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Thrust faulting has been suggested as a viable mechanism of debris transport at many glaciers, often inferred from the presence of up-glacier dipping bands of debris that emerge at the ice surface to form ridges of basally derived material. However, modelling indicates that the development of thrust faults is mechanically inhibited because stresses are much lower than that required for shear failure, a prerequisite for thrust faulting, and field measurements fail to detect thrust-related displacement. The mechanism for the emplacement of these ridges that appear at the surface of many polythermal valley glacier termini remains open to question. This study re-examines the origin of debris ridges on the surface of Storglaciären, a polythermal valley glacier in northern Sweden, using field observations, ice microstructural analyses, sediment grain size analysis, stable isotope composition of the ice, and modelling. We find no evidence of discrete displacement across the debris bands that produce the ridges, nor do we find evidence that folding might be responsible. We propose that the bands originate at the base of the glacier by one of two mechanisms, perhaps in combination: (i) refreezing of meltwater near the thermal transition in basal ice, and (ii) injection into tensile fractures periodically opened at the base due to high fluid pressure and then freezing. In either case, separation from the base occurs due to high fluid pressure and freezing introduces ice below the debris bands, which are then transported forwards due to basal shear and upwards due to longitudinal compression, and revealed by surface ablation.

Microstructures provide key insights into understanding the mechanical behavior of ice. Crystallographic preferred orientation (CPO) develops during plastic deformation as ice deforms dominantly by dislocation glide on the basal plane, modified and often intensified by dynamic recrystallization. CPO patterns in fine-grained ice have been relatively well characterized and understood in experiments and nature, whereas CPO patterns in “warm” ( T > - 10 ∘ C ), coarse-grained, natural ice remain enigmatic. Previous microstructural studies of coarse-grained ice have been limited to c -axis orientations using light optical measurements. We present the first study of a axes as well as c axes in such ice by application of cryo-electron backscatter diffraction (EBSD) and do so in a shear-dominated setting. We have done this by developing a new sample preparation technique of constructing composite sections, to allow us to use EBSD to obtain a representative, bulk CPO on coarse-grained ice. We draw attention to the well-known issue of interlocking grains of complex shape and suggest that a grain sampling bias of large, branching crystals that appear multiple times as island grains in thin sections may result in the typical multimaxima CPOs previously identified in warm, coarse-grained ice that has been subjected to prolonged shear. CPOs combined from multiple samples of highly sheared ice from Storglaciaren provide a more comprehensive picture of the microstructure and yield a pronounced cluster of c axes sub-normal to the shear plane and elongate or split in a plane normal to the shear direction as well as a concomitant girdle of a axes parallel to the shear plane with a maximum perpendicular to the shear direction. This pattern compares well with patterns produced by subsampling datasets from ice sheared in laboratory experiments at high homologous temperatures up to strains of ∼1.5 . Shear strains in the margin of Storglaciaren are much higher than those in experimental work. At much lower natural strain rates, dynamic recrystallization, particularly grain boundary migration, may have been more effective so that the CPO represents a small, final fraction of the shear history. A key result of this study is that multimaxima CPOs in coarse-grained ice reported in previous work may be due to limited sample sizes and a sampling bias related to the presence of island grains of a single host that appear several times in a thin section.

Los pliegues son estructuras geológicas que se presentan en muchos medios geológicos. Analizar su geometría nos permite obtener una aproximación de la deformación que acomodaron. Los pliegues tipo chevron son útiles para hacer estimaciones de acortamiento en sistemas orogénicos debido a su particular geometría. A pesar de esto, no se ha propuesto un método para calcular la deformación total (dinámica y cinemática) que experimentan estas estructuras. En el presente trabajo se expone un método para el cálculo del acortamiento en pliegues chevron que han sufrido una deformación post flexión o post pandeo, el cual toma como base el modelo propuesto por Ramsay en 1974. El método propuesto es conceptualmente sencillo y general, lo que permite su aplicación para estimar el acortamiento en sistemas orogénicos alrededor del mundo.

The impact of frictional crustal heterogeneities and surface processes on the evolution of the ancient thin-skinned Mexican fold-and-thrust belt (MFTB) is investigated and quantified using numerical finite-element simulations. These simulations explore the evolution, total shortening, and structural styles of deforming orogenic wedges. The models include spatial lateral variations in the internal friction, and the topographic erodibility ( K ), using the power law incision rule. The MFTB exhibits paleogeographical elements manifested as lateral facies changes across two platforms units and two sedimentary basins that show different degrees of deformation. The observed kinematics of the MFTB requires, based on the parameters considered in this study, a rheologically heterogeneous crust, with at least laterally varying internal friction. Yet only limited syntectonic and post-tectonic erosion are necessary to explain the geometries and internal total shortening constrained by field observations and interpretations. Application of the analytical and homogeneous critical Coulomb wedge (CCW) theory to the MFTB closely predicts the external wedge geometry but fails to predict the observed shortening of the individual litho-tectonic units and structural styles.

Microstructures provide key insights into understanding the mechanical behavior of ice. Crystallographic preferred orientation (CPO) develops during plastic deformation as ice dynamically recrystallizes, with the dominance of intracrystalline glide on the basal plane. CPO patterns in fine-grained ice have been relatively well characterized and understood in experiments and nature, whereas CPO patterns in "warm" (T > −10 ºC), coarse-grained, natural ice remain enigmatic. Previous microstructural studies of coarse-grained ice have been limited to c-axis orientations using light optical measurements. We have developed a new sample preparation technique, by constructing composite sections, to allow us to use electron backscatter diffraction (EBSD) to obtain a representative, bulk CPO on coarse-grained ice. We suggest that a grain sampling bias of large, branching crystals that appear multiple times as island grains in thin section may result in the typical multiple maxima CPOs previously identified in warm, coarse-grained ice that has been subjected to prolonged shear. CPOs combined from multiple samples of highly sheared ice from Storglaciären provide a more comprehensive picture of the microstructure and yield a pronounced cluster of c-axes sub-normal to the shear plane and elongate or split in a plane normal to the shear direction, and a concomitant girdle of a-axes parallel to the shear plane with a maximum perpendicular to the shear direction. This pattern compares well with patterns produced by sub-sampling data sets from experimentally sheared ice at high homologous temperatures up to strains of ~ 1.5. Shear strains in the margin of Storglaciären are much higher than those in experimental work. At much lower natural strain rates, dynamic recrystallization, particularly grain boundary migration, may have been more effective so that the CPO has been continuously reset and represents a smaller, final fraction of the shear history, rather than the entire finite strain history.

Glaciers, ice sheets and ice caps represent tectonic systems driven by gravity. Their movement can be studied in real time and the rheological properties and strength of ice determined from laboratory experiments and field measurements. All glacial ice has primary stratification, exhibited by variations in grain size, bubble content and debris content. As it deforms, with deformation dominated by plastic flow and recrystallization, accompanied locally by fracture under tension, a suite of structures develops that reflects the primary fabric of the ice and the anisotropy that develops as a result of cumulative deformation. Initial variations in solid impurity content and strain dependent anisotropy as a result of a crystallographic fabric give rise to effective viscosity increases or decreases compared to isotropic polycrystalline ice of about a factor of ten. Foliation develops from inherited (mostly stratification) or introduced (mostly ice veins or fracture traces) fabric elements and from dynamic recrystallization. It is largely dependent on the accumulated strain, which is highest at the base and near the margins of glaciers, ice sheets and ice streams. Folds develop largely passively due to initial amplification of irregularities in the primary stratification, to variations in flow with time or to inhomogeneous flow associated with shear zones and ductile accommodation around open fractures. Buckle folds and boudinage, mostly on a small scale, occur where viscosity contrast is large, mostly in basal ice. Thrusting and wrench faulting are documented in surging glaciers but theoretically most unlikely and rare or absent elsewhere. Many structures interpreted as faults are not due to shear failure but rather result from shear displacements during opening and closing of tensile fractures.

We used illite Ar/Ar dating to obtain absolute ages of folds and shear zones formed within the Mexican Fold–Thrust Belt (MFTB). The methodology takes advantage of illite dating in folded, clay-bearing layers and the ability to obtain accurate ages from small-size fractions of illite using encapsulated Ar analysis. We applied our approach to a cross-section that involves folded Aptian–Cenomanian shale-bentonitic layers interbedded with carbonates of the Zimapan (ZB) and Tampico–Misantla (TMB) Cretaceous basins in central-eastern Mexico. Basinal carbonates were buried by syn-tectonic turbidites and inverted during the formation of the MFTB in the Late Cretaceous. Results from folds and shear zones record different pulses of deformation within this thin-skinned orogenic wedge. Mineralogical compositions, variations in illite polytypes, illite crystallite size (CS), and Ar/Ar ages were obtained from several size fractions in limbs and hinges of the folds and in the shear zones. 1Md-illite polytype (with CS of 6–9 nm) dominates in two folds in the TMB while 2M1-illlite (with CS of 14–30 nm) dominates in the third fold, in the ZB, and in the fold/shear zone. From west (higher grade) to east (lower grade): Ar retention ages indicate shearing occurred at ~84 Ma in the westernmost shear zone, folding at ~82 Ma in the ZB with subsequent localized shearing at ~77 Ma, and Ar total gas ages constrain the time of folding at ~64 Ma on the west side of the TMB and ~44 Ma on the eastern edge. These results are consistent with the age and distribution of syn-tectonic turbidites and indicate episodic progression of deformation from west to east.

We examine the influence of initial fabric on the development of fractures in experiments in which clay cakes are sheared over two horizontal base plates that move past each other in wrench-fault motion. An anisotropic fabric in the kaolinite was produced during the preparation of the clay cakes for these experiments. To produce the fabric, the clay was either screeded perpendicular to the underlying fault, screeded parallel to the underlying fault, or molded to develop a cake-parallel fabric that is isotropic in the horizontal plane. The influence of the initial fabric is striking. With cake-parallel fabric, both Riedel (R) and conjugate Riedel (R′) shears develop, the former being much more pronounced than the latter. With a fault-parallel fabric, only R shears developed, while with a cake- and fault-perpendicular fabric, only closely spaced high-angle shears developed initially, with more widely spaced R shears forming later. There is a difference in the orientation of the R shears in the three types of fabric, with the angle being smallest for a fault-parallel fabric (mean of initial fractures of 15°) and largest for a fault-perpendicular fabric (mean of initial fractures of 25°). The high-angle shears have an initial average angle of 80° for cake-parallel fabric and 100° for fault-perpendicular fabric. Only in the case of the high-angle fractures for a fault-perpendicular fabric is there significant rotation after formation. It is likely that the difference in fracture orientation attributed to anisotropy of the initial fabric of the clay also has its counterpart in nature and is not confined to fractures formed in shear zones.

The Mexican fold-and-thrust belt in central Mexico has overall characteristics that fit the critical tectonic wedge model. It is thin-skinned, forward propagating, tapers toward the toe (the east), and displays an overall decrease in deformation toward the toe. The internal structures and heterogeneity of deformation are not typical of fold-and-thrust belts, however, due to the presence of two large carbonate platforms, flanked by more thinly bedded basinal carbonates. Kilometer-scale thrusts dominate deformation in the platform carbonates (a more brittle behavior), and mesoscopic buckle folds and associated cleavage dominate deformation in the basinal carbonates (a more ductile behavior). Total shortening across the belt, including both platforms and basins, is ∼55%–65%, with higher values in the basins than in the platforms and a concentration of deformation near the platform borders. The dominant mechanism of folding in the basinal rocks is buckling, with thin chert horizons behaving as single layers and limestone and shaly limestone interbeds buckling as multilayers, with a dominant chevron style. A significant shear component of the deformation is indicated by monoclinic fold symmetry, with a consistent sense of vergence of top toward the foreland. We estimated strain and strain history from mesoscopic analysis of fold geometry and internal strain distribution at several locations across the basin and used this information used to assess the overall kinematics and progressive deformation in the basins, which involve both shortening and shear components. The implications of this for the kinematics of the fold-and-thrust belt are discussed.

The effects of postemplacement deformation on originally nonplanar dikes were tested experimentally and compared to field data from the Rainy Lake zone (Ontario, Canada) where irregular fold-like structures are observed. The analyses reveal that two processes are responsible for the irregular dike geometries. First, dikes intrude with varying orientations as they propagate across layers with rheology contrasts, thus developing nonplanar profiles. Second, with postemplacement ductile deformation, dikes that are more competent than the host rocks are folded and refracted in variable amounts, with the amount depending on the initial dike geometry, the heterogeneities in the host, and the regional strain. Deformed dikes and veins are often used as kinematic markers to evaluate regional tectonics, using methods that generally assume that veins had planar walls prior to deformation. The experimental results indicate that caution should be taken when using this approach, and they provide criteria to distinguish between intrusive and deformation structures in intensely deformed areas where distinction between the two is not readily apparent.

Analysis of mesoscopic structures and related veins allows the history of deformation and role of fluids to be established for part of the central Mexican Fold-Thrust Belt (MFTB). The MFTB developed in mostly carbonate rocks with prominent lateral facies changes associated with two platforms and two basins. Fluids played a key role in the deformation, both physically and chemically, by reducing strength and inducing extensional fracturing through raised pore pressure and by providing the medium for solution transfer on the grain-scale. Veins preserve portions of water related to deformation as fluid inclusions. Lithology and lithological variations strongly control deformation styles, with thrusts dominant in the platforms and folds dominant in the basins, and also influence fluid behavior by controlling both porosity and permeability. Structural observations allow distinguishing veins (dominantly calcite) of several generations, emplaced early, during and late/after deformation (V1–V3 respectively). δ13C and δ18O analyses in calcite from veins and host rock show that the veins confined within thrust slices are isotopically buffered by the host rock and differ in isotopic composition from veins emplaced along major thrusts or crosscutting thrust slices. δD analyses in fluid inclusions and clay minerals strongly suggest rock interaction with meteoric fluids in the west (hinterland) and with fluids close to SMOW in the less deformed eastern (foreland) side of the cross-section. By focusing on a single stratigraphic interval exposed across the width of the fold-thrust belt, we propose a conceptual model that explains the differences in vein-rock isotopic composition, the differences of in isotopic composition of aqueous fluids active during deformation, and the progression of clay dehydration reactions as being related to variations in temperature and intensity of deformation in a growing tectonic wedge.

Despite the fact that most fold–thrust belts around the world share many features, successfully explained by the critical wedge model, the details of their geometric evolution and tectonic style development are poorly understood. In the classic section of the southern Canadian Rocky Mountains the dominant tectonic style consists of imbricate thrust sheets with relatively little internal deformation of the individual slices. In the Mexican fold–thrust Belt (Central Mexico), the age of deformation, the overall structural pattern and the total amount of shortening are similar, but the individual thrust sheets exhibit much more internal deformation as manifest by metre-scale buckle folds. One of the differences between these localities is the lateral variation of facies resulting in massive platform limestone separated by thinly-bedded basinal limestone in the Central Mexico section. Strain is concentrated toward the margins between platforms and basins. In Canada, thick platform carbonates form continuous resistant units across the Front Range. Possible reasons for the differences in tectonic style between the two sections include the dominant lithology, distribution of lithologies, taper angle of the tectonic wedges, amount of friction along the basal detachment and the degree of anisotropy of the basin facies rocks. Fold–thrust belts are common features that developed on the edges of orogenic belts worldwide. Many have been well studied and their characteristic features established (Dahlstrom 1970; Chapple 1978; Boyer & Elliott 1982; Price & Fermor 1985; McClay 1992). These features include the classic notions of forward propagation of thrusts, piggyback style, a basal decollement, and decreasing intensity of deformation towards the foreland. Most fold–thrust belts are wedge-shaped, and since the mid 1980s, the critical taper model of their development (Davis et al. 1983; Dahlen et al. 1984; Dahlen 1990) has become generally accepted. Analogue and numerical models have been developed that are able to capture many of the basic features of fold–thrust belts and that are broadly consistent with the critical taper theory (e.g. Huiqi et al. 1992; Stockmal et al. 2007). From early studies it has been clear that most of the large folds in fold–thrust belts are intimately related genetically to the thrusts, and extensive studies of thrust-related folds, in the field and in analogue and numerical models, have established the nature of this relationship (e.g. Dahlstrom 1969, 1970; Elliott 1976; Suppe 1983; McClay 1992; Wilkerson et al. 2002; Spratt et al. 2004). Despite the many similarities among fold–thrust belts in general, there are significant differences among belts and along strike within individual belts. These reflect, among other things, differences in the strength of the basal detachment and the composition and distribution of the rocks in the fold– thrust wedge. In this contribution we compare basic elements of the structures developed roughly synchronously in two widely separated parts of a single orogenic belt: the North American Cordillera. The sections we compare are a well-known section across the southern Canadian Rocky Mountains and a new section across the Sierra Madre Oriental in central Mexico. The dimensions of the fold– thrust belts in these two locations are similar, and both sections are dominated by carbonates. The structural style, however, is quite different. We compare structures in the two locations and speculate on what may be responsible for the differences.

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.

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

Oblate strains are often observed in meso-scale ductile shear zones and this is generally taken to indicate narrowing across the shear zone during formation. Volume loss is one mechanism that could produce shear zone narrowing. However, not all shear zones display characteristics consistent with volume loss, and in such cases, the narrowing must be accomplished by the extrusion of material from within the shear zone. To explore the relationship between shear zone geometry, volume loss, and extrusion, shear zones were mathematically modeled. Results demonstrate the important influence of pure shear and volume loss on controlling the geometry, displacement, and vorticity of ductile shear zones. Further, volume loss does not preclude extrusion unless, for a given volume loss, the strain is of a specific geometry. Extrusion is a likely mechanism important in shear zone development, even if volume loss occurs. Extrusion presents strain compatibility problems because, unlike crustal-scale shear zones, meso-scale ductile shear zones do not possess a free surface. If extrusion occurs, bulk strain compatibility can be maintained if shear zones interlink in anastomosing arrays or change in thickness, though not all shear zone systems display such characteristics. Modeling results elucidate the deformation style of shear zone in the northwest Adirondacks in NY and in the Kebnekaise region in northern Sweden.

Previously proposed models of vertical-walled transpression implicitly assume that material extrudes upwards during deformation. This assumption is not necessarily valid at all scales given that: (a) in areas of diverse lithologies, local zones of relatively rigid materials may cause extruding material to deflect around those zones, and (b) ductile strain often forms anastomosing geometries of shear zones. Therefore, it is possible that a local extension direction in otherwise classic transpression may be nonvertical for rocks deforming at depth. Using an analogue experiment, we have modeled a transpression zone with localized nonvertical extrusion. This extrusion is accomplished by the addition of a side ‘leak’ that allows sideways extrusion in addition to vertical extrusion. The net extension direction depends on the material's position within the deforming zone, resulting in a significant range of lineation orientations with deformation. The strain patterns produced by transpression with localized nonvertical extrusion may explain the wide array of lineation orientations found in some natural ductile transpression zones.

Structural analysis indicates that, for a given set of conditions, subtle differences in layer configuration and rheology can result in major differences in fold kinematics. We studied three hand-sample scale folds from the Maryland Valley and Ridge province in an effort to understand the accommodation of folding on meso- and microscopic scales, and to constrain the deformation histories of the folds sampled. Two single-layer folds and one multilayer fold were taken from two nearby outcrops of the Wills Mountain Anticline, and are therefore interpreted to have developed under virtually identical pressure and temperature conditions. Although all three folds are dominated by carbonate material, structural fabrics are unique for each fold, indicating rheological contributions to and local structural control of fabric development. Both single layer folds have asymmetric vein distributions that are consistent with strain directions expected from asymmetric flexural flow. Slickenlines (on one sample) and cleavage in an adjacent layer (in the other sample) support this interpretation. The first of these folds appears to have undergone late-stage hinge tightening, as evidenced by the development of crosscutting bed-normal stylolites. In contrast, veins, stylolites, and the intracrystalline deformation in the multilayer fold are suggestive of (symmetric) tangential longitudinal strain followed by heterogeneous sub-horizontal flattening. The three folds are interpreted to be buckle folds, with differing mechanisms accommodating strain within the competent layers.

Theory predicts that stretching lineations in an ideal vertical transpressional zone should be either vertical or horizontal. Many field descriptions of transpressional zones, however, indicate a range of lineation orientations between these extremes. Several theoretical models have been developed to explain such departures from expected lineation orientation, and we discuss these in the context of a field example from the Archean Superior Province in the North American craton. Existing models are insufficient to explain obliquely plunging lineations in this example because: (1) obliquely plunging lineations cannot be accounted for by shear zone boundary effects imposed by a no-slip condition, (2) foliations and lineations vary independently, (3) the vorticity-normal section is subhorizontal, limiting possibilities for inclined simple shear, (4) high vorticity is needed for finite strains and lineations to match previously proposed triclinic models, but vorticity is relatively low, and (5) juxtaposed east and west plunging lineations are unlikely in the previously proposed triclinic models. Because existing theoretical models are not applicable to our field example, we contemplate a new model to explain obliquely plunging lineations within quasi homogeneous transpression.

Small-scale ductile shear zones are well developed in amphibolites of Caledonian nappes of the Swedish Lapland. Strain in the shear zones is recorded by the shapes of originally sub-equant aggregates of plagioclase feldspar. Classical strain analysis on two-dimensional (xz) profiles across the shear zones leads to results that indicate large volume losses associated with the shear strain. Large volume losses are, however, incompatible with geochemical analysis, which shows little chemical variation across the shear zones. By including data from xy and yz sections, information on the three-dimensional state of strain can be obtained. These data indicate that deformation is not confined to a two-dimensional plane—particle extension within the shear zone, parallel to the shear plane and perpendicular to the shear direction has also occurred. The state of strain and strain path in the shear zone can be modeled in a general way by combining stretches in x, y and z with simple shearing and volume loss. End member models correspond to plane strain with volume loss (kX=kY=1; kZ

To investigate deformation and fluid-flow in an actively deforming tectonic wedge, we model the evolution of a large, two-dimensional (100 km long, 5 km thick), mechanically and hydrologically homogeneous and isotropic pile of sedimentary strata that is deformed to become a thrust wedge. We compare both ‘dry’ and ‘wet’ cases, in order to illustrate: (1) the relative importance of fluids on wedge evolution, and (2) the effect of brittle deformation on fluid-flow. We use an elastic–plastic constitutive relation, including a Mohr–Coulomb failure criterion and a non-associated flow rule, and coupled fluid flow, with bulk rock properties that approximate typical foreland sedimentary strata. Simulations are made both with and without dilation. The model is fully dynamic, but inertial forces remain small. Results show that deformation within the wedge is accommodated by reverse-slip movement on shear bands, which migrate in both directions through the wedge as both fore- and back-thrusts. The model has features predicted by the critical-taper theory: (1) overall wedge geometry; (2) crudely self-similar growth during evolution; (3) more intense deformation toward the rear of the wedge. The models show strong overall in-sequence faulting behavior with major thrusts isolating relatively undeformed packages, which are moved in a piggyback manner upon the active thrusts. Intermittent out-of-sequence faulting does however occur, in order to maintain the wedge taper. Fluid-flow in the deforming wedge is dominated by topography, but is also strongly affected by dilational plastic deformation. In all simulations, there is focused fluid flow within fault zones. When mechanical time-stepping is shut off (uncoupled), flow systems evolve to steady-state where inflow equals outflow. By subtracting the two ‘states’ we isolate the mechanical fluid response from the total coupled system response. The mechanical fluid response is manifest as contours of head and pressure difference and focusing of fluid-flow in areas of active deformation. In non-dilational simulations, these zones are quite localized and are a result of the contrasting stresses in and adjacent to a brittle shear zone. In simulations with dilation, the zones of head and pressure difference are broad and regional in extent. They are ten times greater in magnitude than in non-dilation simulations, and they track a rock-damage front that moves through the wedge.

A recent paper in this Journal by Bruce Hobbs, Klaus Regenauer-Lieb and Alison Ord [Hobbs, B., Regenauer-Lieb, K., Ord, A., 2008. Folding with thermal–mechanical feedback. Journal of Structural Geology 30, 1572–1592] presents an alternative theory to the traditional Biot-Ramberg theory for folding of viscous rocks that involves non-equilibrium thermodynamics and thermal–mechanical feedback. The authors convey a strong message throughout their paper that the folds produced by this theoretical and numerical modelling are geologically realistic and provide a better explanation for many natural folds than the traditional theory. They promise the same approach for boudinage, and present this folding paper as part of a “unified framework for rock deformation processes”. Readers of the Journal of Structural Geology might be led to conclude that this paper provides a good alternative model for folding of rocks. Our discussion will disagree, on four counts.

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.

The Steep Rock Group of northwest Ontario's Wabigoon subprovince is one of the world's thickest Archean carbonate platform successions. It was deposited unconformably over a 3001-2928 Ma gneissic terrane, and contains a remarkable group of biogenic and oolitic limestones, dolostones, micrites, and karst breccias capped by a thick paleosol developed between and over karst towers. The presence of aragonite fans, herringbone calcite, and rare gypsum molds suggests that the carbonate platform experienced at least local anaerobic and hypersaline depositional conditions. This sequence shows that a combination of chemical and biological processes was able to build a carbonate platform 500 m thick by 3 billion years ago. The carbonate platform is structurally overlain by a mixture of complexly deformed rocks of the Dismal Ashrock forming a mélange with blocks of ultramafic volcaniclastic rocks, mafic volcanics, carbonate, tonalite, lenses of Fe-ore rock, and metasedimentary rocks, in a shaly, serpentinitic, and fragmental ultramafic volcaniclastic matrix. The mélange shows evidence of polyphase deformation, with early high-strain fabrics formed at amphibolite facies, and later superimposed brittle fabrics related to the final emplacement of the mélange over the carbonate platform. An amphibolite- through greenschist-grade shear zone marks the upper contact of the mélange with overlying mafic volcanic and tuffaceous rocks of the ca. 2932 Ma Witch Bay allochthon, interpreted as a primitive island arc sequence. We suggest an evolutionary model for the area that begins with rifting of an arc sequence (Marmion Complex of the Wabigoon arc) that initiated subsidence and sedimentation on the Steep Rock platform and its correlatives that extend for a restored strike length exceeding 1000 km. Shallow water carbonate sedimentation continued until the platform was uplifted on the flanks of a flexural bulge related to the approach of the Witch Bay allochthon, representing collision of the rifted arc margin of the Wabigoon subprovince with the Witch Bay arc. Mélange of the Dismal Ashrock was formed as off-axis volcanic rocks were accreted to the base of the Witch Bay allochthon prior to its collision with the Steep Rock platform.

The movement of the hangingwall over the footwall at thrust ramps produces a variety of structures found commonly at different scales in fold-and-thrust belts. The nature of the structure depends on the relative rigidity of the hangingwall and footwall, friction along the fault, fault ‘dip’, fault displacement, and the boundary conditions of the deformation. Structures include fault-bend style folds, fault-propagation style folds and wedge folds. We investigate the initial stages of development of such structures using the finite-difference code FLAC. The rock layers are represented as continua with elastic-plastic Mohr-Coulomb constitutive relations, and the fault and bounding bedding planes are assigned normal and shear stiffnesses and coefficients of friction. Under conditions with all layers compressed, antisymmetric fault-propagation style folds develop in both the hangingwall and footwall. With rigid footwall and restricted far-field slip in the hangingwall, a single fault-propagation style fold develops. With far-field displacement of the hangingwall allowed, broader antisymmetric wedge folds develop if hangingwall and footwall are deformable, and a single fault-bend style fold develops if the footwall is rigid. All structures become accentuated with increasing slip on the fault. Where both hangingwall and footwall are deformable, the deformation reduces the ramp angle and tends to minimize distortion of the rock adjacent to the fault. Fault-propagation style folds, paired wedge folds and fault-bend style folds are common in nature, and small-scale examples can be found in various stages of development. Continued slip on thrust faults may lead to the mature structures commonly seen in fold-and-thrust belts.

Geological objects that do not deform homogeneously with their matrix can be considered as inclusions with viscosity contrast. Such inclusions are generally treated as initially spherical or ellipsoidal. Theory shows that ellipsoidal inclusions deform homogeneously, so they maintain an ellipsoidal shape, regardless of the viscosity difference. However, non-ellipsoidal inclusions deform inhomogeneously, so will become irregular in shape. Geological objects such as porphyroblasts, porphyroclasts and sedimentary clasts are likely to be of this kind, with initially rectilinear, prismatic or superelliptical section shapes. We present two-dimensional finite-element models of deformed square inclusions, in pure shear (parallel or diagonal to the square), as a preliminary investigation of the deformation of non-ellipsoidal inclusions with viscosity contrast. Competent inclusions develop marked barrel shapes with horn-like corners, as described for natural ductile boudins, or slightly wavy rhombs. Incompetent inclusions develop ‘dumb-bell’ or bone shapes, with a surprising degree of bulging of the shortened edges, or rhomb to sheath shapes. The results lead to speculation for inclusions in the circle to square shape range, and for asymmetric orientations. Anticipated shapes range from asymmetric barrels, lemons or flags for competent inclusions, to ribbon or fish shapes for incompetent inclusions. We conclude that shapes of inclusions and clasts provide an important new type of strain marker and competence criterion.

Many folds in rocks display angular profiles with sharp hinges and straight limbs. Previous studies by many authors have demonstrated that such fold shapes in multilayers result from an intrinsic anisotropy possessed by layered or foliated rocks. Our results of two-dimensional finite-element modeling show that folds with sharp hinges and straight limbs may also develop in isolated competent layers under suitable rheological conditions. Two basic material properties that affect fold shape are non-linearity and anisotropy. Viscous-plastic flow, power-law flow (strain-rate softening), and strain softening are three types of non-linear behavior. All lead to folds with sharp hinges and straight limbs. Anisotropy has a similar influence on fold geometry. Angularity of folds in isolated competent layers increases with increasing non-linearity of the layer or increasing anisotropy of the layer, and a quantitative relationship between fold angularity and degree of non-linearity or anisotropy may be established. Virtually identical angular fold shapes may be produced by either non-linear or anisotropic layer behavior. The strain distribution associated with these shapes is very different, however. For non-linear behavior, strain is focused in the fold hinges and minimal on the limbs, and for anisotropic behavior the reverse is the case, with shear in the limbs being dominant. These differences suggest that it should be possible to distinguish non-linear from anisotropic rheological behavior using layer shape and strain pattern.

In the traditional geology curriculum, developed before plate tectonics, the emphasis was on rocks, minerals, fossils, and structures, which were largely studied separately. This approach was appropriate for the descriptive/cataloging phase of the earth sciences. The development of plate tectonics has changed the zeitgeist within the geological community so that processes are now viewed as being at least as important as materials. We have restructured our undergraduate curriculum in this spirit. The new curriculum begins with two courses in geodynamics, one emphasizing the solid mechanics of Earth, the other the fluid aspects. A key element of these courses has been to bring together basic physical topics, such as stress and strain, that are of fundamental importance in a variety of earth-science applications and present them in a unified way, emphasizing geologic applications. We also added courses in basic geochemistry, geologic time, and a “capstone” course in Earth systems. We retained several of the ...

Small-scale ductile shear zones in high-grade metagabbros from the Caledonides of northern Sweden provide a good opportunity for examining the development of fabric and strain in rocks undergoing progressive non-coaxial deformation. The rocks contain strain markers in the form of aggregates of plagioclase feldspar. A selected shear zone is analyzed assuming monoclinic symmetry, which requires for the zone as a whole variable simple shear, variable volume change, or variable simple shear and volume change plus a homogeneous deformation. Both size of the feldspar aggregates and aspect ratio of the aggregates plotted against orientation indicate significant volume decrease (in excess of 50%, and perhaps as much as 80%) towards the centers of the zones. The data are broadly consistent with a deformation path involving constant shear strain across the zone followed by variable volume change or by a path of progressive loss of volume with increasing shear strain followed by a homogeneous strain. They are not consistent with a single path involving rates of shear strain and volume loss that are in constant proportions. There is little difference in chemical composition between the shear zone and the surrounding rock. An analysis of the concentrations of elements in the shear zone and wall rock suggests that volume changes were slight (< ± 15%). The discrepancy between the values of volume change estimated from strain analysis and geochemical analysis may be resolved either by geochemical equilibration being attained after deformation or by a departure from the geometric constraints of the monoclinic ‘band’ model for shear zones. Modification of marker particle size and shape during dynamic recrystallization may also partly account for the apparent volume loss.

Folds in rock provide opportunities for establishing how fabric and deformation mechanisms vary with local variations in strain and strain rate, and strain distribution in folds should, in principle, provide information on rheological conditions during folding. To investigate this, we use finite element models to simulate buckling in single-layer folds in incompressible, power-law materials in plane strain. Numerical results show that the pattern of strain variation in buckle folds is sensitive to variations in the power-law exponent, n L , of the stiff layer. For a given amplitude, wavelength/thickness, and ratio of viscosities, m , of layer to matrix, strain and strain gradient along the axial trace increase more rapidly away from the neutral surface (on both sides) for power-law ( n L > 1) materials than for Newtonian ( n L = 1) materials. Buckling strain is superimposed on early uniform layer-parallel shortening, which becomes greater as initial amplitude decreases and as n L and m decrease. The effect of this superposition can best be described by reference to the infinitesimal neural surface (INS) and finite neutral surface (FNS), which vary in position with degree of overall shortening. The INS and FNS move from the outer arc towards the inner arc during buckling, the latter following the former. Thus, at any stage of folding, the layer can be divided into three zones with different coaxial, non-linear strain histories. Fabric in natural folds is expected to reflect such unsteady flow conditions.

On the basis of experiment and theory, we expect rocks to deform in a linear fashion when diffusive processes control deformation, and nonlinearly in most other situations. The geometric characteristics of buckle folds in layered materials are dependent on rheological parameters, and in particular depend strongly on the stress exponent, n L , of the stiff layers involved. Thus, information about the deformation rocks have undergone and their rheological state during deformation can be obtained by studying fold shapes and strain distributions. This is important because there is uncertainty in extrapolating laboratory-derived flow laws to the very slow natural strain rates and large strains found in nature.

The contrast in rheological properties between layers of different composition or texture, and between stiff inclusions and their matrix, gives rise to perturbations in flow that result in structures. Theory and modeling allow us to understand the conditions necessary for such structures to form and, conversely, we can use the form of the structures to infer possible rheological conditions for the rocks during natural deformation. We review here several structures and their use as indicators of rheological behavior, based on theory, numerical and experimental models, and observations on natural structures.

Information about rheology can potentially be gained from analyzing the shapes of folds in isolated buckled layers. We employ two-dimensional finite element models of incompressible flow in power-law viscous fluids to investigate this. We first show that the shape of the initial perturbation has relatively little effect on the final shape of the folds when the buckling instability is high. We find that the most significant factor affecting the shapes of single-layer folds is the stress exponent, n L , in the flow law of the layer. The hinges of outer arcs become sharper as n L increases and the limbs become relatively longer and straighter. These differences can be expressed quantitatively by defining a curvature index, ki , which has a value of 0 for a fold formed by circular arcs and 1 for a chevron fold. Results show a dependence of ki on n L that is well-defined for L / h > 10, with ki increasing with n L . Data for experimentally-produced folds are consistent with the numerical results. Limb dip, ki and L/h can all be readily measured on natural folds and provide a basis for comparing the shapes of natural and computer-simulated folds. The data for small folds in siltstone layers in shales in the central Appalachians are consistent with highly non-linear flow of the stiff siltstone layers during buckling.

Folds contain information about the deformation rocks have undergone and the condition of the rocks during deformation. How much of this can we decipher? The geometrical characteristics of folds and the strain distribution within them are perhaps the key to unlocking this information. If the folded layers were originally planar, they reflect inhomogeneous deformation, but without additional originally planar or linear markers of other orientations, we cannot determine the state of strain. The strain in a parallel fold, for example, can be accommodated either by flexural slip or by tangential longitudinal strain, two quite different but geologically realistic strain distributions among an infinite number of possible ones. There are some constraints imposed by fold shapes on possible strain distributions. Fold asymmetry reflects, in most circumstances, sense of shear strain parallel to the general orientation of the folded layering. Problems may arise in shear zones with large strains and in foliated rocks in which kink bands develop at small strains. Information on the orientations of principal stresses cannot be obtained from folds, unless the strain is small and the mechanism of folding understood, as for kink bands, or unless the bulk flow is of constant vorticity, which is difficult to demonstrate. The distribution of measured values of wavelength/thickness in a population of single-layer folds, together with measures of strain and estimates of amplification, can be used to estimate the viscosity ratio of the stiff layer to its matrix and the degree of non-linearity in the flow law, if effective power-law flow is assumed. Information on the rheological state of rocks at the time of folding can also be obtained from the pattern of curvature variation in individual single-layer folds, as demonstrated by the use of computer simulations of folding. Where applied, such methods indicate that the slow natural flow of rock involved in folding is mostly consistent with non-linear power-law rheology, as expected from the results of experimental rock deformation involving crystal-plastic deformation mechanisms.

North- to northwest-directed crustal shortening across the western Quetico subprovince and its boundary regions produced a complex deformation sequence within the Quetico belt and resulted in concentrated zones of dextral ductile shear in the boundary regions within the adjacent greenstone–granite terranes. In this paper, we review and introduce new data on the regional deformation features and their geometries and discuss the history of generation of these features. We attribute the deformation sequence to differential partitioning of shortening and shear strains during dextral transpression associated with oblique convergence and accretion along the southern margin of the Superior Province.The turbiditic wacke in the western Quetico subprovince, now typically amphibolite-facies schist and migmatite, underwent an early deformation stage that included recumbent folding (F1) and the generation of an S1 bedding-parallel foliation. This event is most evident along the northern and southern boundaries of the subprovince, but it is also recognized in the lower grade metasedimentary rocks in the adjacent Wawa and Wabigoon subprovinces. In these subprovinces, F1 folding may have been associated with higher level thrusting and allochthonous emplacement of greenstone units. Despite our F1 designation of this event, it it unlikely that this deformation was synchronous across the subprovinces.Widespread upright folding of the overturned limbs of F1 folds produced moderately to gently plunging F2 folds with east–west-trending axial planes. F, folds, with an associated L, stretching lineation subparallel to fold hinges, are well developed along the southern and northern margins of the Quetico subprovince and in the metasediments of the adjacent Wawa subprovince. During this event, ductile dextral shear was concentrated in steeply dipping east–west-trending shear zones in the Wawa subprovince and in the region of the Rainy Lake – Seine River fault along the Quetico–Wabigoon subprovince boundary. In the northern Wawa subprovince, shear was strongly concentrated in relatively incompetent, steeply dipping metasedimentary and tuffaceous units interlayered with more competent greenstone units. Concentrated zones of ductile shear are not evident within the Quetico subprovince away from its boundary regions. However, emplacement of syntectonic plutons in the central Quetico reoriented F2 folds which were then refolded by large regional F3 folds during continued regional shortening.

Anisotropy of magnetic low-field susceptibility (AMS) and anisotropy of anhysteretic susceptibility (AAS) of the middle Precambrian Thomson formation, east-central Minnesota, generally record structural features, but orientations and axial ratios of the susceptibility ellipsoids depend on local magnetic mineralogy. The northern structural zone of the Thomson formation is characterized by an upright, east-west slaty cleavage. The susceptibility ellipsoids plot in the flattening field of a Flinn diagram, with minimum axes normal to the cleavage. AAS ellipsoids are coaxial with AMS ellipsoids, but are generally more prolate. Anisotropy increases with mean susceptibility at one outcrop, interpreted as mixing of weakly anisotropic, paramagnetic chlorite with more strongly anisotropic magnetite. A second outcrop shows the opposite pattern: anisotropy decreases with increasing mean susceptibility, suggesting that here the magnetite may be relatively isotropic, or possibly post-deformational. The southern structural zone of the Thomson formation is characterized by a subhorizontal foliation (S 1 ) and an upright crenulation cleavage (S 2 ). Here, the magnetic susceptibility minima are coincident with poles to S 1 , and maxima are parallel to the S 1 S 2 intersection. Correlation between directional susceptibilities and mean susceptibility (technique of Henry and Daly, 1983) suggests that paramagnetic minima are poles to the early foliation, while (possibly interdeformational) magnetite minima record the second deformation. This interpretation, however, is not supported by the AAS fabric, by thin section observations, or by high-field measurements of paramagnetic susceptibility. Alternatively, both directional and mean susceptibilities may be controlled by syndeformational chlorite.

Buckle-fold development in single layers has been simulated by means of a two-dimensional finite element model incorporating incompressible power-law viscous fluids. We have produced folds with the stiff layer initially parallel to and oblique to the bulk maximum shortening direction and investigated how fold growth rates and geometry vary as functions of viscosity ratio and power-law exponent. Results for single-layer buckling and the related instabilities of boudinage, mullions and inverse folding closely match theoretical predictions in terms of initial growth rates, which depend strongly on the power-law exponent. The results further show that the fold shape is sensitive to changes in the power-law exponent of the stiff layer. The most striking feature is that the fold hinges become sharper and the limbs become relatively longer and straighter as the exponent increases. Asymmetry of folding can arise in bulk pure shear. Models of asymmetric folds with inclinations of the stiff layer of up to 30° have been made. The results indicate that initial growth rates are independent of the inclination of the stiff layer. Results of this work suggest that a number of geometrical properties of folds may be useful in providing information on the rheological behavior of rocks deformed under very slow natural conditions.

We measured strain at more than 60 locations in metasedimentary and metavolcanic rocks of the Vermilion district, an E-W trending Archean greenstone belt in Minnesota. Strain ellipsoid orientations and shapes correlate strongly with N-S location in the belt, but magnitudes do not. Flattening strains occur near the present Vermilion fault (which bounds the greenstone belt to the north) with constrictional strains to the south. The observed strain patterns can be mathematically modeled by deformation paths which produce the flattening strains (with west plunging λ1 axes) by dextral shear of the constrictional strains (with east plunging λ1 axes). Using reasonable geologic constraints, the shear plane must dip to the north with a subhorizontal shear direction. Structures throughout the district also indicate dextral shear. A geometrical finite element program uses the measured strains to destrain the rocks and find the configuration which most closely satisfies strain compatibility equations. The linear E-W strain patterns and minor rotations about horizontal axes during the deformation preclude origin of the greenstone belt by infolding and shear off the flanks of a rising granitic diapir. By accounting for rotations which result in the (deformed) curvature of the original surface, a true estimate of 50% N-S shortening across the belt can be made. The data and deformation models favor the origin of the Vermilion district rocks at a convergent margin, most likely as a N-dipping subduction zone complex with shallow slab dip. The origin of the constrictional strains remains enigmatic.

Structural analysis in the northern margin of the Quetico subprovince (part of the Archean Superior Province of the Canadian Shield) in Minnesota reveals that the main deformation involved polyphase folding (F1 recumbent and nappe-like, and F2 upright, east–west trending, and tight to isoclinal) during regional ductile transpression and amphibolite-facies metamorphism. A younger deformation, developed during the latter stages of regional transpression, resulted in the generation of localized ultraphyllonites along the steeply dipping Rainy Lake – Seine River fault (RLSRF), the major fault separating the Quetico subprovince from the Rainy Lake wrench zone (a wedge-shaped block between the Quetico and Wabigoon subprovinces). The transpression involved north–south shortening and east–west dextral shear. The presence of shear zones in amphibolite-facies wall rock south of the fault and in lower grade ultraphyllonites within the RLSRF suggests that localization of shear occurred by work and (or) reaction softening, possibly enhanced by the influx of fluids during regional cooling. The youngest structures in the wall rock are conjugate brittle faults oriented similarly to the youngest ductile shear structures in the RLSRF, indicating that the zone of transpression widened following the stage of strain localization, possibly due to work hardening during continued regional cooling. Widening of the zone of deformation was accompanied by an increase in the relative intensity of the north–south shortening component of transpression, revealed by chloritized necks of boudinaged quartz ribbons, quartz and calcite microfabrics, and flattening strains. Protracted ductile flow and localized greenschist-facies conditions in the RLSRF, which occurred during widening of the zone of deformation and rotation of the kinematic frame (to produce north–south shortening structures), are best explained by an influx of fluid phases.Mesostructures and quartz microfabrics in late tectonic (but synkinematic) peraluminous leucogranitoid intrusions and host schist 10 km south of the RLSRF record north–south shortening, but not east–west dextral shear, and further support late north–south shortening across the RLSRF.Tectonic settings for the RLSRF include (i) a suture between distinct lithotectonic terranes or (ii) a zone of localization of deformation within the northern margin of the Quetico subprovince following collision between the Quetico and Wabigoon terranes.

Buckle folding of single competent rock layers is typically accommodated by tangential longitudinal strain (TLS), which involves layerparallel stretching in the outer arc of a fold and layer-parallel shortening in the inner arc, with a neutral surface separating the two. We describe a folded fibrous calcite vein in which the primary crystallographic fabric, with c-axes perpendicular to the vein walls, allows only for layer shortening by twinning on 0112; twinning cannot lead to layer stretching. As there are no other deformation mechanisms that allowed for stretching in the outer arc, the folding must have occurred without the development of a neutral surface in the layer. Intensity of twinning increases from the outer edge of the folded vein to the inner edge, suggesting that a neutral surface existed at the outer edge. This is supported by approximate measurements of twinning strain that show a strain gradient consistent with TLS, and that indicate also about 10% layer-parallel shortening prior to fold growth.

Flexural flow produces a continuous deformation equivalent to that of a flexed deck of cards of vanishing card thickness. It is assumed to be a common process by which parallel folds are formed in nature in competent, anisotropic rock layers. The strain pattern produced by flexural flow contrasts strongly with that produced in parallel folds formed by the bending or buckling of isotropic competent layers, but it has not been documented in nature. Two basic questions are thus: (1) If anisotropy is sufficiently large, will flexural flow develop, and (2) if so, are competent rocks sufficiently anisotropic for them to respond to deformation by flexural flow? A measure of planar anisotropy, A , is given by the ratio of viscosity in shortening to viscosity in shear. Finite-element modeling indicates that if the value of this ratio is greater than about 50, the strain pattern in a buckle fold in a competent layer will be almost indistinguishable from one of flexural flow. Such a high anisotropy, however, is unlikely in natural competent layers. The most texturally anisotropic rocks tend to be those rich in phyllosilicates, and these usually behave in an incompetent fashion. Common competent rocks, such as those rich in quartz and feldspar, are rarely significantly anisotropic. A strong preferred crystallographic orientation as a result of crystal-plastic flow can produce anisotropic layer behavior, but A is unlikely to exceed ∼10, which is insufficiently great for folding by flexural flow. We thus conclude that flexural flow is unlikely in single competent layers. The aggregate response to deformation, however, of composite layers of alternating competent and incompetent rocks may be highly anisotropic and on average correspond to flexural flow.

At a depth of about 75 m in the lower part of the accumulation area of the Barnes Ice Cap there is a change from fine-grained ice with a weakly-oriented c-axis fabric to coarser ice with a broad single-maximum fabric. At a depth of about 150 m the single maximum becomes elongate perpendicular to the direction of bubble elongation, and then splits into two distinct maxima making an angle of about 40–45° with respect to one another. At greater depths a third and finally a fourth maximum appear, forming the well known diamond pattern. Mean crystal size does not seem to increase in the transitions from one to two and thence to three maxima, but it may become more uniform. Crystal size does increase in the transition from three to four maxima, however. At the base of the glacier there is a 10–20 m thick layer of unusually-bubbly, fine-grained white ice with a strong single-maximum fabric.The depths to the transitions increase up-glacier and in place of the single-maximum fabric a small-circle pattern is found. Down-glacier the depths to the transitions decrease, systematically eliminating the higher zones. Thus in the lower part of the ablation area, ice with a four-maximum fabric appears at the surface.The independent variables governing these fabric transitions appear to be temperature T, stress τ, and cumulative strain ³oc. In a tentative stability diagram showing the fields in which given fabrics are stable in T–τ–³oc space, multiple-maximum fabrics occur at high temperatures (> —10°C) and at moderate to high stresses, weakly-oriented fabrics at low stresses or low cumulative strains, broad single-maximum fabrics at moderate stresses or moderate cumulative strains, and strong single-maximum fabrics at high stresses or large cumulative strains.

The active portion of the Portuguese Bend landslide complex is approximately 3 km2 in area and 30-50 m thick. Measured displacement rates range from less than 1 to greater than 30 mm/day on different parts of the landslide, with total displacements over the last 30 yrs ranging from about 10 to greater than 150 m. Six types of breccia, each locally with a foliated matrix, were recognized in the active landslide complex and are absent outside the landslide complex. Slide-body breccias are of two types, the first formed by extensional fracturing during bulk "pure shear" at the top of the landslide (slide-top breccia) and the second by flow of tuffaceous shales and fracture of embedded siliceous shales during simple shear deep in the landslide to the basal decollement (slide-bottom breccias). Slide-margin breccias, also in simple shear, are produced on the lateral margins of individual slide blocks accompanying wrench-fault motion. Other breccias (fault-ramp breccias) are formed during motion over ramps. Coll...