Strain geometry,microstructure and mineral chemistry in metagabbro shear zones: a study of softening mechanisms during progressive mylonitization

The strain geometry, microstructure and metamorphism is described from two minor shear zones from the Chatelaudren metagabbro, N. Brittany. A serially slabbed shear zone reveals a strain geometry consistent with simple shear deformation. Strain calculations based on X trajectory angles coincide with those obtained from elliptical mineral clusters at high values of strain only. Strain profiles typically show a broad low-strain region with a narrow high-strain peak at the centre of the zones. Microstructures typically show distinct grain size reduction in both amphibole and feldspar towards the high strain region of shear zones, and this is discussed in terms of deformation mechanisms related to strain softening. A palaeostress estimate based on recrystallized feldspar grain sizes gives a differential stress of 32 MPa for the low strain region and 119 MPa in the shear zone centre. Electron probe analyses reveal chemical and mineralogical changes accompanying metamorphism within the shear zones. This suggests local conditions favourable for ionic diffusion and the activity of fluids is implied.     

Ri-μi/μm-Ci dependent oblateness of deformed pebbles in the Baraitha conglomerate,Central India and tectonic implications

Strain analysis of the Baraitha conglomerate is attempted by direct measurements on extracted pebbles and by micrometric analysis. The overall deformation is of flattening type, with thek value lower by more than half in the matrix than in the pebbles. The viscosity contrast between pebbles and matrix (μ _i/μ_m) is in the ratio of 2:1 and the bulk deformation appears to be strongly controlled by C_i (concentration of pebbles expressed as percentage). The total shortening (≃35%) in the Baraitha conglomerate is comparable with the shortening accomplished in the folding of the overlying Bijawar Group volcanosedimentary sequence. The bulk strain axesX _t, Y_t andZ _t, as determined from the analysis of the deformed conglomerate, are unsymmetrically oriented with reference to folds formed by oblique flexural-slip with neitherX _t norY _tcoincident with the fold hinges. The lack of transection of folds by cleavage again suggests flattening deformation. The extension in theY _tdirection is greater in the matrix than in the pebbles.     

Shear-zone geometries in experimentally deformed clays: the influence of water content,strain rate and primary fabric

In experimentally deformed clays, three parameters which influence the geometry of shear zones are water content, strain rate and the orientation of the shear zones with respect to any primary fabric. The shear zones are the main microstructures induced in the clays within the experimental conditions (triaxial compression at water contents between 20 and 35% w/w and strain rates between 10⁻⁴ and 10⁻⁸ s⁻¹). Small changes in water content produce a significant change in the appearance of shear zones: structures in wetter sediments are more complex and more numerous than those in drier sediments which tend to produce a small number of discrete planar shear zones. Strain rate is a far less important influence on shear-zone geometry. The orientation of the zones with respect to a primary fabric is also significant. Shear zones which are parallel to the fabric have a simpler geometry than zones which intersect the fabric at a high angle. Knowledge of these factors may help interpret the conditions within which shear zones formed in naturally-deformed soft sediments.     

Strain analysis in folds (Infrahelvetic complex,Central Alps)

Strain analysis based on initially uniformly oriented elliptical particles in an oolitic limestone (Blegi oolite) was used to study the homogeneity of the state of strain on various scales, kinematics of folding and deformation mechanisms. A computer (reduced means) method for strain analysis is presented which is based on deforming a population of ellipses with shape and orientation properties of measured undeformed ooids. The strain values obtained with this method are within an accuracy of about 10% (in terms of axial ratios) and are in good agreement with the ones obtained with existing graphical methods. The state of strain is homogeneous on the scale of a thin section, handspecimen and outcrop, provided that regions around relatively strong fossils and regions of marked variations in lithology are avoided. Whole rock strains and strains as indicated by ooids alone are similar. Strain patterns in folds in limestones embedded in sandstones, shales and marl are compatible with bending accompanied simultaneously with a shortening perpendicular to the axial surface. The shortening may be attributed to the shear strains related to fold asymmetry and overthrusting. Strains on the outer arcs of a competent dolomite layer compare well with theoretical and experimental fold models; strain patterns include complex contact strains and change along the fold hinge line across a transverse fault which was active during the folding process. Strains parallel to the hinge line are more or less uniform but do not necessarily represent a plane strain state. Volume change took place during deformation. It was accomplished by pressure solution processes, the pressure solved material being partly redeposited. Pressure solution accounts for only a relatively small fraction of the bulk finite strain and was accompanied by plastic flow. Intracrystalline deformation together with grain boundary sliding and/or grain boundary migration went hand in hand with recrystallization (noteably grain growth).     

Progressive deformation and reorientation of fold axes in a ductile mylonite zone: the woodroffe thrust

The structural geometry of a mylonite zone (the Woodroffe thrust) and the country rock in its immediate vicinity is described. Mylonitic schistosity formed axial planar to folds in country rock foliation and contains a mineral elongation lineation which is constant in orientation. However, the fold axes (and associated intersection lineation) spread in orientation within the mylonitic schistosity but with a strong maximum parallel to the mineral elongation lineation. It is demonstrated that the fold axes formed initially at approximately 90° to mineral elongation but rotated with increase in strain towards it. Where this phenomenon was homogeneous on a macroscopic scale, rotation of large blocks of country rock across zones of mylonitization accompanied reorientation of fold axes within the mylonite.The controversy of progressive simple versus progressive pure shear for mylonite zones is discussed in the light of recent fabric and other evidence. It is concluded that the inhomogeneous forms of both progressive pure shear and progressive simple shear played a part and that the former dominated initially but gradually gave way to the latter until brittle rupture with large simple-shear displacements on a zone lubricated by the formation of pseudotachylite, brought granulite over amphibolite facies rocks.     

Heterogeneous constrictional deformation in a ductile shear zone resulting from the transposition of a lineation-parallel fold

We use new (micro-)structural, petrofabric, strain and vorticity data to analyze the deformation path in a mesoscopic quartz mylonite zone. The mylonite zone resulted from the complete transposition of a stretching lineation-parallel isoclinal fold. Symmetric cleft-girdle quartz c-axis fabrics were recorded in the middle domain, which occupies the inner limbs of the precursor isoclinal fold, while asymmetric cleft- and crossed-girdle fabrics were observed in the upper and lower domains that represent the outer limbs. Constrictional strain, with increasing k values towards the middle domain, is inferred from petrofabric and 3D strain data. Oblique grain shape fabrics yield vorticity estimates of 0.72–0.90 in the zone. However, in the middle domain, pure shear dominated deformation is suggested by orthorhombic crystallographic fabrics. Strain rate is constant throughout the zone; a strain decrease towards the zone center implies that deformation ceased earlier in the middle domain. The data indicates that fold transposition and subsequent mylonitization started as pure-shear-dominated constrictional deformation and progressively changed to simple-shear-dominated, plane strain. During this flow path the asymmetric quartz c-axis fabrics likely developed by depopulation of cleft-girdle maxima rather than from the synthetic rotation of fabric maxima itself.     

Evolution of a calcite marble shear zone complex on Thassos Island, Greece: microstructural and textural fabrics and their kinematic significance

The deformation history of a monophase calcite marble shear zone complex on Thassos Island, Northern Greece, is reconstructed by detailed geometric studies of the textural and microstructural patterns relative to a fixed reference system (shear zone boundary, SZB). Strain localization within the massive marble complex is linked to decreasing P–T conditions during the exhumation process of the metamorphic core complex. Solvus thermometry indicates that temperatures of 300–350°C prevailed during part of the shear zone deformation history. The coarse-grained marble protolith outside the shear zone is characterized by symmetrically oriented twin sets due to early coaxial deformation. A component of heterogeneous non-coaxial deformation is first recorded within the adjacent protomylonite. Enhanced strain weakening by dynamic recrystallization promoted strong localization of plastic deformation in the ultramylonite of the calcite shear zone, where high strain was accommodated by non-coaxial flow. This study demonstrates that both a pure shear and a simple shear strain path can result in similar crystallographic preferred orientations (single c-axis maximum perpendicular to the SZB) by different dominant deformation mechanisms. Separated a-axis pole figures (+a- and −a-axis) show different density distributions with orthorhombic texture symmetry in the protolith marble and monoclinic symmetry in the ultramylonite marble consistently with the observed grain fabric symmetry.     

Strain refraction in layered systems

Strain refraction across competence contrasts is presented as a simple model consisting of two components, a homogeneous strain and a heterogeneous simple shear. For Newtonian materials, the ratio of the layer-parallel simple shear component in adjacent layers is the inverse of their viscosity ratio. Strong changes in ellipsoid size, shape and orientation are predicted across viscosity contrasts.The geological implications of strain refraction theory are considered within the context of the ‘cleavage/strain debate’. The particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies (and kinematic histories) should appear to be subparallel to the XY planes of measured strain ellipsoids. Competent rocks should develop dominantly layer-orthogonal strain, and incompetent layers shear-dominated deformation. A variety of structural features ranging from cleavage refraction, changing lineation orientations, folds transected by cleavage, changes from coaxial to non-coaxial deformation, and ramp-flat fault geometry may be the result of stress and strain refraction in rocks.     

Local shear zone pattern and bulk deformation in the Gran Paradiso metagranite (NW Italian Alps)

The Gran Paradiso nappe of the northwestern Alps mostly consists of augen gneisses derived from the Alpine deformation of Permian granitoids. The regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top-to-west sense of shear. The granitoid protolith is preserved in the kilometre-scale low-strain domain of the Piantonetto Valley and mainly consists of a porphyritic metagranite including joints, leucocratic dykes and biotite-rich schlieren. In this low-strain domain, the Alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite. The shear zones mostly dip towards S–SE in a shallow (shear zones ₁) to steep inclination (shear zones ₂). The shear zones show typical features that can be explained by reactivation of pre-existing joints and planar compositional heterogeneities. Palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening. The kinematics of individual shear zones depends on the orientation of the original heterogeneities (acting as nucleation planes) and by partitioning of strain components at the kilometre-scale with concentration of the flattening component to the Piantonetto low-strain domain. The strain geometry and the kinematics of individual shear zones within Piantonetto are not directly connected to the top-to-west sense of tectonic transport observed elsewhere in the Gran Paradiso nappe. However, the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the Gran Paradiso massif. We conclude that the shear zone network of the Piantonetto Valley is representative of the incipient stages of ductile deformation of a granite nappe. Even if its architecture is determined by the arrangement of pre-existing structural and compositional heterogeneities, aspects of the large-scale bulk strain can be derived from this local shear zone pattern.     

Strain and competence contrast estimation from fold shape

A new method to estimate strain and competence contrast from natural fold shapes is developed and verified by analogue and numerical experiments. Strain is estimated relative to the nucleation amplitude, A_N, which is the fold amplitude when the amplification velocities caused by kinematic layer thickening and dynamic folding are identical. A_N is defined as the initial amplitude corresponding to zero strain because folding at amplitudes smaller than A_N is dominantly by kinematic layer thickening. For amplitudes larger than A_N, estimates of strain and competence contrast are contoured in thickness-to-wavelength (H/λ) and amplitude-to-wavelength (A/λ) space. These quantities can be measured for any observed fold shape. Contour maps are constructed using existing linear theories of folding, a new nonlinear theory of folding and numerical simulations, all for single-layer folding. The method represents a significant improvement to the arc length method. The strain estimation method is applied to folds in viscous (Newtonian), power-law (non-Newtonian) and viscoelastic layers. Also, strain partitioning in fold trains is investigated. Strain partitioning refers to the difference in strain accommodated by individual folds in the fold train and by the whole fold train. Fold trains within layers exhibiting viscous and viscoelastic rheology show different characteristic strain partitioning patterns. Strain partitioning patterns of natural fold trains can be used to assess the rheological behaviour during fold initiation.     

The geometry and kinematics of flow perturbation folds

Minor folds formed synchronous with ductile deformation in high strain zones can preserve a record of the scale and kinematics of heterogeneous flow. Using structures associated with WNW-directed Caledonian thrusting in N Scotland, we show that localised perturbations in flow resulted in the generation of predominantly cylindrical minor folds with hinges lying at low angles to the transport direction. These define a series of larger-scale fold culminations (reflecting ‘surging flow’) or depressions (reflecting ‘slackening flow’) that are bisected by transport-parallel culmination and depression surfaces. The fold patterns suggest a dominance of layer-normal differential shearing due to gradients in shear strain normal to transport. Culmination surfaces are marked by along-strike reversals in the polarity of structural facing and vergence of minor folds which, contrary to classic fold patterns, define reverse asymmetric relationships. Culmination surfaces separate folding in to clockwise (Z folds) and anticlockwise (S folds) domains relative to the transport lineation. The dip of fold axial planes systematically increases as their strike becomes sub-parallel to transport resulting in a 3D statistical fanning arrangement centred about the transport direction. Thus, mean S- and Z-fold axial planes intersect precisely parallel to the transport lineation and potentially provide a means of determining transport directions in cases where lineations are poorly preserved. Culminations display convergent fold patterns with fold hinges becoming sub-parallel to transport towards the culmination surface and underlying detachment, whilst axial planes define overall concave up listric geometries which are bisected by the culmination surface. Thus, around culminations and depressions there are ordered, scale-independent relationships between transport direction, shear sense, fold facing, vergence and hinge/axial plane orientations. The techniques described here can be applied and used predictively within any kinematically coherent system of ductile flow.     

Structural pattern in the Precambrian rocks of Sonua-Lotapahar region,North Singhbhum,eastern India

In the western part of the North Singhbhum fold belt near Lotapahar and Sonua the remobilized basement block of Chakradharpur Gneiss is overlain by a metasedimentary assemblage consisting of quartz arenite, conglomerate, slate-phyllite, greywacke with volcanogenic material, volcaniclastic rocks and chert. The rock assemblage suggests an association of volcanism, turbidite deposition and debris flow in the basin. The grade of metamorphism is very low, the common metamorphic minerals being muscovite, chlorite, biotite and stilpnomelane. Three phases of deformation have affected the rocks. The principal D1 structure is a penetrative planar fabric, parallel to or at low angle to bedding. No D1 major fold is observed and the regional importance of this deformation is uncertain. The D2 deformation has given rise to a number of northerly plunging major folds on E-W axial planes. These have nearly reclined geometry and theL ₂lineation is mostly downdip on theS ₂surface, though some variation in pitch is observed. The morphology of D2 planar fabric varies from slaty cleavage/schistosity to crenulation cleavage and solution cleavage. D3 deformation is weak and has given rise to puckers and broad warps on schistosity and bedding. The D2 major folds south of Lotapahar are second order folds in the core of the Ongarbira syncline whose easterly closure is exposed east of the mapped area. Photogeological study suggests that the easterly and westerly closing folds together form a large synclinal sheath fold. There is a continuity of structures from north to south and no mylonite belt is present, though there is attenuation and disruption along the fold limbs. Therefore, the Singhbhum shear zone cannot be extended westwards in the present area. There is no evidence that in this area a discontinuity surface separates two orogenic belts of Archaean and Proterozoic age.     

Calcite texture development in experimentally induced ductile shear zones

Specimens of fine grained micritic limestone were deformed in plane strain geometry in pure shear, a combination of simple and pure shear, and in simple shear. Temperatures were 400° C and 500° C, confining pressure was 100 MPa. In the experiments with a simple shear component strain is concentrated and approximately homogeneous in a 2–3 mm wide shear zone. Shear displacement is documented by marker lines and circles. Shear strain γ varies between 0.84 and 1.56. Strain is recorded by flattening of individual grains, defining a foliation normal to the axis of principal finite shortening ε ₁. No twinning is observed on a macroscopic scale. X-ray and neutron diffraction techniques were used to characterize texture before and after deformation. All specimens display strong preferred orientation as documented by 0006, 10¯14 and 11¯220 pole-figures, c axes pole-figures display three maxima in the ε₁–ε₃ plane. If the axes of the strain ellipsoids are used as a coordinate system textures in pure and simple shear are similar but there is considerable monoclinic distortion in simple shear which is attributed to the noncoaxial strain path.     

Usage of strain and vorticity analyses to interpret large‐scale fold mechanisms along the Sanandaj–Sirjan HP‐LT metamorphic belt,SW Iran

3D finite strain analyses and kinematic vorticity measurements were carried out on the Loghon Anticline within the HP‐LT Sanandaj–Sirjan metamorphic belt (Neyriz area, SW Iran). Rƒ/φ and Fry methods were used on the strain markers (e.g. deformed fossils) to interpret geometric relationships between the fold axis, strain ellipsoid axes and shear zone boundaries. The results indicate the predominance of prolate strain in the anticline. Quantitative kinematic analyses show that the W_k parameter is 0. 67 ± 0. 06 (i.e. pure‐shear dominated non‐coaxial flow). This study quantitatively supports the establishment of a dextral transpressive system, which is responsible for the development of the large‐scale right‐lateral shear zones that strike sub‐parallel to the major folds. Flexural shear combined with regional dextral‐shear is suggested to be the most common mechanism of folding in this area. Copyright © 2011 John Wiley & Sons, Ltd.     

Rheological and kinematical responses to flow of two-phase rocks

Numerical simulations have been performed to investigate the strain-dependent behaviour of rheological and kinematical responses to flow of two-phase rocks using the commercial finite-difference program FLAC2D. It was assumed that the two phases have Maxwell rheology. Plane strain and velocity boundary condition, which produces a simple shear deformation, were also assumed. Two types of geometries were considered: strong phase supported (SPS) and weak phase supported (WPS). We calculated strain-dependent variations of effective viscosity and partitioning of strain rate, vorticity and kinematic vorticity number during deformation in both SPS and WPS structure models.The results show that the strain-dependent behaviour is largely influenced by the geometry of the composite. SPS models show both strain hardening and strain softening during the simulations, with strain hardening preceding strain softening. A critical shear strain is necessary to begin the strain softening behaviour. Strain hardening and strain softening are accompanied by a reduction and an increase of the partition of strain rate into the weak phase, respectively. On the other hand, WPS models show only weak strain hardening and strain softening, being the strain-dependent behaviour close to a steady state flow. In addition, the following results are obtained on vorticity and kinematic vorticity number; (1) in both SPS and WPS models the partition of vorticity into weak phase increases with progressive shear strain, i.e. the strong phase becomes less rotational, (2) in SPS models weak inclusions changes from sub-simple shear to super-simple shear with progressive strain, whereas the strong matrix changes from super-simple shear to sub-simple shear, (3) in WPS models the strong inclusions with high viscosity contrasts are less rotational but can be in super-simple shear condition to high strains.The observed strain-dependent behaviours have been compared with previous proposed analytical models. The degree of agreement is variable. Balshin and Ryshkewitch–Duckworth models are only applicable to SPS models. Ji-generalized mixture rule model is applicable to both models.The results suggest that polyphase rocks with SPS structure during ductile shear deformation respond as strain softening materials, after an initial strain hardening stage that may drive to the strain localization into the material.     

The development of asymmetrical folds in a cross-laminated siltstone

A group of folds in alternating pelites and cross-laminated siltstones is described. An interpretation of the finite strain state, in the competent silt layers, is proposed on the basis of an analysis of the angle between cross-lamination and the principal surface of accumulation. Strain magnitudes are greatest in the fold hinge where domains of layer parallel shortening and layer parallel extension are separated by a neutral surface. Strain magnitudes in the fold limbs are small and are largely related to the development of the asymmetry of the folds. In the incompetent pelitic layers, strain in the fold limbs has a large, layer parallel shear component. Deformation in the pelites is accompanied by, and presumably partially achieved by, migration of quartz from areas where there is a tendency for volume to decrease, to areas where it is tending to increase. This process involves local increases in volume of more than 50%.A kinematic model is proposed for development of the folds. It involves early development of small symmetrical folds followed by their modification to asymmetrical, parasitic structures on the limbs of later folds. In the late stages of folding, continued shortening perpendicular to the axial surface orientation is achieved by development of a conjugate crenulation cleavage.     

Strain and rotation in a multilayered fold

Strain was estimated in a fold of Cambrian interlayered siltstones and pelites by determining the preferred orientation of chlorite grains with an X-ray goniometer. Strains so obtained and the postulate that continuity be preserved allowed unfolding of the fold and the determination of rigid body rotations that accompanied the strain. Petrologic investigation showed no sign of major differential volume changes in the siltstones, and this in conjunction with measured strains led to the conclusion that one of the silty layers making up the fold was not, originally, a bed of uniform thickness but a lenticular body, probably representing a single ripple on a ripple-marked tidal flat.Unfolding by piecemeal fitting of unstrained domains shows that none of the principal axes of strain lie consistently parallel to or at right angles to the fold axis. Rock material was displaced with components orthogonal to the profile plane as well as parallel to it. Strain due to compaction during an early history of increasing sediment overburden cannot be separated from strain during tectonic deformation. Its influence is most clearly seen in differential volume change between siltstones and pelites. Additional differential volume changes within pelite beds could have occurred at any time of the deformational history.A comparison of the orientation of strain and rotation axes in the two limbs of the fold, and also comparison of the same orientations in Eulerian coordinates (Cartesian coordinates in the observed fold) and in Lagrangean coordinates (Cartesian in the unfolded fold) make it probable that episodes of relatively uniform strain both preceded and followed the buckling episode that produced the sharp hinge in the competent silt-stone. The siltstone may have been less indurated and thus no more competent than the pelite during early deformation.     

Three-dimensional model and late stage warping of the Plattengneis Shear Zone in the Eastern Alps

The Plattengneis shear zone is a 250–600 m thick, flat lying, Cretaceous, eclogite facies, mylonitic shear zone, with north-over-south transport direction, that is exposed over almost 1000 km² in the Koralpe region along the eastern margin of the Alps. Although the shear zone is one of the largest in the Alps, its role in the Eoalpine metamorphic evolution and the subsequent exhumation of the region, remain enigmatic and its large-scale geometry is not well understood. The outcrop pattern suggests that the shear zone is made up of a single sheet that is folded into a series of open syn- and antiforms with wavelengths of about 10 km. Eclogite bodies occur above, within and below the shear zone and there is no metamorphic grade change across the shear zone. In the south, the fold axes strike east–west and plunge shallowly to the east. In the north, the fold axes are oriented in north–south direction and form a dome shaped structure of the shear zone. Total shortening during this late stage warping event was of the order of 5%. Indirect evidence constrains this folding event to have occurred between 80 and 50 Ma and the fold geometry implies that the final exhumation in the Koralpe occurred somewhat later than further north. Interestingly, the shear zone appears to strike out of the topography in the south and dip into the topography in the north, so that north of the shear zone only hanging-wall rocks are exposed and south of it only foot-wall rocks. Possibilities for the geometric relationship of the Plattengneis shear zone with the surrounding south dipping detachments are discussed.     

Strain localization in sandstone during embryonic stages of shear-zone evolution

Geometric analysis of nested Riedel structures was used to identify and quantify strain localization processes within faulted Navajo sandstone. The analysis shows systematic deviation from the basic Riedel geometry complying with the Mohr–Coulomb criterion. Using cross-cutting relations amongst deformation bands within the Riedel structures, and comparing the orientations of the deformation bands to theoretical strain calculations, we identify two coupling deformation mechanisms involved in the early stages of shear-zone evolution, namely, granular flow and discrete faulting. Both mechanisms localize during strain accumulation, and the granular flow facilitates considerable change in the initial geometry of the Riedel structures. The analysis demonstrates a systematic sequence, by which new Riedel structures form after a constant amount of shear strain takes place in the sandstone. Analysis further indicates that granular flow is the major deformation mechanism during early stages of shear-zone evolution and discrete faulting is the dominant mechanism during later deformation stages.     

Strain analysis and fold shape in a limestone layer and implications for layer rheology

Folds are developed in thin limestone layers within slates of the McKay Formation exposed to the east of the Rocky Mountain Trench, British Columbia, Canada. They possess geometrical characteristics expected of development by buckling. Strain in the profile plane of a selected fold is similar to that predicted by tangential longitudinal strain, except that magnitudes are too low for the observed curvature. This is attributed to inhomogeneity of strain on the scale of measurement, largely because of pressure solution. Material removed by pressure solution from the inner arc of the fold appears to form veins perpendicular to the hinge, a direction of tectonic stretching. Bedding-parallel stylolites developed diagenetically prior to tectonism.Layer-parallel shortening during the initiation of buckling was less than 20%, and probably less than 10%. The mean arclength/thickness ratio is 6.5 and 7.1, with a dispersion of 0.48 and 0.37 for local and regional populations of 29 and 212 folds, respectively. Application of buckling theory to this data suggests that folding followed a non-linear flow law. The viscosity contrast between limestone and slate would be higher and the power law exponent lower, if initial irregularities in the layers were in the form of a constant amplitude spectrum rather than one of white roughness. The data do not allow a choice of initial amplitude spectrum to be made, nor do they closely constrain estimates of n the power law exponent and viscosity contrast.Deformation in the limestone layers was accommodated by intracrystalline flow (twin gliding), pressure solution, and extensional veining (the last two linked by diffusive mass transfer). The first two dominated deformation in the profile plane of the fold and the last, in association with fracturing, allowed for extension parallel to the hinge. Experimental and theoretical considerations suggest that deformation by a combination of these processes should be non-linear. The non-linear flow law deduced from buckling analysis is consistent with expectations based on observations of active deformation mechanisms.