Competent Layers Research Articles

A paradoxical situation in determining relative competence from wavelength/arclength ratios within buckle folded multilayers

By measuring the ratio of wavelength to arclength of folds in closely associated disharmonically folded competent layers, it is possible to determine which layer has undergone a greater layer-parallel strain and has a smaller competence. This method may lead to a paradoxical situation. For example when foliated quartzite and mica schist layers are folded together, the mica schist laminae show a much larger buckle shortening than the quartzite layers. On the other hand, the geometry of folds in quartzite indicates that quartzite was more competent than the mica schist. The structure can be explained by different modes of buckling, general buckling in quartzite layers and internal buckling in strongly anisotropic mica schists.

Induced Seismicity Monitoring of an Underground Salt Cavern Prone to Collapse

Within the framework of a large research project launched to assess the feasibility of microseismic monitoring of growing underground caverns, this specific work focuses on the analysis of the induced seismicity recorded in a salt mine environment. A local seismic network has been installed over an underground salt cavern located in the Lorraine basin (Northeast of France). The microseismic network includes four 3-components and three single component geophones deployed at depths between 30 and 125 m in cemented boreholes drilled in the vicinity of the study area. The underground cavern under monitoring is located within a salt layer at 180 m depth and it presents a rather irregular shape that can be approximated by a cylindrical volume of 50 m height and 180 m diameter. Presently, the cavern is full of saturated brine inducing a significant pressure on its walls (~2.0 MPa) to keep the overburden mechanically stable. Nevertheless some small microseismic events were recorded by the network and analyzed (approximately 2,000 events in 2 years of recording). In October 2005 and April 2007, two controlled pressure transient experiments were carried out in the cavern, in order to analyze the mechanical response of the overburden by tracking the induced microseismicity. The recorded events were mainly grouped in clusters of 3–30 s of signal duration with emergent first arrivals and rather low frequency content (between 20 and 120 Hz). Some of these events have been spatially located by travel-time picking close to the actual cavern and its immediate roof. Preliminary spectral analysis of isolated microearthquakes suggests sources with non-negligible tensile components possibly related to fluid-filled cracks. Rock-debris falling into the cavern from delamination of clay marls in the immediate roof is probably another source of seismic excitation. This was later confirmed when the most important seismic swarms occurred at the site during May 2007, accompanied by the detachment of more than 8 × 104 m3 of marly material on top of the cavern roof. In any case, no clear evidence of classical brittle ruptures in the most competent layers of the overburden has been observed during the analyzed period. Current work is focused on the discrimination of all these possible mechanisms to better understand the damage processes in the cavern overburden and to assess its final collapse hazard.

Controls of folding on different scales in multilayered rocks

Folding at different scales is ubiquitous in orogenic belts, and small-scale folds are commonly used to constrain the geometry of larger-scale folds. Analyses of viscous multilayers of N equally-thick stiff layers show that the rate of fold amplification and the dominant wavelength to layer thickness increase with N. This suggests that large-scale multilayer folds would initiate earlier than small-scale minor folds in a single layer or a few layers. How then do the latter amplify at a faster rate than the former to become minor folds on the limbs of larger-scale folds? To answer this question, analytical models of fold initiation in multilayers that contain stiff layers with different thicknesses and viscosities were studied. The models comprise five alternating stiff/soft viscous layers, with a thinner or thicker central stiff layer, in a soft confinement by either viscous half spaces or finite layers against rigid frictionless platens. One or two maxima may occur in the amplification rate – wavelength spectrum: if two, the stronger buckling instability may be either that of the multilayer or a single layer. Early initiation of single-layer folds in a central thin layer is favoured if the multilayer is narrowly confined, and if the layer is significantly stiffer than the adjacent stiff layers. Folds then develop on two scales, creating potential ‘minor’ and ‘major’ folds. In models with a thicker central layer, amplification rate decreases as the layer thickness increases. Unusually thick competent layers in a confined multilayer do not act as ‘control units’ that enhance folding; instead they impede folding.

Three-level Tectonic Model for Intraplate Earthquakes

Earthquake is a process concerning the long-term accumulation and abrupt release of energy in nature. On the basis of the analysis of the characteristics of intraplate earthquake, we present a three-level tectonic model for intraplate earthquake. It consists of a deep level uplift structure of the asthenosphere, a middle level sandwich structure, i.e. ductile-rheological layer sandwiched between competent layers and a shallow brittle faulting structure in the upper crust level. The uplifting of the asthenosphere heats and necks the lithosphere of the local area around the upcoming epicenter. It supplies the thermal energy and mechanical energy to the source area of the upcoming earthquake. It increases the temperature and lowers the viscosity of the ductile-rheological layer in the sandwich structure. The thermal energy increases the competent difference between the high-conductive layer and the high-resistive layers. The thermal and mechanical energy concentrates at the boundary area between the competent and incompetent layers. Active faulting above the sandwich structure accelerates the thinning of the lithosphere and highly concentrates the stress at the boundary between the competent and incompetent layers. When the active fault reaches the boundary of the sandwich structure, it triggers the intensive intraplate earthquake—inducing an abrupt release of the strain energy accumulated at the boundary area. On the basis of the model, we design a primary monitoring system for earthquake prediction by the information received from the three-level tectonic system.

Heterogeneous strain behaviour in competent layers during folding in transpressive regimes

The understanding of folding processes in transpression is crucial in structural geology. However, the study of folding in multilayers where adjacent strata exhibit high competence contrast is impossible if the observations are only restricted to geometrical features. Nevertheless, using detailed three dimensional finite strain analyses it is possible to constrain the solutions. Strain data from the deformed northern Portugal Ordovician quartzites show that folding mechanisms in transpression are highly variable. For the more deformed materials found, not only in the short limbs, but also in the thinner layers of the long limbs, the strain ellipsoids orientations can be explained by a combination of flexural shear (related to the folding process) and sinistral wrench component parallel to the bedding plane (a pervasive regime in the southern branch of the Ibero-Armorican Arc). In the thicker layers of the long limbs, orthogonal flexure seems to be the dominant mechanism. Concerning the strain ellipsoid shape, the constrictional forms predominate mainly in the long limbs. Strain partitioning in transpressive regimes is important at the mesoscopic fold scale, especially in asymmetric folds. In the more deformed short limb, the data indicate that transpression tends to be always an active mechanism, while in the long limb the regional wrench component it's often less important.

Numerical modelling of the effect of matrix anisotropy orientation on single layer fold development

The influence of matrix anisotropy of variable orientation on single layer folding is investigated using finite element models. Both linear (Newtonian) and power-law viscous materials are considered. The results show that the available isotropic analytical solution, when modified to include an appropriate approximation for the anisotropic viscosity, accurately predicts growth rates at small amplitude for planar anisotropy oriented at α = 45° to the competent layer for a wide range of normal viscosity ratios between single layer and matrix ( μ c = 10, 100) and degrees of anisotropy ( δ = normal viscosity/shear viscosity = 2, 12, 25). For high normal viscosity ratio ( μ c = 100), the deviation from the analytical solution for other orientations increases with increasing degree of anisotropy but still remains relatively small (<5% for δ = 25). For low normal viscosity ratio ( μ c = 10), the differences for high δ are more significant and for α ≠ 0°, 45°, or 90° also depend on the imposed boundary conditions. However, if carefully applied, the analytical solution does provide a benchmark test for numerical codes that include oblique anisotropy. The numerical models at both small and finite amplitude show that a tight control on the boundary conditions is crucial for experiments with anisotropic materials, especially when the anisotropy is oblique to the boundaries. Analogue experiments with anisotropic materials, where boundary conditions are more difficult to control, must therefore be designed and interpreted with caution. Matrix anisotropy initially oriented obliquely with regard to the maximum shortening direction results in asymmetric buckle folds in the single layer and asymmetric chevron folds in the matrix, even if the deformation is purely coaxial. This is true for both linear and power-law materials and for a range of boundary conditions, both free and constrained. Asymmetric natural fold structures in anisotropic material do not therefore necessarily imply a component of non-coaxial flow.

Fluid-assisted grain boundary sliding in bedding-parallel quartz veins deformed under greenschist metamophic grade

Iron formation rocks of Quadrilátero Ferrífero, Brazil, were deformed at greenschist facies. Quartz grains in bedding parallel veins were sheared and deformed by a combination of mechanisms assisted by aqueous fluids. Veins in the outcrop appear to be stretched parallel to the compositional layering. The overall vein shapes resemble those of boundinage and pinch and swell. In thin sections, veins show microstructures similar to those observed in hand samples, where domains of large quartz crystals are pulled apart for several millimeters. The voids between quartz fragments are filled with domains of polycrystalline quartz. The microstructural and orientation data show that the strain imposed on the vein as a rigid and competent layer was not accommodated in the quartz polycrystals exclusively by crystal plastic deformation or dynamic recrystallization. The new grains are strain-free, with straight boundaries and with weak to random crystallographic fabrics. We interpret these features to have resulted from a combination of processes, which included grain boundary sliding accomplished by solution transfer. We propose that the coeval operation of both mechanisms allows the aggregate to deform at higher strain rates without necking of the vein layer in a type of flow similar to those described in superplastic regimes.

Structures of the Zagros fold-thrust belt in Iran

The Zagros fold-thrust belt in Iran forms the external part of the Zagros active orogenic wedge. It includes a sequence of heterogeneous latest Neoproterozoic--Phanerozoic sedimentary cover strata, ∼7 to 12 km thick and composed of alternating incompetent and competent layers, overlying Precambrian crystalline basement with a complex pre-Zagros structural fabric. Balancing structures of the cover statigraphic units in the Zagros fold-thrust belt requires in-sequence and out-of-sequence involvement of the Precambrian basement in the deformation. Six detailed balanced and retrodeformable cross-sections, which are constructed based on geological and geophysical data across various sectors of the belt, show fault-bend and fault-propagation folds interpreted to have formed by slip on their subjacent thrusts. Out-of-sequence, basement-rooted thrusts, as the interpretations in the constructed cross-sections suggest, have breached the cover/basement interface and, using incompetent cover strata for propagation, cut across the cover structures and have created associated new folds superimposed upon the pre-existing structures. This style of deformation, which has resulted in structural complexity of the belt, characterizes a ∼200 to 300 km-wide zone of distributed, partly synchronous, deformation with along-strike and across-strike variations. It also implies that the Zagros fold-thrust belt, as the external part of the orogenic wedge, is still in its subcritical condition with internal deformation to achieve a critical taper. In creating structural complexity, in addition to out-of-sequence thrusting, mechanically weak layers (evaporites and mudstones) of the cover strata have played a significant role by providing several detachment horizons. Shortening estimates across the belt are variable; based on the constructed cross-sections and their restorations, minimum shortening estimates range from 16 percent to 30 percent in different sectors of the belt.

Effect of state of compensation on the relaxation of crustal plateaus on Venus

Building on Nunes et al. (2004), which examined the end‐member case of viscous relaxation of fully compensated crustal plateaus on Venus, we now investigate viscous relaxation for the other end‐member case of uncompensated topography. The approach consists of modeling viscous flow in a crust‐mantle system by means of analytic and finite element methods. Differential stresses produced by uncompensated plateau topography concentrate beneath the entire domain of the plateau and extend deep into the mantle. Loss of topography occurs via fast subsidence due to isostatic adjustment, and a slower lateral flow due to spreading of the thickened crust. Rims arise from the isostatic adjustment, and rim dimensions depend on the thickness of the competent layer, controlled by the thermal state of the system. Rim dimensions after 1 Myr are ∼1 km high and ∼300 km wide for a surface temperature of 740 K and thermal gradient of &lt;10 K/km, and ∼600 m high and ∼100 km wide for hotter crustal conditions. Although models predict the marginal circumferential (hoop) folding observed at crustal plateaus, they do not account for the large radial grabens also observed. Adding a subcrustal zone of time‐dependent buoyancy causes a delay in the isostatic adjustment. During this period of delay, viscous flow in a relatively hot crust leads to a reduction in rim amplitude once isostatic adjustment takes place. Only cooler conditions in this model can reproduce the wide and high rims observed at crustal plateaus on Venus. Consequently, phase transitions in the lower crust may produce sufficiently large variations in buoyancy to affect the surface topography and are more compatible with a cooler thermal state, but they cannot explain the thickened crust or the interpretation of gravity data showing current isostatic support. Lateral variation in heat flow (and thickness of the competent layer) may help address the latter and needs to be explored through both lithospheric and mantle convection models.

Influence of viscosity contrast and anisotropy on strain accommodation in competent layers

Folds with different values of viscosity contrast and anisotropy have been simulated using the finite-difference code FLAC™. The kinematical analysis of these folds has enabled conclusions to be reached about strain accommodation mechanisms. The sequence of strain patterns in all the folds analysed only differs in the intensities of the different mechanisms involved, which depend on the mechanical properties of the folds. The order of the different strain patterns in the sequence is the same, regardless of the anisotropy and viscosity contrast. Strain accommodation in folds follows the patterns of tangential longitudinal strain, flexural flow and layer shortening. Nevertheless, no combination of these strain patterns can explain the shape of the folded layer at the inflection point and the high strain intensity values in the inner arc. These problems can only be solved by considering a variant of longitudinal tangential strain that is less intense than has classically been thought and combined with a heterogeneous distribution of flexural flow and layer shortening across the layer. The dependence of the different folding mechanisms on the mechanical properties has been used to devise a graphical method for estimating viscosity contrast and anisotropy from the intensities of the strain patterns in the sequence.

Microstructures in a banded iron formation (Gua mine, India)

This paper provides a detailed documentation of microstructures developed in the banded iron formation (BIF) of Gua mine, located in the Bonai Synclinorium (eastern India), where the rocks have been subjected to three deformations (D1 to D3). Folded iron ores, quartz strain fringes around rigid core objects and folded iron ore layers, and refracted quartz veins are described from samples taken from D2 folds in the banded iron formation. Orientations of microstructures are compared with mesoscopic structures to interpret the generations of ore minerals, planar structures and the time relationship between deformation and development of different microstructures. The mechanism of D2 folding is worked out and its bearing on microstructure development is discussed. The D2 folds are inferred to have developed by a combination of tangential longitudinal strain in the competent layer, flexural flow in the incompetent layers and flexural slip at the interface between layers of differing competence. Homogeneous flattening strain superposed the earlier strain, which led to modification of the folds in the competent layer from class 1B to 1C. This strain is quantified and is found to be higher in the limb than the hinge of a fold. Diffusive mass transfer by solution and bulging dynamic recrystallization in quartz are inferred as the dominant deformation processes during folding. Moreover, based on comparison with published deformation microstructure maps, the microstructures of the present study are estimated to have developed between 300 and 350 °C temperatures at a strain rate of 10−14–10−12 s−1, which are geologically realistic conditions for naturally deformed rocks.

A search for distribution of competent layers under tailings by Multi-channel Analysis of Surface Wave (MASW) – A case history

The Multi-channel Analysis of Surface Wave (MASW) method is a shallow seismic method which may complement the traditional seismic refraction method used in engineering applications. Introducing this newly-developed method, this paper presents a case history of an MASW survey in a tailings dam, where the original topography has been covered with tailings (red mud) from an aluminium refinery over the years. An eight-line 3.6 kilometre MASW survey was carried out in the tailings dam of Queensland Alumina Limited in Boyne Island near Gladstone, Queensland to estimate the depth of the competent layer for construction of a new tailings dam wall. The field procedure used was similar to the off-end shooting method used for seismic reflection surveys. This was further simplified by the use of a purpose-built 24-channel land streamer. The seismic source was a weight dropping system mounted on a vehicle, which drops a 50kg steel weight from a height of 0.8 metres. The recording system used was a Seistronix RAS-24 seismograph. The data were analysed and inverted using the SurfSeis software by Kansas Geological Survey. The depths of the original surface and underlying clay are profiled in terms of their S-wave velocities. The result shows the effectiveness of the MASW method even where velocity reversals are present.

Fluid flow during accretion in sediment-dominated margins: Evidence of a high-permeability fossil fault zone from the Internal Ligurian accretionary units of the Northern Apennines, Italy

We report here a detailed structural study carried out in the Internal Ligurian Units of the Northern Apennines, Italy, formed during the building of the Alpine accretionary complex through subduction of the sediment-filled Ligure–Piemontese oceanic basin. The deformation mechanisms associated with fluid migration across an accretion-related fault zone have been studied through a detailed analysis of different generations of syn-tectonic veins. Hydrofracturing occurred mainly sub-parallel to bedding in unlithified to semi-lithified sediments. Transient, upward-directed fluid injection locally connected the décollement-parallel veins through bedding-normal hydrofractures of lithified sandstone layers. A third vein system comprises fibrous hydrofractures developed on the limbs of accretion-related folds. Crosscutting vein sets and the peculiar features of each identified vein set suggest that deformation was intricately associated with lithification and diagenetic processes. Dehydration-produced fluids transiently injected the lithifying sediments leading to local stress permutations. The proposed model provides a “ramp-flat” migration of fluids in which fluid flow is enhanced along high permeability, less cohesive layers, leading to the development of regional dilated hydrofracture channels like those recognized along the décollement zone of modern margins. The more competent layers are truncated by high angle fractures representing the transient connectivity that existed between horizontal conduits.

Kinematic analysis of symmetrical natural folds developed in competent layers

Analysis of symmetrical folds developed in competent layers yields a kinematic folding model based on the study of the geometric characteristics of the folded layer, finite strain measurements and c-axis preferred orientation of detrital quartz. The meter-scale folds that we studied were formed during the first phase of Variscan deformation in the northwestern Iberian Massif. The strain sequence of the proposed model begins with initial layer shortening (ILSH) followed by tangential longitudinal strain (TLS), flexural flow (FF) and, finally, flattening (FL). In each fold, the intensity of each strain pattern has a small variation. FF is much less important than TLS and usually occurs after the latter due to the geometric incompatibilities developed during TLS. Neutral surface migration cannot completely explain the misfit between the strain measurements of the natural folds and the strain values predicted by the combination of the different strain patterns. This problem may be solved by considering a heterogeneous distribution of ILSH and FF, together with a modified tangential longitudinal strain. Heterogeneous area change across the fold profile could have also had an influence.

Effects of overburden on joint spacing in layered rocks

When a layered system of rock beds is subjected to a sufficiently large extensional strain, joints form in the competent layers. Previous analyses have shown that the ratio between joint spacing and competent layer thickness decreases as the applied strain increases. Further, if the entire interface between the competent layer and the matrix fails in shear (slip), no new joints can form and a lower bound on the joint spacing is reached. In this paper, a joint spacing analysis is developed to explicitly account for the effects of overburden depth. The resulting model is a pair of non-linear equations that can be solved for the characteristic joint spacing as a function of layer thickness, applied strain, and overburden depth. The model results show that, for a given applied strain, the joint spacing first decreases and then increases with depth. This behavior is controlled by the opposing effects of depth-increasing shear strength along the competent layer–matrix interface and depth-increasing compressive prestress. The analysis also reveals that the lower bound (saturation) joint spacing is strongly dependent on depth. Within the choice of realistic physical parameters, predicted values of the saturation-spacing-to-layer-thickness ratio span the range of values observed in the field.

Central Zagros fold‐thrust belt (Iran): New insights from seismic data, field observation, and sandbox modeling

We present five generalized cross sections across the central Zagros fold‐and‐thrust belt (Iran). These sections show that the fold geometry varies significantly both horizontally and vertically. The style is closely related to the changes in the mechanical behavior of the lithostratigraphic horizons and, in particular, to the presence of intermediate décollements within the sedimentary pile. Restoration of the sections shows amounts of shortening of the same order from one section to the other. However, it appears to be unequally distributed, suggesting variations in basal décollement shear strength. Analogue modeling has been performed to systematically investigate the effect of an intermediate décollement level at different depths on the style of folding. The models demonstrate that the position of intermediate décollements is an important factor controlling both structural style and fold wavelength. Models with shallow intermediate décollement show regularly and widely spaced anticlines. In these models, the fold wavelength depends directly on the thickness of the dominant competent layer and short‐wavelength superficial structures mask broad anticlines at depth. Models with deep intermediate décollement are characterized by the rapid propagation of deformation (with small rate of shortening) along this décollement influencing localization of forthcoming anticlines in the upper levels. Such propagation favors the development of duplexes and multiwavelength folds. On this basis, fold kinematics in central Zagros is discussed using the variation of structural style along different folds as an indicator of the sequence of deformation. Detachment folding is the main folding style at least for the initial stages of deformation and thrust faults developed only at later stages. Some of these faults, branched on décollement levels, express the progression of folding, whereas others are linked to late basement faults cutting through early structures. In general, the décollement levels are activated sequentially from deeper horizons to shallower ones. However, in one case (Gachsaran décollement) a shallow décollement is activated during the early stages of folding and then abandoned during the subsequent evolution.

Coeval folding and boudinage under plane strain with the axis of no change perpendicular to the layer

Plane-strain coaxial deformation of a competent plasticine layer embedded in an incompetent plasticine matrix was carried out to improve our understanding about the evolution of folds and boudins if the layer is oriented perpendicular to the Y-axis of the finite strain ellipsoid. The rock analogues used were Beck’s green plasticine (matrix) and Beck’s black plasticine (competent layer), both of which are strain-rate softening modelling materials with a stress exponent n=ca. 8. The effective viscosity η of the matrix plasticine was changed by adding different amounts of oil to the original plasticine. At a strain rate $$\dot e$$ of 10−3 s−1 and a finite strain e of 10%, the effective viscosity of the matrix ranges from 1.2×106 to 7.2×106 Pa s. The effective viscosity of the competent layer has been determined as 4.2×107 Pa s. If the viscosity ratio is large (ca. 20) and the initial thickness of the competent layer is small, both folds and boudins develop simultaneously. Although the growth rate of the folds seems to be higher than the growth rate of the boudins, the wavelength of both structures is approximately the same as is suggested by analytical solutions. A further unexpected, but characteristic, aspect of the deformed competent layer is a significant increase in thickness, which can be used to distinguish plane-strain folds and boudins from constrictional folds and boudins.

Late-to-post collisional brittle–ductile deformation in the Himalayan orogen: Evidences from structural studies in the Lesser Himalayan Crystallines, Kumaon Himalaya, India

The Chiplakot Crystalline Belt (CCB) is a broad shear zone located within the meta-sedimentary zone of the Lesser Himalaya. Evidence from mesostructures and microstructures reveals development of this shear zone in a compressional ductile tectonic regime during continental collision in the Himalaya by top-to-SW shearing along NE-dipping shear surfaces. The shear surfaces are parallel to both the upper and basal thrust planes of the CCB. A hitherto unrecognized, late-to-post collisional brittle–ductile tectonic regime, comprised of two discrete phases is established from dynamic analysis of the kink bands, extensional shear fractures, foliation boudinage, competent layer boudinage, quartz veins and associated flanking folds, and brittle normal and reverse faults in the CCB. Kink bands, foliation parallel slip and general shear were major consequences of this late-to-post-collisional brittle–ductile tectonic episode, and were associated with the intrusion of a swarm of quartz veins. Quartz veins associated with flanking folds are developed at high angles with respect to the mylonitic foliation and parallel to the axial plane of the kink bands throughout the CCB. The geometry of the flanking folds along the quartz veins is a consequence of the fracturing and not vice-versa. The precise shape of the folds depends on the nature of general shearing along the wall of quartz veins and various orientations to the shear direction.

Layering stratigraphy of eastern Coprates and northern Capri Chasmata, Mars

Distinct competent layers are observed in the slopes of eastern Coprates Chasma, part of the Valles Marineris system on Mars. Our observations indicate that the stratigraphy of Coprates Chasma consists of alternating thin strong layers and thicker sequences of relatively weak layers. The strong, competent layers maintain steeper slopes and play a major role in controlling the overall shape and geomorphology of the chasmata slopes. The topmost competent layer in this area is well preserved and easy to identify in outcrops on the northern rim of Coprates Chasma less than 100 m below the southern Ophir Planum surface. The volume of the topmost emplaced layer is at least 70 km 3 and may be greater than 2100 km 3 if the unit underlies most of Ophir Planum. The broad extent of this layer allows us to measure elevation offsets within the north rim of the chasma and in a freestanding massif within Coprates Chasma where the layer is also observed. Rim outcrop morphology and elevation differences between Ophir and Aurorae Plana may be indicative of the easternmost extent of the topmost competent layer. These observations allow an insight into the depositional processes that formed the stratigraphic stack into which this portion of the Valles Marineris is carved, and they present a picture of some of the last volcanic activity in this area. Furthermore, the elevation offsets within the layer are evidence of significant subsidence of the massif and surrounding material.

From XY tracking to buckling: axial plane cleavage fanning and folding during progressive deformation

Folding of axial plane cleavage can occur during progressive deformation without a change in the overall background flow. Two field examples of upright (Lachlan Fold Belt, SE Australia) and recumbent (Naukluft Nappe Complex, central Namibia) folds are presented, in which strongly refracted pressure solution cleavage in competent layers on the fold limbs is buckled as a result of ongoing fold amplification. Finite element modelling confirms that cleavage refraction on limbs can be sufficient for cleavage planes to be subsequently shortened and therefore folded. Cleavage refraction is unequally developed on opposite limbs of asymmetric folds formed by oblique shortening of a layer in coaxial flow or by folding in a more general shear environment. The differences in finite strain on opposite limbs can be quite marked even when the fold shapes themselves are not obviously asymmetric. For folding in simple shear flow, as specifically modelled here, refraction is only strong on the fold limb that rotates against the imposed sense of shear. In known shear environments, this provides a potential kinematic indicator in folded units at relatively low strain (e.g. in simple shear, γ of around one), where other higher-strain indicators, typical of mylonites, are not yet sufficiently developed or are equivocal.