temperature 500 c during exhumation of the host rocks the duration from the beginning of exhumation to the apparent resetting of the phengite k ar system was different between the two metamorphic sequences and significantly longer in the ishigaki than the nishiki

offscraping and underthrusting of sediment at the deformation front of the barbados ridge deep sea drilling project leg 78a on leg 78a we drilled sites 541 and 542 into the seaward edge of the barbados ridge complex and site 543 into the adjacent oceanic crust the calcareous ooze marls and muds at sites 541 and 542 are lithologically and paleontologically similar to the upper strata at site 543 and are apparently offscraped from the down going plate a repetition of miocene over pliocene sediments at site 541 documents major thrust or reverse faulting during offscraping the hemipelagic to pelagic deposits offscraped in the leg 78a area include no terrigenous sand beds but they contain numerous neogene ash layers derived from the lesser antilles arc hence this sequence is quite unlike the siliciclastic dominated terranes on land that are inferred to be accretionary complexes the structural fabric of the offscraped deposits at sites 541 and 542 is disharmonic probably along a decollement with an underlying acoustically layered sequence suggesting selective underthrusting of the latter the acoustically layered sequence correlates seismically with pelagic strata cored at site 543 on the incoming oceanic plate cores recovered from the possible decollement surface at both sites 541 and 542 show scaly foliation and stratal disruption approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits in the incoming section a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata in the leg 78a area the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the barbados ridge complex

well documented travel history of mesozoic pelagic chert in japan from remote ocean to subduction zone the mino tanba belt in southwest japan a segment of the cordilleran type orogenic chain of jurassic east asia is composed mainly of a middle upper jurassic subduction accretion complex in which triassic and lower jurassic bedded radiolarian cherts occur as large allochthonous units structurally interlayered with middle upper jurassic clastic rocks high resolution microfossil conodont and radiolaria research has identified very low average sedimentation rates of about 0 5 g cm 1000 yr in the chert units similar to those of modern pelagic sediments accumulated in open ocean environments judging from the low average sedimentation rate high purity of biogenic silica long duration of continuous deposition 50 m y and wide along strike extent 1000 km the bedded radiolarian cherts in the mino tanba belt are best understood as ancient pelagic sediments that accumulated in open ocean environments accordingly the alleged origin in smaller marginal basins is untenable upward lithologic change from bedded chert to overlying siliceous mudstone in the uppermost portion of chert sequences suggests the gradual landward approach of the oceanic plate toward a trench the tectonic interlayering of these cherts and coarse grained terrigenous elastics is a secondary feature that was added through duplexing underplating in the subduction zone on the basis of the primary stratigraphy and field occurrence of triassic bedded chert in the mino tanba belt newly proposed are an idealized oceanic plate stratigraphy and a generalized travel history of a cordilleran type bedded chert from its birth at a mid oceanic ridge to its demise at a subduction zone

tectonics and hydrogeology of accretionary prisms role of the decollement zone abstract at convergent margins the decollement zone comprises the plate boundary and is marked by profound structural disharmony by changes in stress orientation and by a discontinuity in plate velocity the decollement zone initiates in a weak sediment layer typically a low permeability hemipelagic mud lying below a more rapidly deposited stronger more permeable trench turbidite sequence at the deformation front trench turbidites tend to be offscraped whereas the finer grained hemipelagic and pelagic sediments are more likely to be underthrust offscraped materials may undergo only limited burial whereas underthrust deposits can be deeply buried undergo high pressure metamorphism and are more likely to be preserved in the stratigraphic record burial rates associated with underthrusting are high exceeding 20 km my with high rates of underthrusting sediment descending below the decollement zone is probably buried faster than it can dewater resulting in emplacement of relatively high porosity deposits at depth fluids flowing from modern decollement zones have migrated substantial distances laterally and tapped deep sources the episodicity of flow suggests pumping of fluid by cycles of dilation and flow to the decollement zone followed by failure and fluid expansion along the decollement zone possible pressure gradients below the decollement zone allow flow upward into it while maintaining minimum effective stress along it the thickness of sediment underthrust beneath the decollement zone determines whether these deposits are emplaced by diffusive underplating with stratal disruption and efficient dewatering or by coherent underplating with the formation of macroscopic duplexes and transfer of fluid to the base of the accretionary prism deformation mechanisms affecting accreted sediment depend on depth of entrainment into the decollement zone and duration of residence

the gem lake shear zone cretaceous dextral transpression in the northern ritter range pendant eastern sierra nevada california the gem lake shear zone is a northwest striking steeply dipping dextral transpressional shear zone that provides the first direct evidence for dextral deformation in wall rock pendants in the central part of the sierra nevada batholith the gem lake shear zone is a minimum of 30 km in length and extends at least from the north end of the northern ritter range pendant to the southeast edge of the ritter range pendant the amount of displacement on the zone is uncertain but matching fault slivers of pennsylvanian marble in the northern ritter range pendant to similar exposures north of the mount morrison pendant suggests a minimum dextral offset of 20 km the gem lake shear zone was active in early late cretaceous time from before 91 ma the age of the syntectonic granodiorite of kuna crest to at least 80 ma 40ar 39ar age of syndeformational mica from the shear zone in the ritter range pendant deformation in the gem lake shear zone is characterized by combined dextral simple shear and subvertical stretching which are variably partitioned in anastomosing high strain zones in the shear zone at gem lake predominantly dextral deformation is indicated by porphyroclast asymmetries s c fabric and asymmetric crenulations a component of subvertical stretching is indicated by a moderately to steeply plunging stretching lineation in a segment of the shear zone at kuna crest a strongly developed stretching lineation indicates predominantly subvertical stretching with a lesser component of dextral strike slip the gem lake shear zone is considered to be part of a proposed regional system of shear zones in the eastern sierra nevada the sierra crest shear zone system this dextral transpressional system was active prior to and synchronous with intrusion of the late cretaceous sierra nevada batholiths deformation ceased shortly after pluton emplacement the sierra crest shear zone system includes possibly related shear zones as far north as saddlebag lake pendant and at least as far south as oak creek pendant and the owens valley indicating a possible strike length greater than 150 km

the hiding exposure effect revisited a method to calculate the mobility of bimodal sediment mixtures abstract predicting seabed mobility is hampered by the limited accuracy of sediment transport models when the bed is composed of mixed sediments the hiding exposure he effect modifies the threshold of motion of individual grain classes in sediment mixtures and its strength is dependent on the grain size distribution however an appropriate method of predicting this effect for bimodal sediment mixtures remains to be developed the prototypical example of a bimodal mixture is that consisting of a well sorted sand and gravel for the fine and coarse fractions respectively through a comprehensive series of laboratory experiments the he effect has been quantified for a full range of sand gravel mixtures from pure sand to pure gravel the choice of which has been underpinned by an integrated study of offshore geophysical and sedimentological data found in coastal and shelf seas in the sand gravel mixtures used in the present study the critical shear stress needed to mobilise the sand and gravel fractions increased by up to 75 and decreased by up to 64 respectively compared to that needed to mobilise well sorted sediment of similar size the he effect was found to be dependent on the percentage of gravel coarse mode present in the bimodal mixture whereby the effect for the mixture is the weighted sum of the he effect for the fine and coarse modes

slip partitioning by elastoplastic propagation of oblique slip at depth oblique motion along tectonic boundaries is commonly partitioned into slip on faults with different senses of motion the origin of slip partitioning is important to structural geology tectonophysics and earthquake mechanics partitioning can be explained by the upward elastoplastic propagation of oblique slip from a fault or shear zone at depth the strain field ahead of the propagating fault separates into zones of predominantly normal reverse and strike slip faulting the model successfully predicts the distribution of fault types along parts of the san andreas and haiyuan faults

modelling the bulk viscosity of two phase mixtures in terms of clast shape abstract the rheological behaviour of many rock types will depend on how mixtures of different minerals and rock clasts behave together this paper develops two dimensional modelling of the bulk viscosity of two phase viscous mixtures with different shape fabrics in pure shearing based on self consistent mechanics and behaviour of inclusion matrix systems an equation is presented for the bulk viscosity in terms of the phase viscosity contrast phase fractions and clast shape results are given for idealised two dimensional mixtures of circles squares or ellipses with a variety of phase viscosity contrasts in pure shearing parallel perpendicular to the shape fabric the normalised bulk viscosities of all these mixtures fall between the theoretical upper and lower viscous bounds given by homogeneous strain rate and homogeneous stress respectively but nearer to the upper bound with increasing shape fabric mixtures with a particular geometry are shown to have a constant relative horizontal distance between these two bounds as proposed by p d bons using graphs for the normalised bulk viscosities of mixtures with variably elliptical aligned shape fabrics the results for fabrics orthogonal to pure shearing can be extrapolated to a diagonal fabrics in pure shearing and b orthogonal or diagonal fabrics in simple shearing these reveal the bulk viscous anisotropy of mixtures with shape fabric in two dimensions it is speculated that during a progressive finite deformation two phase mixtures might undergo changes in bulk viscosity over time with steady stiffening during pure shear and perhaps cyclic stiffening and softening during simple shear the two dimensional models provide an approximation to the three dimensional properties of mixtures with shape fabrics with potential applications to rocks including conglomerates

flow variation in layered rocks subjected to bulk flow of various kinematic vorticities theory and geological implications abstract flow variation is studied analytically in rocks with layers of varied competence and varied orientation with respect to constant bulk flow fields of given kinematic vorticity 0 w k 1 the results explicitly demonstrate that flow varies significantly from layer to layer and with time in any individual layer even though the bulk flow is constant any flow regime can occur and the instantaneous stretching axes isa spin all the time reverse sense of non coaxiality can occur in the same layer at different times and in different layers at the same time this study further reveals the significance of heterogeneity and non steadiness in natural flows the assumption of a time independent flow history for natural deformation is highly unrealistic the spinning of the instantaneous stretching axes isa and the variation of flow regimes have bearings on the progressive development of fabrics and structures such as folded boudins refolded folds in shear zones and double rotation of porphyroclasts that are often interpreted as being due to multi deformational events

global radiolarian zonation for the pliensbachian toarcian and aalenian jurassic radiolarians from 220 samples in queen charlotte islands b c williston lake b c east central oregon baja california sur southern spain austria slovenia turkey oman japan and argentina were studied in order to construct global zonation for the pliensbachian toarcian and aalenian stages well preserved faunas from continuous stratigraphic sections in queen charlotte islands provide the most detailed record for this time interval and all collections are tied to north american ammonite zones or assemblages collections from nearly all other areas lack independent dating except for early toarcian carbon isotope dating in slovenia and late aalenian ammonites in spain a database of 197 widely distributed updated taxonomic species was used to construct a unitary association ua zonation for the interval a global sequence of 41 uas was obtained for the top of the sinemurian to the base of the bajocian the first and the last uas represent the late sinemurian and the early bajocian respectively the remaining 39 uas were merged into nine zones four early pliensbachian one late pliensbachian one early toarcian one middle late toarcian and two aalenian according to prominent radiolarian faunal breaks and ammonite data the new zones are the canutus tipperi katroma clara zone latest sinemurian earliest pliensbachian zartus mostleri pseudoristola megaglobosa hsuum mulleri trillus elkhornensis and gigi fustis lantus sixi zones early pliensbachian eucyrtidiellum nagaiae praeparvicingula tlellensis zone late pliensbachian napora relica eucyrtidiellum disparile zone early toarcian elodium pessagnoi hexasaturnalis hexagonus zone middle and late toarcian higumastra transversa napora nipponica zone early aalenian and mirifusus proavus transhsuum hisuikyoense zone late aalenian these zones can be correlated worldwide and link previously established ua zonations for the hettangian sinemurian and the middle to upper jurassic the new zonation allows high resolution dating in the studied interval and provides a solid basis for analyzing faunal turnovers and the paleobiogeography of jurassic radiolarians

book reviewthe techniques of modern structural geology vol 1 strain analysis j g ramsay and m i huber academic press london 1983 xiii 307 pp us 55 00 28 00 hardcover

flow perturbations a tool to study and characterize heterogeneous deformation the classification of the myriad of small scale structures that are used in tectonic analysis is presently based on their geometry which makes it difficult to discover transitions and groups amongst them an alternative would be to classify structures according to the flow type by which they form but this is difficult although most structures form by heterogeneous flow modelling studies are often focussed on bulk homogeneous flow since the mathematical treatment of heterogeneous flow is cumbersome also heterogeneous deformation patterns seem to occur in as many types as there are small scale structures this paper introduces the use of the geometry of flow perturbations to improve our understanding of heterogeneous flow and to allow classification of structures based on kinematics a flow perturbation is the deviation of a heterogeneous flow pattern from the background homogeneous flow perturbation patterns can be visualised as open or closed loops of vectors which occur in pairs rows or groups of four they usually cross cut rheological interfaces and can be subdivided into those that cross interfaces twice or four times most geological structures belong to the first group while initiating buckle folds belong to the second group q 2005 elsevier ltd all rights reserved

a rigid circular inclusion in an anisotropic host subject to simple shear abstract the solution for velocity and rate of deformation in an incompressible homogeneously anisotropic viscous host embedding a cylindrical rigid inclusion subjected to a far field shearing parallel to the anisotropy direction is presented the rotation rate of the inclusion is equal to half the ambient shear rate independently of the anisotropy factor the flow in the matrix localizes into conjugate shear bands sub parallel to the anisotropy directions as the anisotropy factor increases the presented anisotropic analytical solution which neglects the bending stiffness approximates the flow in a layered medium in the limit of infinitely thin layers an initially planar layering is deflected adjacent to the inclusion and fold trains propagate into the host with progressing deformation numerical simulations show that the structural development leads to a decrease in the inclusion rotation rate and eventually to inclusion stagnation at high strains the fold shapes become more angular and the fold trains reach further out into the host as the anisotropy factor is increased increasing the layering thickness up to only 9 layers intercepting the inclusion has no significant effect on how the layer inclusion rotation rate evolves with strain a coarse layering in the host leads to a strongly reduced outreach of the fold trains and the absence of tight folds the analytical solution which is derived for a planar anisotropy in the host can be employed to approximate the structural development for a weakly anisotropic host or small deformation structural development of an anisotropic medium plays a major role in determining the system behavior for large deformation

flow and finite deformation of surface elements in three dimensional homogeneous progressive deformations rocks are heterogeneous materials deformation structures and fabrics in the ductile lithosphere can result from the history of the three dimensional flow field or the progressive deformation of some rheologically distinct elements embedded in the 3d flow field or a combination of both the flow and progressive deformation of a rheologically distinct element is generally non steady even if the hosting 3d flow field is steady vorticity decomposition is the key to following the progressive history of the deformation of specific elements a general theoretical background to the problem is presented flow and progressive deformation of surface and layer like elements in 3d homogeneous flows are investigated specifically analytical procedures to calculate the sectional vorticity number and the sectional dilatancy number on a general section in a 3d flow field are introduced the theoretical analysis is applied to various examples of sectional flow embedded in different types of 3d homogeneous and steady flow fields implications of this investigation for kinematic analysis of natural rock deformation are discussed all types of material line histories are expected in sectional flows of layer like elements shortened boudins can be formed in a single progressive deformation in sequences of rocks with a well developed transposition foliation common in large scale high strain zones each layer unit may have undergone a unique deformation history during its rotation to the final shear zone boundary parallel orientation in kinematic analysis of such areas it is important to ascribe the correct kinematic significance to structural associations and fabrics

vorticity determination distribution partitioning and the heterogeneity and non steadiness of natural deformations abstract an investigation on vorticity in general heterogeneous and non steady flows is presented the bulk vorticity of a flow is solely determined by the imposed boundary conditions and it exerts no constraints on local flow the distribution of bulk vorticity in different rheological domains and the partitioning of the distributed vorticity into shear induced vorticity and spin are both time dependent this indicates that heterogeneous flows are inevitably non steady in a quantitative example it is demonstrated that even though the imposed boundary movement is a constant simple shear local flow can be non steady spinning and non coaxial super simple shear flow with internal kinematic vorticity number greater than i occurs but the finite strain does not necessarily pulsate the geological implications of this investigation are discussed the significance of heterogeneity and non steadiness in natural deformations are emphasized it is concluded that vorticity distribution and partitioning is the link between different scale structures the internal kinematic vorticity number is not a criterion of finite strain oscillation

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

strain partitioning in banded and or anisotropic rocks implications for inferring tectonic regimes abstract among the many concepts that have contributed to the development of structural geology in the last half century two are particularly important these are i the link between the symmetry of the structure the vorticity of the associated deformation process and the tectonic regime and ii the widespread partitioning of strain however a clear understanding of these concepts and the links between them are not always apparent in structural analyses carried out to determine the prevailing tectonic regime of an area through the use of structural symmetries and kinematic indicators this contribution based on field studies from cap de creus experimental data and theoretical concepts highlights some of the problems encountered when attempting to deduce the type of strain and associated regional tectonic regime from field structures the relation between the symmetry of a structure and its associated kinematics is not unique for example it is demonstrated that in mechanically anisotropic materials symmetry variations do not necessarily reflect marked vorticity variations but rather the angular variations between the kinematic framework and the anisotropy planes because of widespread strain partitioning local and general kinematics do not generally match this mismatch is especially accentuated in rheologically heterogeneous rocks nevertheless correct interpretations are possible by performing continuous multi scale structural analyses in which the impact of strain partitioning and of the presence of material anisotropies and heterogeneities are considered

ocean plate stratigraphy in east and southeast asia abstract ancient accretionary wedges have been recognised by the presence of glaucophane schist radiolarian chert and melange recent techniques for the reconstruction of disrupted fragments of such wedges by means of radiolarian biostratigraphy provide a more comprehensive history of ocean plate subduction and successive accretion of ocean floor materials from the oceanic plate through offscraping and underplating reconstructed ocean floor sequences found in ancient accretionary complexes in japan comprise from oldest to youngest pillow basalt limestone radiolarian chert siliceous shale and shale and sandstone similar lithologies also occur in the melange complexes of the philippines indonesia thailand and other regions this succession is called ocean plate stratigraphy ops and it represents the following sequence of processes birth of the oceanic plate at the oceanic ridge formation of volcanic islands near the ridge covered by calcareous reefs sedimentation of calcilutite on the flanks of the volcanic islands where radiolarian chert is also deposited deposition of radiolarian skeletons on the oceanic plate in a pelagic setting and sedimentary mixing of radiolarian remains and detrital grains to form siliceous shale in a hemipelagic setting and sedimentation of coarse grained sandstone and shale at or near the trench of the convergent margin radiolarian biostratigraphy of detrital sedimentary rocks provides information on the time and duration of ocean plate subduction the ages of detrital sediments becomes younger oceanward as younger packages of ops are scraped off the downgoing plate ops reconstructed from ancient accretionary complexes give us the age of subduction and accretion direction of subduction and ancient tectonic environments and is an important key to understanding the paleoenvironment and history of the paleo oceans now represented only in suture zones and orogenic belts

the single slip hypothesis revisited crystal preferred orientations of sheared quartz aggregates with increasing strain in nature and numerical simulation this study discusses the fabric development in naturally sheared quartz aggregates in comparison to results from texture modeling according to the polycrystalline plasticity theory with particular emphasis on the formation of a single c axis maximum the investigated natural shear zone samples were deformed at about 650 50 c with increasing strain up to 14 and show dynamic recrystallization microstructures of grain boundary migration recrystallization neutron diffraction texture analysis results in c axis pole figures with a single maximum at the periphery of the pole figure this maximum does not align with the shear plane normal towards higher strain but rotates towards an inclined orientation in accordance with the sense of shear such a rotation is inconsistent with the single slip hypothesis and suggests that the formation of this c axis pattern is controlled by multi slip on several slip systems based on the polycrystalline plasticity theory this quartz fabric can develop if combined view the mathml source r view the mathml source z and view the mathml source prism slip dominates and must not be related to the commonly proposed view the mathml source basal slip the multi slip texture development is in agreement with the shear sense interpretation from the asymmetry between well defined quartz fabrics and the foliation for dominant view the mathml source basal slip in quartz and 2 numerical simulations predict a single peripheral maximum perpendicular to the shear plane and two a maxima with a 30 inward position parallel to the shear plane this simulation corresponds to naturally observed cpo patterns of quartz formed at different deformation conditions and it is in agreement with the single slip hypothesis hence our combined natural and numerical data suggest that the single slip hypothesis is a possible explanation for a single c 0001 maximum but not universally true

quartz microstructures and crystallographic preferred orientation which shear sense do they indicate abstract crystallographic preferred orientation cpo and shape fabrics of dynamically recrystallized quartz are currently used as shear sense indicators we show that the cpo and shape fabric in a polyphase rock do not necessarily indicate the shear sense at the global scale of the shear zone but rather at the local scale of the deforming and recrystallizing quartz aggregates in lower amphibolite facies shear zones in the gran paradiso metagranodiorite magmatic quartz grains have recrystallized dynamically by subgrain rotation and grain boundary migration and deform inside a very fine grained feldspar mica matrix the quartz cpo has a peripheral c axis maximum inclined synthetically with the local shear sense the orientation of the surface fabric is related to the orientation of the c axis maximum and the microscopic shear sense in the quartz aggregate the geometry of the surface fabric ranges from monoclinic to symmetric depending on the relative contribution of grain boundary migration it is inferred that flow partitioning between the quartz aggregates and the matrix controls the local kinematics cpos are only reliable shear sense indicators in polyphase rocks if the most highly strained parts are analyzed where spin of the aggregates with respect to the shear zone boundary has ceased

mesozoic tectonic evolution of the proto south china sea a perspective from radiolarian paleobiogeography abstract the proto south china sea pscs represents the lithosphere that formerly occupied the region southeast of south china where the present day south china sea is located the subduction and demise of the pscs have attracted wide attention whereas the pre subduction history particular the mesozoic of the region remains relatively neglected in this paper the radiolarian paleobiogeography of the region is analyzed to identify the paleobiogeographic affinity and paleo connectivity of the pscs with the pacific and tethys oceans during the mesozoic then the continent ocean configuration and tectonic evolution of the pscs and adjacent areas are discussed through the integration of paleobiogeographical results and associated paleomagnetic stratigraphic and petrologic data the main conclusions are as follows 1 during the triassic the pscs was situated in the transition zone between the pacific and tethys oceans and its paleobiogeography was influenced by both oceans the pscs was characterized by complex tectonic overprinting by the pacific and tethyan tectonic regimes 2 in the early middle jurassic the pscs was well connected to the pacific ocean but poorly connected to the tethys ocean consistent with the paleobiogeography of the pscs being mainly constrained by the pacific ocean the pscs was part of the paleo pacific slab 3 during the late jurassic cretaceous the pscs was in a semi closed environment the late jurassic early late cretaceous pscs was part of a west facing andean type subduction zone and the late late cretaceous pscs was situated in an extensional tectonic setting

origin of kinkbands and shear band cleavage in shear zones an experimental study specimens of artificial kcl mica schist have been deformed in simple shear with different initial orientations 10 intervals between 0 and 170 of the mica schistosity s1 relative to the imposed simple shear plane for most experiments the angular shear strain was 30 the strain rate 4 8 10 4s 1 and the stress normal to the imposed shear plane 20 mpa all experiments were conducted at room temperature deformation of the specimens is achieved largely by slip parallel to the s1 mica foliation however two new types of slip surfaces also develop as either a single orientation set or as a pair of conjugate sets these new slip surfaces are kinkbands and a shear band cleavage the type and number of sets of new slip surfaces which develop depends on the orientation of s1 relative to the shortening and extension fields of the instantaneous strain ellipsoid our experiments indicate that kinkbands or shear band cleavage form in shear zones in which s1 is inclined to the margin of the zone which structure forms and whether it occurs as a single set of structures or as a conjugate pair depends on the initial orientation of s1 in situations where s1 is parallel to the margins of the zone neither structure develops if the deformation is a simple shear the presence of kinkbands or shear band cleavage in such zones where s1 is parallel to the margins is an indication that the zones were transtensional or transpressional respectively we conclude that shear band cleavage is not only an excellent indicator of shear sense but also a qualitative indicator of the ratio of s1 normal shortening to s1 parallel shear conjugate shear bands indicate that s1 normal shortening was relatively strong and single shear band sets indicate that s1 parallel shear was relatively strong

the behaviour of deformable ellipsoidal particles in three dimensional slow flows implications for geological strain analysis abstract eshelby s equations governing the motion of a viscous inclusion embedded in a matrix of contrasting viscosity are solved numerically for arbitrarily shaped and arbitrarily oriented ellipsoidal particles the theory has important implications for geological strains and strain analysis specifically it predicts that particles more competent than their matrix will have deformed shapes with larger k values than equivalently shaped and oriented passive markers the effect is significant even for viscosity ratios as low as r 1 5 and increases with increase in r for general applications numerical solutions must be sought nevertheless there are useful exceptions 1 1 the two dimensional deformed shape of viscous particles can be computed graphically provided the matrix strain is irrotational pure shear and the section ellipse is a principal plane of the particle ellipsoid for sections through a three dimensional system with arbitrarily oriented particles this method and those based on sections through an elliptic cylinder is inaccurate the reliability is further diminished for simple shear 2 2 irrotational deformations in three dimensions can be computed on transformed flinn plots if the particle or average particle axial orientations are parallel with the applied flow similar diagrams may be used to make independent estimates of viscosity contrasts in polymict conglomerates

evolution of pinch and swell structures in a power law layer analytical and finite element fem solutions for the necking of a single power law layer up to large finite amplitude are obtained continuous necking of the layer produces pinch and swell structures the layer is either a free plate or embedded in a homogeneous medium an analytical solution for finite amplitude necking based on the assumption that plane sections remain plane psrp agrees well with the fem result for a layer power law stress exponent n 5 and for a ratio of layer to medium effective viscosities m 100 fem simulations for embedded layers verify that psrp for m 20 the presented numerical experiments generate localized necking and pinch and swell structures similar to natural ones for n 5 and m 20 additional weakening mechanisms such as strain softening although likely to be operative in nature are not required to generate natural pinch and swell structures fem experiments with random perturbation of the layer interfaces show that even with strong necking instability the layer is thinned at the swell as well as at the necks affecting strain estimation from pinch and swell geometry 2008 elsevier ltd all rights reserved

the motion of deformable ellipsoids in power law viscous materials formulation and numerical implementation of a micromechanical approach applicable to flow partitioning and heterogeneous deformation in earth s lithosphere abstract earth s lithosphere is heterogeneous in rheology on a wide range of observation scales when subjected to a tectonic deformation the incurred flow field can vary significantly from one rheologically distinct element to another and the flow field in an individual element is generally different from the bulk averaged flow field kinematic and mechanical models for high strain zones provide the relations between prescribed tectonic boundary conditions and the resulting bulk flow field they do not determine how structures and fabrics observed on local and small scales form to bridge the scale gap between the bulk flow field and minor structures eshelby s formalism extended for general power law viscous materials is shown to be a powerful means this paper first gives a complete presentation of eshelby s formalism from the classic elastic inclusion problem to newtonian viscous materials and to the most general case of a power law viscous inhomogeneity embedded in a general power law viscous medium the formulation is then implemented numerically the implications and potential applications of the approach are discussed it is concluded that the general eshelby formalism together with the self consistent method is a powerful and physically sound means to tackle large plastic deformation of earth s lithosphere

a newly proposed radiolarian zonation for the jurassic of japan abstract eight radiolarian zones are established on the basis of biostratigraphic investigations on the japanese jurassic these are theparahsuum sp c archicapsa pachyderma laxtorum jurassicum tricolocapsa plicarum tricolocapsa conexa stylocapsa spiralis cinguloturris carpatica andpseudodictyomitra primitiva zones in ascending order biohorizons reflecting evolutionary lineages are utilized where possible in defining the zonal boundaries in the case of absence of adequate lineages biohorizons of the first or last appearance of characteristic species are applied the zones are compared with the zones previously proposed by several workers in japan

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

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

variations in deformation fields during development of a large volume magmatic arc central sierra nevada california mid to late cretaceous plutons in the central sierra nevada magmatic arc show widely preserved magmatic foliation whereas regionally developed solid state foliation is absent relatively slow cooling of these plutons and expected strain rates 10 14 suggest that the plutons were emplaced in a neutral or weakly extensional deformation regime domains of solid state ductile shear of only slightly younger age than the plutons on the other hand indicate a contractional regime timing of pluton emplacement and movement on the shear zones have been constrained using pb u zircon and 40 ar 39 ar hornblende and biotite geochronology both plutons and ductile shear zones become younger toward the east the four more westerly shear zones which were active between ca 100 and 90 ma show steeply plunging stretching lineations whereas the most easterly and or youngest zones active between ca 88 and 78 ma show mostly oblique and or subhorizontal stretching lineations indicating a change in kinematics at ca 90 ma the above events define a complex deformation pattern in which strain regimes fluctuated in time and space between neutral or weak extensional and contractional we propose a tectonic model in which thenospheric mantle corner flow produced eddy pairs in the mantle corner that transmitted a neutral or weak extensional regime to the overlying crust and facilitated the movement of granitic magma to mid and upper levels probably as dikes via fractures slab flattening caused the neutral or weak extensional regime to move eastward away from the trench increased coupling between upper and lower plates induced by the slab flattening promoted contractional strain in the cooling plutons and domains of ductile shear formed in progressively younger plutons to the east the above events were accompanied by an oblique convergence vector between north america and farallon plates engebretson et al 1985 which imposed a relatively small component of right lateral shear onto the arc that increased with time we estimate that at ca 100 ma the convergence vector made an angle obl 20 to the arc normal and we suggest that around ca 90 ma obl passed through a critical value conceivably 20 oblcrit 30 at this juncture the component of right lateral shear became sufficiently large to induce significant arc parallel strike slip movement on the most easterly shear zones these kinematics continued as the dominant scheme possibly as late as ca 78 ma

kinematics of rock flow and the interpretation of geological structures with particular reference to shear zones abstract current interpretations of structures are generally based on homogeneous and steady deformation models despite the fact that both the heterogeneity of rocks materially rheologically and geometrically and the time dependence of imposed geological conditions give rise to significant heterogeneous and non steady flow in concert with field observations we emphasize that the expectation of heterogeneity and non steadiness is the key to understanding natural deformation and that in order to carry out successful structural analysis and tectonic interpretation it is necessary to recognize the first order distinction between imposed boundary conditions typically used to define the tectonic regime e g transcurrent transpression and the response recorded by rocks within the zone structures and fabrics using s c fabric as an example it is demonstrated how flow with a non zero spinning component resulting from the rheological contrasts and or geologically realistic time dependent boundary displacement can drastically change the ideal geometric and kinematic relations between the fabric and the host zone in agreement with both theoretical analysis and field observation it is shown that natural flow regimes range from pure shear to pure rotation including super simple shear in consideration of the heterogeneity and non steadiness of natural deformation kinematic analysis is justifiable only within a homogeneous domain and steady period flow kinematics and mechanisms are interrelated in that firstly mechanisms provide internal constraints on kinematics ensuring that only certain flows are possible and secondly flow kinematics will favour development of certain mechanisms

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

sustainable transpression an examination of strain and kinematics in deforming zones with migrating boundaries most orogenic belts owe their development to oblique convergence and commonly have many orogen parallel transpressional high strain zones to constrain the tectonic history of orogenic belts by structural and fabric analysis of rocks it is desirable to understand quantitatively the relationship between the boundary conditions and the resulting strain distribution and kinematics in these zones most current models for transpression assume homogeneous deformation confined by boundaries that are fixed to material planes this creates a strain compatibility problem at the margins of the active deforming zone and also requires that the strain rate normal to the zone boundaries increase to implausibly high values soon after the onset of oblique convergence transpressional motion the latter contradicts with the observation that transpressional motion can be sustained throughout an orogeny the assumption that zone boundaries are fixed to material planes is unrealistic the outstanding problems of current transpressional models are resolved in this paper by allowing the zone boundaries to migrate through the rock material the consequence of zone boundary migration for the strain field and kinematics within a transpressional high strain zone is investigated mathematically the implications of the modeling for fabric interpretation are discussed the modeling makes general predictions consistent with observed planar and linear fabric patterns in natural transpressional high strain zones it predicts that foliations in transpressional high strain zones are subparallel to the zone boundaries regardless of variation in the imposed boundary conditions lineations cluster along the great circle girdle subparallel to the average foliation the spread of the lineations may vary from point maxima to complete girdles

heterogeneous deformation and texture development in halite polycrystals comparison of different modeling approaches and experimental data modeling the plastic deformation and texture evolution in halite is challenging due to its high plastic anisotropy at the single crystal level and to the influence this exerts on the heterogeneity of deformation over halite polycrystals three different assumptions for averaging the single crystal responses over the polycrystal were used a taylor hypothesis a self consistent viscoplastic model and a finite element methodology the three modeling approaches employ the same single crystal relations but construct the polycrystal response differently the results are compared with experimental data for extension at two temperatures 20 and 100 degreesc these comparisons provide new insights of how the interplay of compatibility and local equilibrium affects the overall plastic behavior and the texture development in highly anisotropic polycrystalline materials neither formulation is able to completely simulate the texture development of halite polycrystals while at the same time giving sound predictions of microstructural evolution results obtained using the finite element methodology are promising although they point to the need for greater resolution of the individual crystals to capture the full impact of deformation heterogeneities c 2003 elsevier b v all rights reserved

small scale ductile shear zones as transposed rheologically weak domains a numerical modeling investigation and practical application abstract we test the hypothesis that small ductile shear zones are developed from initial rheologically weak domains we regard weak domains as ellipsoidal inhomogeneities and apply eshelby s formalism extended for power law viscous materials to investigate the kinematics and finite strain evolution of the partitioned flow fields in weak domains we show that under an imposed bulk flow field weak domains regardless of their initial shapes and orientations deform into zone like features at relatively small bulk strains and the deformation paths inside weak domains have all characteristics expected in ductile shear zones we apply our model to the cap de creus area spain where abundant small ductile shear zones exist to relate the fabrics inside these shear zones with the regional deformation we take a multi scale approach we assume that the area contained many weak domains which were randomly shaped and oriented initially and were deformed into shear zones eventually in response to the regional flow we constrain the regional flow field by the fabric patterns across the area and compute numerically the partitioned flow fields in individual weak domains the latter are related to fabrics inside shear zones our model reproduces first order features of field observed fabrics although the deformation path of each shear zone was close to simple shearing the deformation of the whole belt was not our approach also resolves the strain compatibility problem for a finite sized shear zone embedded in a far less deformed country rock

the concept of steady state foliation abstract a steady state foliation is thought of as a foliation with constant orientation and intensity in a rock undergoing steady state flow the problem discussed is whether such straininsensitive foliations exist in rocks and how they might be distinguished from the more familiar strain sensitive foliations steady state foliations would be useful indicators of stress or of strain rate yet misleading if used as indicators of finite strain one requirement for maintenance of steady state foliations is operation of processes that locally weaken grain shape preferred orientation or reduce grain aspect ratios or destroy lithological layering during flow these are the inverses of the processes normally discussed in connection with foliation development many such processes are known though the details of how and when they operate efficiently enough to stabilize a foliation are unknown there is however a tendency for some of them to operate more efficiently at high strains high temperatures and in flow regimes with high vorticity natural and experimental high temperature shear zones are accordingly one environment in which steadystate foliations may be encountered an experiment is described in which a thin specimen of para dichlorobenzene was given a large shearing deformation at 90 of its absolute melting point a steady state foliation was not produced but the technique is a promising one for further microstructural studies where in situ observation of material undergoing large deformations is desirable

viscoplastic self consistent and equilibrium based modeling of olivine lattice preferred orientations implications for the upper mantle seismic anisotropy anisotropy of upper mantle physical properties results from lattice preferred orientation lpo of upper mantle minerals in particular olivine we use an anisotropic viscoplastic self consistent vpsc and an equilibrium based model to simulate the development of olivine lpo and hence of seismic anisotropy during deformation comparison of model predictions with olivine lpo of naturally and experimentally deformed peridotites shows that the best fit is obtained for vpsc models with relaxed strain compatibility slight differences between modeled and measured lpo may be ascribed to activation of dynamic recrystallization during experimental and natural deformation in simple shear for instance experimental results suggest that dynamic re crystallization results in further reorientation of the lpo leading to parallelism between the main 010 100 slip system and the macroscopic shear thus modeled simple shear lpos are slightly misoriented relative to lpos measured in natural and experimentally sheared peridotites this misorientation is higher for equilibrium based models yet seismic properties calculated using lpo simulated using either anisotropic vpsc or equilibrium based models are similar to those of naturally deformed peridotites errors in the prediction of the polarization direction of the fast s wave and of the fast propagation direction for p waves are usually 15 moreover overestimation of lpo intensities in equilibrium based and vpsc simulations at high strains does not affect seismic anisotropy estimates because these latter are weakly dependent on the lpo intensity once a distinct lpo pattern has been developed thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow two notes of caution have nevertheless to be observed in using these results 1 the dilution effect of other upper mantle mineral phases in particular enstatite has to be taken into account in quantitative predictions of upper mantle seismic anisotropy and 2 lpo patterns from a few naturally deformed peridotites cannot be reproduced in simulations these abnormal lpos represent a small percent of the measured natural lpos but the present sampling may not be representative of their abundance in the earth s upper mantle

the formation of micafish a modeling investigation based on micromechanics abstract micafish porphyroclasts are common and excellent shear sense indicators in mylonites although their geometries are well characterized by recent studies they are still poorly understood here we simulate the behavior in mylonites of micafish by regarding them as anisotropic power law ellipsoidal inhomogeneities embedded in a power law viscous material subjected to general plane straining non coaxial flows we apply the eshelby s solutions extended for general non newtonian power law materials the anisotropy of mica is defined by two different shear viscosities a weak shear viscosity associated with slip on cleavage and a strong shear viscosity without activating the cleavage slip our modeling reproduces naturally observed geometries of micafish natural micafish geometries require that the weak shear viscosity of mica must be lower and the strong shear viscosity must be greater than the matrix viscosity we demonstrate that regardless of the initial condition of micafish the shape long axis and the cleavage trace observed on the vorticity normal section will most commonly end up at a small antithetic angle relative to the shear plane or parallel to it our modeling results are consistent with the mica rheological properties determined by rock deformation experiments

large strain shearing of halite experimental and theoretical evidence for dynamic texture changes abstract we report results from torsion experiments on polycrystalline halite nacl to shear strains 8 and observe a very complex texture evolution the same behavior was reproduced with polycrystal plasticity simulations suggesting that we capture the underlying mechanisms while crystal shapes gradually rotate into the shear plane crystal orientations change continuously and dynamic texture patterns evolve with increasing shear this is highly significant for ultra large deformation as for example implied from geodynamic modeling for the deep earth where seismic anisotropy patterns may develop and locally disappear again as material is deformed during convection the study also suggests that caution is required when interpreting deformation mechanisms from simple shear preferred orientation patterns

strain compatibility and shear zones is there a problem in analyzing deformation in rocks it is important to ensure that the solutions obtained satisfy strain compatibility this creates a challenge to understanding patterns of strain associated with shear zones in which measured strain may appear incompatible with the strain in the shear zone walls flattening strains are common in natural shear zones with locally straight and parallel boundaries to satisfy compatibility conditions such strains require volume loss across the shear zone or deviations from plane strain with or without discontinuities between the shear zone and the wall rock in the case of shear zones for which there is no evidence of volume loss or discontinuities along the shear zone walls problems of strain compatibility may be resolved if individual shear zones are linked together in an appropriate fashion shear zones commonly occur in anastomosing arrays and simple configurations of such arrays and the strains associated with them are examined it is shown that local transpression with strain compatibility can be accounted for in this way quite complex local strain patterns can develop in simple arrays

partitioning of non coaxiality in deforming layered rock masses abstract partitioning of non coaxiality and other flow parameters in deforming layered rock masses with nearly incompressible newtonian rheology is examined in two dimensions by the finite element method the results show that layers with viscosity contrast in suitable orientation deform by different and even by a reverse sense of non coaxiality this remarkable partitioning of non coaxiality is due to a large rotation rate of instantaneous stretching axes in a heterogeneous body as a result of geometry changing through deformation rather than through rotation of material lines

information from folds a review abstract folds are spectacular geological structures that are seen in layered rock on many different scales to mark 30 years of the journal of structural geology we review the information that can be gained from studies of folds in theory experiment and nature we first review theoretical considerations and modeling from classical approaches to current developments the subject is dominated by single layer fold theory with the assumption of perfect layer parallel shortening but we also review multilayer fold theory and modeling and folding of layers that are oblique to principal stresses and strains this work demonstrates that viscosity ratio degree of non linearity of the flow law anisotropy and the thickness and spacing distribution of layers of different competence are all important in determining the nature and strength of the folding instability theory and modeling provide the basis for obtaining rheological information from natural folds through analysis of wavelength thickness ratios of single layer folds and fold shapes they also provide a basis for estimating the bulk strain from folded layers information about folding mechanisms can be obtained by analysis of cleavage and fabric patterns in folded rocks and the history of deformation can be revealed by understanding how asymmetry can develop in folds by how folds develop in shear zones and how folds develop in more complex three dimensional deformations

scale dependence strain compatibility and heterogeneity of three dimensional deformation during mountain building a discussion plate motion at convergent margins causes crustal shortening and orogenic thickening relative motion that is oblique to the plate margins is an inevitable consequence of plate kinematics on a sphere and results in non coaxial three dimensional deformation that cannot be approximated to simple shear models of mountain building that include a smoothly varying component of pure shear shortening allow strain compatibility to be maintained by deforming the upper free surface of the earth without disrupting the material continuum however such models do not reflect accurately the nature of deformation in many areas of high strain in the upper crust which are characterized by interconnected arrays of kinematically linked faults that can be active on several scales of magnitude simultaneously as brittle deformation increases the coherence of the material continuum is highly reduced in such situations strain compatibility is maintained by partitioning the deformation amongst structures of varying kinematic significance over a wide range of scales and not by smooth variations in strain magnitude acting on a single scale across a material continuum there is a marked tendency for such partitioned domains to be oriented parallel or sub parallel to the orogenic grain alignment of domains in this way represents a strong structural anisotropy which acts as a highly significant boundary condition that controls deformation at subordinate scales finite strain observed within an individual domain at a given scale need not therefore display the same magnitude or orientation as bulk finite strain at the plate scale and consequently data must be collected from as large an area as possible to relate outcrop scale structures to global scale tectonics

anisotropic convection with implications for the upper mantle on the basis of polycrystalline theory describing the plasticity in olivine and enstatite the flow in a convection cell has been simulated using a finite element formulation the spatial variations in anisotropic properties are computed from the textures that evolve with the flow a kinematically constrained equilibrium based assumption is used to partition the macroscopic deformation among crystals within an aggregate we model the convection for one specific cell geometry and two sets of boundary conditions a complete map of textures throughout the cell is obtained the textured convection cell is structurally very heterogeneous and textures along streamlines do not correlate with the finite strain the results of the simulations indicate that during up welling a strong texture develops rapidly it convects during spreading and is attenuated during subduction results are compared with features of the upper mantle in our predictions the pattern of preferred orientation during spreading is inclined to the flow coordinates due to deformation by simple shear this is contrary to hess 1964 intuition that 001 slip planes of olivine orient themselves parallel to the flow planes yet the pattern is consistent with natural fabric data significant differences are observed as a function of depth within the cell the variations in the p wave velocities in this textured model mantle are analyzed and correspond well with observed seismic data

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

deformation of granitic rocks experimental studies and natural examples crustal deformation is inherently complex because the crust is inhomogeneous and anisotropic on a wide variety of scales advances in our understanding have come from a combination of field observations laboratory experiments and theoretical models what necessarily links all these investigations is a focus on process crystal plasticity dislocation creep is one of several grain scale deformation processes or mechanisms each process is dominant over a certain range of conditions in the crust has a particular form of the flow law and produces a characteristic set of microstructures characterizing the deformation mechanisms for different minerals over a range of conditions allows one to link the mechanical behavior or flow law associated with that process to the microstructures it produces the information on microstructures can be used to help interpret the thermomechanical history of naturally deformed rocks and flow laws can be used to create more realistic models predicting crustal behavior in different tectonic settings iterations among field theoretical and experimental approaches are critical for making progress in our understanding the deformation process of crystal plasticity is the main focus of this volume crystal plastic deformation of the crust is in many ways more complex than that of the mantle because most crustal rocks are polyphase aggregates and the strengths of common crustal silicates such as quartz feldspar and mica are very different at a given set of conditions yield strength envelopes for the crust as shown schematically in figure 1 are commonly plotted using the dislocation creep flow law for quartz which inherently assumes that quartz is the controlling phase and undergoes steady state flow but is the flow law for dislocation creep of quartz a good approximation for the deformation of a granite let alone a crustal plum pudding of regions and layers of different lithologies the crust

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

an analytical solution in 2d for the motion of rigid elliptical particles with a slipping interface under a general deformation a mathematical model for rigid inclusions with a slipping interface immersed in a general 2d homogeneous deformation is developed under bulk pure shear inclusions are expected to rapidly approach the stretching axis when compared to the behaviour of inclusions with no slip at the interface the derived model predicts synthetic and antithetic motion into a stable orientation under simple shear and thereafter the inclusion makes an antithetic angle with the shear direction under simple shear rotation rates can be higher or lower than those of no slip inclusions depending on orientation a direct relationship between object inclination to the shear direction and the vorticity of the bulk flow is predicted the model compares well with published analogue and numerical experiments

the radiolarian evidence for the accretion of the fu saki formation with the inferred oceanic plate stratigraphy a case of weakly metamorphosed accretionary complex in ishigaki jima southern ryukyu arc japan abstract the island of ishigaki jima located in the western part of the southern ryukyu arc japan is underlain by a basement comprising the tumuru and fu saki formations the former is a pelitic glaucophane schist with a metamorphic age of 220 190 ma and the latter is a weakly metamorphosed accretionary complex composed mainly of chert mudstone and sandstone with minor amounts of limestone and mafic rocks the fu saki formation was weakly metamorphosed at 140 ma latest carboniferous early jurassic microfossils have been obtained from the limestones cherts and siliceous mudstones of this formation but no fossils have been collected from the phyllitic mudstones the radiolarian fauna of the phyllitic mudstones described herein indicates a late pliensbachian early toarcian early jurassic age this result when combined with existing data enables the reconstruction of an oceanic plate stratigraphy showing a succession of in ascending order upper carboniferous triassic cherts sinemurian lower pliensbachian siliceous mudstones and upper pliensbachian lower toarcian phyllitic mudstones and sandstones the radiolarians from the phyllitic mudstones are important in constraining the timing of the accretion of the fu saki formation to the base of the tumuru formation

microtectonics and 39ar 40ar dating of high pressure metamorphic rocks of the south ryukyu arc and their bearings on the pre eocene geodynamic evolution of eastern asia abstract the pre eocene metamorphic rocks found in the yaeyama islands the southern part of the ryukyu arc have disputed relationships with the metamorphic formations of japan and taiwan they are divided into the tomuru and fusaki formations 1 1 the tomuru formation is composed of high pressure hp metamorphic rocks derived from oceanic sediments and the uppermost oceanic crust the first deformation phase is characterized by a synmetamorphic nw se trending stretching lineation l1 kinematic analysis shows that the deformation corresponds to a subhorizontal ductile shear directed along l1 from the northwest to the southeast 2 2 the fusaki formation or chaotic formation is an olistostrome of triassic or lower jurassic radiolarite olistoliths the age of the matrix is assumed to be late jurassic it is slightly schistose and only a s vergent deformation phase is observed it is assumed that the hp schists overthrust the mesozoic olistostrome 39ar 40ar step heating experiments have been carried out on phengite and na amphibole from the tomuru formation phengite and zoned barroisite crossite give well defined plateaus at 225 4 8 ma and 237 6 3 ma respectively they are interpreted as the age of the ductile deformation crossite yields an integrated data of 104 8 8 3 ma interpreted as the age of the thrusting of the hp schists above the chaotic formation the lithostratigraphy petrology microtectonics and 39ar 40ar ages of the hp schists and the chaotic formation are similar to the permian sangun hp schists and the jurassic tanba olistostrome of the inner belt of southwestern japan respectively there the sangun schists also overthrust the tanba olistostrome during the mesozoic orogeny the same tanba like mesozoic olistostrome is also found in taiwan but was intensely deformed and metamorphosed in miocene times the geodynamic collision model proposed to account for the late permian to early triassic orogen of southwestern japan is extended to the south ryukyu and taiwan conversely the mesozoic sanbagawa hp schists and the upper jurassic to lower miocene turbidites and olistostrome sanbosan shimanto zones of southwestern japan are neither exposed in the south ryukyu arc nor in taiwan hypotheses are discussed that call upon tectonic erosion or non deposition to account for this absence

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

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

numerical modeling of the motion of rigid ellipsoidal objects in slow viscous flows a new approach abstract a simple algorithm for modeling the rotation of rigid ellipsoidal objects in viscous flows based on jeffery s 1922 proceedings of the royal society of london a102 161 179 theory is presented and is implemented in a fully graphic mathematics application mathcad http www mathsoft com the orientation of ellipsoidal objects is specified in terms of polar coordinate angles that can be easily converted to the trend and plunge angles of the three principal axes rather than the euler angles with the mathcad worksheets presented in the supplementary data associated with this paper modeling the rotation paths of individual rigid objects the development of inclusion trail geometry within syn kinematic porphyroblasts and the development of preferred orientation and shape fabrics for a population of rigid objects becomes as easy a task as using a spreadsheet the shape and preferred orientation fabrics for a population of rigid objects can be presented in both a three dimensional form and a two dimensional form allowing easy comparison between field data and model predictions the modeler can customize the type and format of the output to best fit the purpose of the investigation and to facilitate the comparison of model predictions with geological observations application examples are presented for various types of modeling involving rigid objects

comments on the growth of accretionary wedges uplift within accretionary foreland wedges has been explained by the development of duplexes which provides a mechanism for underplating in submarine accretionary wedges with little deformation in the upper part of the wedge we interpret duplex development beneath the costa rica forearc a zone widely considered to be a prime example of nonaccretion because of the apparent absence of an imbricate fan of thrusts at the toe of the wedge large scale structural geometry of accretionary wedges is compatible with low angle trenchward dipping backstops in centrist to reverse angle backstops used commonly in models of return flow within wedges which have been invoked to explain the presence of high p t metamorphic rocks we suggest that emplacement of high p t metamorphic rocks are better explained in a collisional setting where known uplift rates are high than by flow within noncollisional accretionary wedges

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

accreted oceanic materials in japan abstract the phanerozoic circum pacific orogenic belts contain numerous ocean derived materials accreted through plate converging processes japanese islands in particular display various kinds of oceanic materials of different origins including fragments of seamounts oceanic reef limestone morb like rocks and oceanic mantle and pelagic sediments the compilation of these rocks in many subduction complexes of late permian to the present led to following conclusions accretion processes work effectively only for materials primarily composing the upper portion of subducting oceanic crust i e layer 1 and layer 2 many fragments of seamount with alkali basalt 600 hot spot seamount 26 oceanic reef limestone 291 morb like basalt 200 and numerous cherts more than 1000 are recognized as ancient oceanic materials accreted to the japanese islands however gabbros and mantle materials of layer 3 and lower parts of the oceanic lithosphere scarcely occur in subduction accretion complexes except for a few examples of back arc basin or fore arc origin accretion occurs episodically in southwest japan oceanic materials were accreted intermittently in 1 a end permian 2 b middle late jurassic 3 c late cretaceous times 4 d at ca 50 ma and 5 e in miocene times while in northeast japan and hokkaido this occurred in b middle late jurassic c late cretaceous and f early cretaceous times in contrast to the general belief on accretion of younger oceanic plates the majority of japanese subduction accretion complexes were formed during the subduction of plates up to 160 ma old the accretionary events in end permian and middle late jurassic times coincide with northward collision of ancient island arcs oceanic rises or seamount chains of hot spot origin with the asian continent accretion relevant to subduction of older plates may be controlled by the collision subduction process of these topographic reliefs on an oceanic plate in addition the chronological coincidence with the continent collision amalgamation between the sino korean and siberian platforms and between the sino korean and yangtze blocks also implies collision induced voluminous supply of elastics from back arc regions and its contribution to the formation of huge accretionary complexes accreted fragments of ancient seamounts are much smaller than the average size of modern seamounts this implies that most parts of a colliding seamount are not accreted but subducted together with the underlying oceanic crust to much deeper levels with respect to the metamorphic grades for japanese subduction complexes oceanic materials have less than 1 vol in the zeolite facies 15 20 in the prehnite pumpellyite metagraywacke facies and ca 30 in the greenschist glaucophane schist facies and albite epidote amphibolite fades this relationship indicates that the major process for landward accretion of oceanic materials is not off scraping or sedimentary mixing at the trench but underplating subcretion at much deeper levels of a subduction zone

a micromechanical approach for simulating multiscale fabrics in large scale high strain zones theory and application 1 deformation fabrics in earth s crust and mantle are commonly used to constrain the tectonic history deformation mechanisms and rheological properties of the lithosphere their formation involves heterogeneous and multiscale deformation processes that current single scale models cannot capture here we present a micromechanics based multiorder power law approach mopla for the simulation of multiscale fabrics in crustal scale high strain zones we consider the progressive deformation in a crustal high strain zone on three different scales on the macroscopic scale representing the average assemblage of rock units at a point we regard the rock mass as a continuum made of many first order elements the progressive deformation of first order elements in the macroscopic flow field simulates tectonic transposition on the scale of an individual first order element we regard it as an eshelby inhomogeneity embedded in a poly element continuum we apply eshelby s inhomogeneity formalism for power law materials to relate the flow field inside a first order element to the macroscopic flow field on the scale pertinent to structures observed on the outcrop or smaller scale the partitioned flow fields inside individual first order elements are used to examine the fabric development we implement mopla in mathcad apply the approach to a natural example of the cascade lake shear zone and discuss the implications of multiscale deformation our model predicts lineation patterns observed in natural high strain zones that have remained unexplained by single scale models

thrust sequences and the significance of low angle out of sequence faults in the northernmost moine nappe and moine thrust zone nw scotland in common with most thrust belts the moine thrust zone scotland preserves a foreland propagating sequence of thrusting however significant examples of local to regional overstep sequences are also recognized and their interpretation remains controversial remapping of the northernmost moine thrust zone has defined a thick belt of mylonites in the centre of which lies the ductile wnw directed moine thrust the base of the mylonite belt however is marked by a regionally significant low angle west directed brittle fault here termed the lochan riabhach thrust that truncates thrust related structures in its footwall comparison of the tectonic stratigraphy of the mylonite belt with that preserved in a downfaulted outlier to the west at durness indicates that the lochan riabhach thrust cuts down section in its footwall and therefore appears to be extensional locally it is unequivocally breached by underlying imbricate thrusts we prefer to interpret the lochan riabhach thrust as an extensional fault that formed during a hiatus in thrust stacking we relate this faulting episode to the changes in the stability of the evolving orogenic wedge and reiterate that such extensional faults may be common although possibly unrecognized features in many other thrust belts

how do lineations reflect the strain history of transpressive shear zones the example of the active alpine fault zone new zealand abstract lineations within mylonites exhumed in the hanging wall of new zealand s active alpine fault zone have a complicated relationship to contemporary plate kinematics the shear zone is triclinic and macroscopic object lineations are not usually parallel to the simple shear direction despite high total simple shear strains 150 this is mostly because the lineations are inherited from pre mylonitic fabrics and have not been rotated into parallelism with the mylonitic stretching direction which pitches c 44 in the fault plane furthermore some lineations have been variably rotated depending on whether they are present in shear bands or microlithons which accommodated bulk strains with different vorticities total strains required to obtain parallelism between the finite maximum principal stretching direction calculated from transpression models and these mylonitic lineations are pure shear stretch s 1 3 5 simple shear 11 7

a fundamental problem with the kinematic interpretation of geological structures abstract non steady flows are ubiquitous in nature not only do imposed boundary conditions vary with time and rock rheology change during the course of deformation but also deformation is generally heterogeneous and all of these conditions lead to non steady flow histories in modern kinematic analysis flow apophyses the instantaneous stretching axes and the vorticity vector collectively referred to as the eigen directions of deformation are used in place of bruno sander s kinematic axes for deformation with a steady flow history this practice is well justified and has led to great advances in structural geology but for non steady flow histories the geometrical relationships among eigen directions flow pattern vary with time this makes it inappropriate to correlate an association of structures and fabrics with certain time invariant flow patterns and hence raises the question how should we interpret geological structures and fabrics kinematically without the assumption of homogeneous and steady deformation we suggest that the answer lies primarily in forward modeling of deformation based on a knowledge of rock properties