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 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).     

Three-dimensional strains in fold-hinge zones

Experiments have been carried out to study the orientation and values of the three principal extensions during folding, and to investigate the nature of the deformation paths of the strain ellipsoids at the fold hinges. Single layers embedded in a matrix were deformed by plane strain pure shear with the layers obliquely inclined to the axes of the bulk strain. The strains on the outer and inner arcs of the fold hinges were measured by means of grids on the layer surfaces. The orientation and values of three principal strains during folding depend on the layer orientation and the degree of deformation, and may be different on the outer and the inner arcs. The deformation paths in the outer arc are generally in the constriction field and those in the inner arc lie in the flattening field. The deformation paths have been computed by analyzing the manner by which components of layer shortening, tangential longitudinal strain and elongation parallel to the fold-hinge line combine to give the finite strain.     

Axial directions of folds in rocks with linear/planar fabrics

This paper examines the geological implications of an analysis of constraints on the orientation of fold axes in orthotropic materials. We argue that rocks with penetrative linear and planar shape fabrics may have orthotropic (anisotropic) properties during deformation. Two forms of anisotropy (rheological and structural) could be potentially important in the control of fold axial directions. We discuss a model of deformation of rocks with linear/planar fabrics where, in a single deformation event, major fold axes need not be parallel to minor fold axes and neither need be perpendicular to the principal compressive stress direction. Geological and model examples of anistropic control on fold axial directions are given.     

Strain analysis in Jurassic argillites of the Monte Sirino area (Lagonegro Zone,southern Apennines,Italy) and implications for deformation paths in pelitic rocks

Analysis of strain in Jurassic argillites forming part of the folded and thrusted sedimentary succession of the Lagonegro basin (southern Italian Apennines) has been carried out using ellipsoid-shaped reduction spots as strain markers. Most of the determined finite strain ellipsoids are of oblate type and show a peculiar distribution of the maximum extension direction (X), with maxima either subparallel or subperpendicular to the local fold axes. Using the strain matrix method, two different deformation histories have been considered to assist the interpretation of the observed finite strain pattern. A first deformation history involved vertical compaction followed by horizontal shortening (occurring by a combination of true tectonic strain and volume loss), whereby all strain is coaxial and there is no change in the intermediate axis of the strain ellipsoid. By this type of deformation sequence, which produces a deformation path where total strain moves from the oblate to the prolate strain field and back to the oblate field, prolate strain ellipsoids can be generated and may be recorded where tectonic deformation has not been large enough to reverse pretectonic compaction. This type of deformation history may be of local importance within the study area (i.e. it may characterize some fold hinge regions) and, more generally, is probably of limited occurrence in deformed pelitic rocks. A second deformation sequence considered the superposition of pre-tectonic compaction and tectonic strain consisting of initial layer-parallel shortening followed by layer-parallel shear (related to flexural folding). Also in this instance, volume change during tectonic deformation and tectonic plane strain have been assumed. For geologically reasonable amounts of volume loss due to compaction and of initial layer-parallel shortening, this type of deformation history is capable of producing a deformation path entirely lying within the oblate strain field, but still characterized by a changeover, during deformation, of the maximum extension axis (X) from a position parallel to the fold axis to one perpendicular to it. This type of deformation sequence may explain the main strain features observed in the study area, where most of the measured finite strain ellipsoids, determined from the limb regions of flexural folds, display an oblate shape, irrespective of the orientation of their maximum extension direction (X) with respect to the local structural trends. More generally, this type of deformation history provides a mechanism to account for the predominance of oblate strains in deformed pelitic rocks.     

Quartz deformation mechanisms during Barrovian metamorphism: Implications from crystallographic orientation of different generations of quartz in pelites

The behaviour of quartz during metamorphism is studied based on two case studies from the Barrovian terrains of Sulitjelma in arctic Scandinavia and Loch Tay in the Central Highlands Dalradian of Scotland. Both terrains preserve evidence for metamorphism in pelites involving nucleation and growth of garnet at different times in the deformation history. Data are presented on the size, shape and crystallographic orientation of quartz preserved as inclusions in garnet and as grains in the surrounding matrix. While quartz-grains remain small and dispersed between mica grains, deformation appears to be dominated by grain-boundary sliding accommodated by dissolution–precipitation. At amphibolite facies, textural coarsening occurs by dissolution of small quartz grains and growth of larger quartz grains, coupled with segregation of quartz from mica. As a result, quartz deforms by dislocation creep, developing crystallographic preferred orientations (CPO) consistent with both coaxial and non-coaxial strain. Quartz CPOs with <0001> axes lying parallel to foliation and stretching direction are commonly developed, and best explained by mechanical rotation of inequant (detrital?) quartz grains. There is no evidence for selective entrapment of quartz inclusions in garnet on the basis of quartz crystallographic orientation.     

Flow pattern within a Permian submarine slump recorded by oblique folds and deformed fossils, Ulladulla, south-eastern Australia

The flow pattern within a slump in Permian marine rocks of the southern Sydney Basin, Australia, is recorded by folds and deformed fossils. Abundant brachiopod and bryzoan fossils in the slumped rocks are relatively undeformed, but fossil crinoid stems have been deformed by relative rotation of individual ossicles. Measurement of the strain indicates that the deformation of the crinoids is consistent with flexural flow folding within the slump. Previous models assume that curved slump fold axes remain parallel to the enveloping bedding surface of a slump sheet. Detailed measurements of the orientation of slump folds in this study found fold axes to be oblique to bedding, which is interpreted as a result of folds plunging downward towards the flanks of the slump or slump lobes. In the present model, fold axes are not generally parallel to the strike of the fold axial surface, and this can explain differences between the orientations of slump fold axes and axial surfaces when these are used as directional indicators of slump movement.     

Spiral and staircase inclusion trail axes within garnet and staurolite porphyroblasts from schists of the Bolton Syncline, Connecticut: timing of porphyroblast growth and the effects of fold development

In the Littleton Formation, garnet porphyroblasts preserve three generations of growth that occurred before formation of the Bolton Syncline. Inclusion trails of foliations overgrown by these porphyroblasts are always truncated by the matrix foliation suggesting that garnet growth predated the matrix foliation. In contrast, many staurolite porphyroblasts grew synchronously with formation of the Bolton Syncline. However, local rim overgrowths of the matrix foliation suggest that some staurolite porphyroblasts continued to grow after development of the fold during younger crenulation producing deformations. The axes of curvature or intersection of foliations defined by inclusion trails inside the garnet porphyroblasts lie oblique to the axial plane of the Bolton Syncline but do not change orientation across it. This suggests the garnets were not rotated during the subsequent deformation associated with fold development or during even younger crenulation events. Three samples also contain a different set of axes defined by curvature of inclusion trails in the cores of garnet porphyroblasts suggesting a protracted history of garnet growth. Foliation intersection axes in staurolite porphyroblasts are consistently orientated close to the trend of the axial plane of the Bolton Syncline on both limbs of the fold. In contrast, axes defined by curvature or intersection of foliations in the rims of staurolite porphyroblasts in two samples exhibit a different trend. This phase of staurolite growth is associated with a crenulation producing deformation that postdated formation of the Bolton Syncline. Measurement of foliation intersection axes defined by inclusion trails in both garnet and staurolite porphyroblasts has enabled the timing of growth relative to one another and to the development of the Bolton Syncline to be distinguished in rocks where other approaches have not been successful. Consistent orientation of foliation intersection axes across a range of younger structures suggests that the porphyroblasts did not rotate relative to geographical coordinates during subsequent ductile deformation. Foliation intersection axes in porphyroblasts are thus useful for correlating phases of porphyroblastic growth in this region.     

Development of bedding-normal boudins in the Pilot Mountains, Nevada

Boudins with long axes (BA) oriented subnormal to bedding and to associated fold axes are observed in folded rocks in a thrust sheet exposed near the base of a regionally extensive allochthon in west-central Nevada, USA. Formation of the boudins is related to development of a regional fold-set coeval with major thrusting. The axes of boudins lie at a high angle to bedding, and in some instances, boudins define tight to isoclinal folds which are geometrically associated with the regional deformation. Quartz c-axis fabrics from oriented thin-sections of the boudins indicate extension parallel to the boudin axes (BA).

These relations and other mesoscopic structural data indicate a complex deformational history for boudin development. The history involves thin layers (to become boudins) deformed in folds disharmonic to major structures within the thrust sheet followed by flattening and associated extension parallel to fold axes. During flattening, arcuation occurred within the deforming mass resulting in rotation of fold axes and boudin axes (BA) toward the axis of finite extension (X). Extension parallel to BA recorded in the petrofabrics of boudins records incremental strain axes oriented at a high angle (50°) to the finite X and is probably related to an early plane-strain state associated with disharmonic folding. The finite extension (X) is down-dip in axial planes of major folds formed during thrusting and indicates a northwest to southeast transport for the thrusts.     

Rotation and strain of linear and planar structures in three-dimensional progressive deformation

Rotation and progressive strain have been studied for a sheet embedded in a matrix which undergoes rotational three-dimensional strain under constant volume conditions. The mathematics gives explicit information on the following features:

1. (1) The length and position (relative to a defined coordinate system) of the principal axes of the strain ellipsoid at any stage of the progressive deformation.

2. (2) The position and length of the principal axes in any plane intersecting the strain ellipsoid, also at any stage of the deformation.

3. (3) The position and length of passive markers which initially coincided with the principal axes in an intersecting plane. This is of consequence for the distinction between passively rotating structures and actively forming structures.

4. (4) The shear strain parallel to an intersecting plane or sheet, as indicated by the angular difference between the normal to an intersecting plane at any time and the marker at the same time which initially, however, was parallel to the normal. This layer-parallel shear causes boudins to rotate and the axial plane of buckles to tilt.

The relationships have been expressed quantitatively in the bulk of the paper and illustrated in diagrams. The analysis presented is basic for the study of the deformational behavior of competent sheets of rocks embedded in less competent ones.     

Structures in the banded iron-formation of the southeastern Bababudan Hills, Karnataka, India

The banded iron-formation in the southeastern Bababudan Hills display a macroscopic synformal bend gently plunging towards WNW. The bedding planes in smaller individual sectors show a cylindrical or conical pattern of folding. The dominant set of minor folds has WNW-ESE trending axial planes and the axes plunge towards WNW at gentle to moderate angles, though there is considerable variation in orientation of both axes and axial planes. A later set of sporadically observed folds has N-S trending axial planes. The macroscopic synformal bend within the study area forms the southeastern corner of a horseshoe shaped regional synformal fold closure which encompasses the entire Bababudan range. The minor folds are buckle folds modified to a varying extent by flattening. In some examples the quartzose layers appear to be more competent than the ferruginous layers; in others the reverse is true. The folds are frequently noncylindrical and the axes show curvature with branching and en echelon patterns. Such patterns are interpreted to be the result of complex linking of progressively growing folds whose initiation is controlled by the presence of original perturbations in the layers. Domes and basins have at places developed as a result of shortening along two perpendicular directions in a constrictional type of strain. Development of folds at different stages of progressive deformation has given rise to nonparallelism of fold axes and axial planes. The axes and axial planes of smaller folds developed on the limbs of a larger fold are often oriented oblique to those of the latter. Progressive deformation has caused rotation and bending of axial planes of earlier formed folds by those developed at later stages of the same deformational episode. Coaxial recumbent to nearly reclined fold locally encountered on the N-S limb of the macroscopic fold may belong to an earlier episode of deformation or to the early stage of the main deformation episode. The E-W to ESE-WNW strike of axial plane of the regional fold system in the Bababudan belt contrasts with the N-S to NNW-SSE strike of axial planes of the main fold system in the Chitradurga and other schist belts of Karnataka.     

Application of the technique to ellipitical markers deformed by pressure-solution

Compression and extension axes are deduced from quartz deformation lamellae in a quartzite and a graywacke folded into an asymetrical syncline. Deformation lamellae fabrics in the two sandstones are distinctly different. In the graywacke, regardless of bedding orientation or position on the fold, compression axes are normal or nearly normal to the axial planar rough cleavage. Extension axes generally lie in the cleavage plane, parallel to dip. In most quartzite samples, compression axes are parallel or subparallel to bedding, at high angles to the fold axis and extension axes are normal to bedding. Two samples from the very base of the formation indicate compression parallel to the fold axis with extension parallel to bedding, at high angles to the fold axis. One of these two shows both patterns. The lamellae fabric geometry in these two samples suggests the presence of a neutral surface in the quartzite. The lamellae-derived compression and extension axes are in good agreement with the buckling behavior of a viscous layer (quartzite) embedded in a less viscous medium (graywacke and shale below and shale and carbonate above).     

Progressive deformation and the rotation of contemporary fold axes in the Ballybofey Nappe,north-west Ireland

Internal regions of orogenic belts may be characterized by an alignment of fold axes with mineral elongation lineations. This relationship is commonly interpreted as representing progressive tightening and rotation towards the shear direction of early buckle folds, the hinges of which were initiated orthogonal to this direction. Detailed structural analysis of lower amphibolite facies Dalradian metasediments of the Ballybofey (fold) Nappe, north-west Ireland, shows that an intense S₃ schistosity is developed axial planar to mesoscopic and minor F₃ folds. In areas of low D₃ strain, F₃ fold axes plunge gently towards the north-east, whereas in regions of greater strain plunges are towards the south-east subparallel to the constant mineral lineation. Minor folds which initiated at angles of 70–80° from the mineral lineation subsequently rotated towards the shear direction in a consistent clockwise sense. Progressive and variable non-coaxial deformation oblique to the original mean F₃ orientation has resulted in a unimodal distribution pattern of fold axes. Analysis of the angular rotation of fold axes enables estimates of the bulk shear strain to be evaluated and models of progressive deformation to be assessed.     

Structural analysis of a small area in the Wagonga beds at Narooma,N.S.W.

The small area surrounding Narooma provides an example of a departure from the ideal geometry of superposed fold systems. Three systems of folds are recognized, the second set being by far the best developed. Axial planes of these folds lack a preferred orientation so that the fold style is polyclinal and B‐axes are variable. Evidence of extension parallel to B is widespread. Some dynamic aspects of the second deformation are discussed.     

Magnetic fabric in folded sills and lava flows. A case study in the Permian basalts of the Anayet Massif (Pyrenean Axial Zone, Spain)

The shallow intrusive bodies and lava flows emplaced within the Permian upper red unit in the Anayet Massif, represent a magmatic episode that occurred about 255 Ma (Saxonian) in the Pyrenean Axial Zone (northern Spain). Anisotropy of magnetic susceptibility (AMS) measurements, in both igneous bodies and their host rocks, allow us to infer the existence of magnetic fabrics of tectonic origin linked to the main cleavage-related folding episode. The relationship between the susceptibility axes and the field structures is the criterion that permits to differentiate normal from inverse magnetic fabrics in the igneous samples. The structural interpretation of all AMS data taken from the igneous bodies and sedimentary host rocks, is in accordance with a folding model which include: (i) flattening associated with cleavage formation during fold amplification in incompetent layers (host pelites), responsible for a magnetic lineation at high angles with respect to the regional folding axis and (ii) buckling in competent (conglomerates and igneous bodies) levels, responsible for a magnetic lineation parallel to the regional fold axes.     

Microscopic deformation mechanisms associated with mica film formation in cleaved psammitic rocks

At Islesboro, Maine, cleavage is well developed in low greenschist-facies siltstones and interbedded pelites of early Paleozoic age. The siltstones contain a spatial sequence of mica film structures that corresponds to increasing intensity of mesoscopic cleavage. In the most weakly foliated rocks, cleavage is defined by the preferred orientation of individual mica particles. In siltstones displaying slightly higher strain, these particles are accompanied by short, discontinuous mica film segments, thought to have formed by the recrystallization of early pore-space layer silicates. In moderately cleaved rocks, these segments link up to form lengthened mica films by a process thought to include intergranular fracture, transgranular fracture and layer silicate crystallization. In strongly cleaved rocks, the lengthened mica films become longer and thicken appreciably by solution transfer of quartz and residual accumulation of layer silicates and opaque minerals. Layer silicate crystallization is evident at all stages of mica film development, but is especially marked by the growth of decussate mica inside late-stage, thick mica-rich layers. This sequence of mica film development is probably characteristic of fine-grained psammitic rocks, and may not necessarily occur in carbonate-rich or mica-rich rocks.     

Determination of geologic strain using randomly oriented strain markers of any shape

Strain markers are geologic bodies within a rock which, during the deformation of that rock, have retained their identity but did not differ from their surrounding material in their mechanical behaviour. In sections they appear as closed contours. If it can be assumed that there was no preferred orientation of the markers, the axial-strain ratio in a planar section can be determined. Two lines are drawn through the centre of each marker, parallel respectively to the two axes of the sectional strain ellipse. If aj and cj are the segments defined by the intersection of these two lines with the contour of a marker j, then the axial-strain ratio is the logarithmic average of all aj/cj. As shown in an example, this method can also be used if the markers are ellipsoids. A statistical test of initial isotropy is proposed for this special case.     

The structure of the Sierras Australes (Buenos Aires Province, Argentina): An example of folding in a transpressive environment

The Sierras Australes of eastern Argentina record the progressive suturing of the Patagonian terrane with the South American craton during Permo-Triassic time. On the South American side of the suture, fold axes and axial plane cleavage show a systematic variation in orientation across the region, rotating counterclockwise from the southern and central segments to the northwestern segments. These data, in combination with finite strain measurements indicative of extension parallel to fold axes, suggest progressive, horizontal, simple shear with a left-lateral sense. Thus, the suturing probably had a strong, transpressive component. This conclusion explains the characteristic sigmoidal shape of the sierras and is supported by comparison with scale-model experiments conducted by other investigators. Furthermore, this interpretation is in agreement with paleomagnetic data.     

Superposed deformations in the Eastern Alps: strain analysis and microfabrics

Superposition, mainly of two Eoalpine (Cretaceous) deformation events, can be observed in vertical sections and horizontal traverses throughout a decollement zone in the Eastern Alps consisting of crystalline basement and autochthonous and allochthonous cover stacked in several nappes. The first event is an episode with a non-coaxial strain history caused by W-directed thrusting, the second a N-directed shortening by folding and fold imbrication. Strain analysis based both on deformed pebbles and on the preferred orientation of phyllosilicate grains (March theory), microfabric analysis, and observations of the relative timing of deformation and metamorphism, together indicate that the superposition of structures caused by the two events amounts to one continuous, progressive act of deformation. We attribute gradual and consistent variations in intensity and axial ratio of the tectonic strain to a history influenced by: (a) the position of samples in the pile; (b) the relative importance of the strain increments caused by the two events; and (c) rock ductility. We interpret variations of the c-axis fabrics of quartz in the same way, and draw tentative kinematic conclusions for the Eoalpine orogeny in the Eastern Alps.     

Pressure-solution deformation of the purgatory conglomerate,rhode island (u.s.a.): quantification of volume change,real strains and sedimentary shape factor

Pressure solution has caused substantial volume redistribution within the Purgatory Conglomerate from Rhode Island. Material has been removed from quartzite cobble surfaces parallel to the fold axes and mostly redeposited as fibrous pressure shadows at the long axis terminations of the cobbles. In the hinges of folds, 23% of the mean cobble volume has been removed, and in highly deformed and overturned fold limbs up to 55% volume reduction has occurred. The initial cobble shape and orientation can be measured at an undeformed locality and the deformation path can be easily deduced; thus these are real cobble volume reductions. Apparent volume losses (initial shapes not removed) range from 70% to 89%. Real strains for cobbles (axial ratios ranging from 1:0.65:0.38 to 1:0.47:0.15) have values of e_x, e_y, e_z which range from 0%, −20%, −11% to 0%, −37%, −42%, respectively, depending on structural position. The conglomerate itself has been extended parallel to the fold axes (e_r), but the extension is not recorded by the cobble shapes.