Fold Hinges Research Articles

Rock rheology and sharpness of folds in single layers

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

A structural record of the emplacement of the Pozanti-Karsanti ophiolite onto the Menderes-Taurus block in the late Cretaceous, eastern Taurides, Turkey

The Aladag region of eastern Taurides, Turkey, is characterized by an imbricated thrust structure developed during late stage emplacement of the Pozanti-Karsanti ophiolite onto the Menderes-Taurus block in the late Cretaceous. The mid to late Cretaceous dynamothermal metamorphic sole and the underlying unmetamorphosed mélange, here named the Aladag accretionary complex, were accreted to the base of the Pozanti-Karsanti ophiolite during intra-oceanic subduction, transport and final obduction of the ophiolite onto the Menderes-Taurus block. In the dynamothermal metamorphic sole, intensity of deformation and degree of metamorphism increase from the base to the top, and at least three episodes of foliation, lineation and fold development are recognized. The asymmetry of quartz c-axis fabrics, tightness and asymmetry of folds of the same generation, and curvature of fold hinge lines increase from base to top, indicating that non-coaxial progressive deformation prevailed during the development of the metamorphic sole. The mélange is divided into three major thrust fault-bounded tectonic slivers, each of which is characterized by distinctive types of matrix and block lithologies, structures and deformation style. Kinematic analyses of the dynamothermal metamorphic sole and the mélange reveal that the tectonic transport direction of the Pozanti-Karsanti ophiolite during its emplacement was from north-northwest to south-southeast, suggesting that the Pozanti-Karsanti ophiolite was derived from a Neo-Tethyan ocean to the north of the Menderes-Taurus block.

Rotation of fold-hinge lines associated with simple shear during southerly directed thrusting, Ouachita Mountains, southeastern Oklahoma

The Broken Bow uplift in southeastern Oklahoma contains the oldest rocks exposed in Ouachita orogenic belts and has experienced tour phases of deformation. First generation folds are tight, overturned, Sverging. and have a well developed slaty cleavage. Second-generation folds are coaxial with first-generation structures, are open to tight, are inclined, and reveal S-vergence. Northerly dipping faults truncate second generation fold limbs. Third-phase structures, including recumbent folds and pencil structures, are interpreted to be contemporaneous with S-directed thrusting. The hinges of earlier folds documented in the area least affected by thrusting are horizontal and trend E-W. The hinges of folds of the same generation within fault zones plunge 30–50 ° toward the north or northwest. Thus, it is concluded that the earlier folds have been passively rotated during S-direeted thrusting, as have the contemporaneous folds and pencil structures. A simple-shear model associated with S-directed thrusting is proposed to explain these geometrical relations. The rotated fold hinges provide quantifiable shear-strain gauges which can be used to quantify shear strain in the Broken Bow uplift as well as in other orogenic belts. A model for shear-strain calculation is developed using the general geometric relationship between fold-hinge lines, the shear direction, and the shear plane.

Tectonic and metamorphic events in the westernmost Grenville Province, central Ontario: new results from high-precision U–Pb zircon geochronology

The U–Pb isotopic ages of metamorphic zircons from deformed amphibolitized metadiabase dykes and associated unstrained anatectic pegmatites have been determined to constrain the times of postdyke ductile flow related recrystallization and deformation. Samples from the Central Gneiss Belt of the Grenville Province in the southern Georgian Bay region provide a means of linking areas formed during common episodes of ductile flow. Three zircon fractions from one attenuated dyke in the Moon River domain gave identical concordant ages of 1062, 1065, and 1064 ± 2 Ma, while those from another gave identical concordant ages of 1052, 1051, and 1050 ± 2 Ma for the time of metamorphism. Zircons from a folded dyke from the Go Home domain gave identical concordant ages of 1049, 1047, and 1046 ± 2 Ma, while a single overgrowth fragment from an unstrained anatectic pegmatite in the fold hinge gave a concordant age of 1047 ± 2 Ma, as if folding, metamorphic zircon growth and postfolding pegmatite are part of the same process. An anatectic pegmatite in a metadiabase boudin neck in the Ahmic domain gave two zircon fractions with identical concordant ages of 1079 and 1080 ± 2 Ma that match the concordant 1078 ± 2 Ma age of a single overgrowth type grain from the host migmatite. Zircons from the metadiabase have similar but less precise ages due to a low level of uranium and a low abundance. U–Pb results for the Love Lake granite situated at the Parry Sound–Ahmic domain boundary are equivocal but may imply folding of this structure after 1068 Ma, the age of contained titanite. Either the upper or lower end of the ca. 1050–1372 Ma zircon discordia may correspond to granite emplacement. Zircons from a dyke that postdates a granulite-grade migmatization in the Parry Sound domain record a metamorphism at 1114 ± 2 Ma but older inherited grains are present in other fractions. Ductile flow in the Go Home and Moon River domains occurred in the interval 1047–1064 Ma during which the latter was probably emplaced, whereas earlier (1080 Ma) ductile flow in the Ahmic domain may relate to the time of overthrusting of the Parry Sound domain. Parry Sound domain dyke metamorphic ages imply a lack of post 1114 Ma migmatization, and a predyke migmatization consistent with a more exotic derivation of this domain. A [Formula: see text] Ma age for a postmigmatite crosscutting pegmatite dyke sets an upper limit on the age of a late buckling event in the Ahmic domain.

Palaeomagnetic analysis of plunging fold structures: Errors and a simple fold test

The conventional corrections for bedding dip in palaeomagnetic studies involve either untilting about strike or about some inclined axis—the choice is usually governed by the perceived fold hinge orientation. While it has been recognised that untilting bedding about strike can be erroneous if the beds lie within plunging fold structures, there are several types of fold which have plunging hinges, but whose limbs have rotated about horizontal axes. Examples are interference structures and forced folds; restoration about inclined axes may be incorrect in these cases. The angular errors imposed upon palaeomagnetic lineation data via the wrong choice of rotation axis during unfolding are calculated here and presented for lineations in any orientation which could be associated with an upright, symmetrical fold. This extends to palaeomagnetic data previous analyses which were relevant to bedding-parallel lineations. This numerical analysis highlights the influence of various parameters which describe fold geometry and relative lineation orientation upon the angular error imparted to lineation data by the wrong unfolding method. The effect of each parameter is described, and the interaction of the parameters in producing the final error is discussed. Structural and palaeomagnetic data are cited from two field examples of fold structures which illustrate the alternative kinematic histories. Both are from thin-skinned thrust belts, but the data show that one is a true plunging fold, formed by rotation about its inclined hinge, whereas the other is an interference structure produced by rotation of the limbs about non-parallel horizontal axes. Since the angle between the palaeomagnetic lineations and the inclined fold hinge is equal on both limbs in the former type of structure, but varies from limb to limb in the latter, a simple test can be defined which uses palaeomagnetic lineation data to identify rotation axes and hence fold type. This test can use pre- or syn-folding lineations and could be useful in areas of non-coaxial folding.

Geometry and Origin of Fault-Related Folds in Extensional Settings

The majority of folds in extensional tectonic settings are associated with normal fault systems over a wide range of scales. The hinges of longitudinal folds are subparallel to associated faults, whereas those of transverse folds are oriented at a high angle to the related normal faults. Longitudinal folds include drag folds (hanging-wall synclines and footwall anticlines), reverse-drag folds (hanging-wall anticlines and footwall synclines), and rollover folds (hanging-wall anticlines). Drag folds form as a result of fault propagation into monoclinal warps present at the fault tips; some may form as a result of frictional drag and differential compaction. Reverse-drag folds develop because displacement decreases with distance from the fault surface. Rollover folds are pro uced by movement along gravity-driven listric faults in thick sedimentary successions. Drag folds have a smaller wavelength than reverse-drag or rollover folds and may be superimposed on these larger structures. Basin-scale synclines are the largest type of transverse folds and are manifested in plan view by basin outlines that are concave toward the border fault. These folds form because displacement is greatest near the center of the map trace of the border fault and decreases toward its along-strike ends; this is a scale-invariant feature of most normal faults. Transverse folds are also associated with segmented fault systems: hanging-wall synclines are located near the centers of fault segments, whereas hanging-wall anticlines are found at segment boundaries where the fault segments ommonly overlap. Some transverse folds may be caused by movement along an undulatory fault surface. Many folds in extensional tectonic settings form syndepositionally and control stratal thickness and facies relationships.

Late syndepositional deformation and detachment tectonics in the Barberton Greenstone Belt, South Africa

The thick‐bedded, competent quartzose sandstones, conglomerates, and siltstones of the Moodies Group (<3,225 Ma) influence structural style in the ∼3.1–3.5 Ga Barberton Greenstone Belt. Moodies Group rocks are folded into a train of tight, subparallel, doubly plunging, overturned synclines separated by narrow, strike‐parallel fault zones. Structural analysis along a transect across the central part of the greenstone belt suggests that much of the volcanic‐sedimentary greenstone sequence was buckle‐folded above a detachment within altered ultramafic rocks located at a depth of 3–6 km. Early development of brittle structures concentrated shortening and displacement near the fold hinges and in the anticlinal fault zones. Penetrative strain during folding was low. The sense of displacement along faults, the original fold vergence, and the provenance of Moodies Group sandstone and conglomerate suggest that tectonic transport was directed toward the interior of the belt from its northwestern and southeastern margins. Orogenic contraction due to folding alone was at least −56% and subsequent ductile shortening strain accounted for an additional contraction of approximately −10%. Buoyant rise of the Kaap Valley Tonalite to the north at approximately 3.22 Ga and lateral intrusions of sheetlike granitic plutons along both sides of the preserved greenstone belt at 3.1 and 3.0 Ga, respectively, imposed significant ductile strain on the greenstone belt margins and obscured or displaced the orogenic hinterlands of the facing fold‐and‐thrust belts. The syn‐and post‐Moodies tectonic history of the central Barberton Greenstone Belt involves the formation and deformation of several shortlived alluvial to shallow marine sedimentary basins on unstable “protocontinental” crust. The probable extensional origin of at least some of these basins and their subsequent shortening, buckle folding, and displacement above a subhorizontal detachment emphasizes the dominance of horizontal tectonics in the late‐stage deformation of this Archean greenstone belt.

Controls on hinge‐parallel extension fracturing in single‐layer tangential‐longitudinal strain folds

The stress history results from a published viscous layer folding solution are used as the basis for a fracture mechanics analysis of the factors that control hinge‐parallel extension fracturing in tangential‐longitudinal strain folds. The analysis incorporates published results for the change in sedimentary rock mode I fracture toughness at increasing confining stress to examine the relationship between regional strain rate, depth of burial, pore fluid pressure, initial crack size, layer viscosity, and the amount of fold shortening required for the propagation of a bed‐perpendicular, hinge‐parallel extension fracture. Tangential‐longitudinal strain folding of layers can occur at all scales in a foreland thrust system and is the result of the buckling and bending of stratigraphic units during the development of décollement, fault bend, and fault propagation folds. Hinge‐parallel extension fractures oriented perpendicular to bedding are a common fracture set observed in tangential‐longitudinal strain folds. The fractures propagate as a result of local tensile stresses that develop by the stretching of layers in the outer arc of fold hinges during bending. We considered a range of geologically reasonable boundary conditions to show that at one extreme, fracturing can occur as a result of only minor shortening by folding to the other extreme where a tight fold can form with no associated extension fracturing. For folds formed at shallow depths, where the confining stress on the system is less than the bending stresses in the layer and where the confining stress has not greatly increased the fracture toughness of the rock, hinge‐parallel extension fractures can grow under hydrostatic fluid pressure conditions. As depth increases, however, much higher pore fluid pressures are required to cause fracturing under similar strain rates. The observed controls are used to hypothesize how hinge‐parallel extension fracturing in fault bend folds can vary spatially and temporally across a thrust belt as a function of strain (thrusting) rate, the amount of bending at thrust ramps, and the depth of folding.

Synkinematic contact metasomatism: implications for the timing of pluton emplacement and regional deformation in the Scanlon shear zone, south‐eastern California

ABSTRACTStructures in rocks altered by the infiltration of magmatic fluids provide key constraints on the relative timing of plutonism and regional deformation in the central Old Woman Mountains, south‐eastern California. In this well‐exposed area the Scanlon shear zone, a belt of attenuated, shallowly dipping, amphibolite facies Palaeozoic strata, is in contact with two tabular, Late Cretaceous (∼74 Ma) granitoid plutons. The shear zone contains massive wollastonite‐bearing skarns ∼40 m above the contact with the Old Woman granodiorite. Field evidence, petrological data and stable isotope compositions indicate that the mineral assemblages in these skarns formed when the granodiorite crystallized and exsolved water‐rich, magmatic fluids. Structural features in the skarns, which include transposed wollastonite foliations, syntectonic vesuvianite and garnet porphyroblasts, and quartz ± wollastonite veins, afford an opportunity to monitor the regional strain at the time of pluton crystallization. These structures yield a broad range of timing relationships that indicate synkinematic mineral growth with deformation ending first, at c. 74 Ma. The metasomatic structures are kinematically compatible with the regional deformation observed in other lithologies and no post‐tectonic features overprint the fabrics in the skarns. Observations of isoclinally folded and boudinaged dykes and pegmatites, granodiorite saddle reefs in isoclinal fold hinges, and undeformed, locally pegmatitic dykes also yield a broad range of timing relationships indicative of synkinematic plutonism, with deformation ending first. Our results show that coupling petrological and stable isotope data with structural analysis is effective for unravelling the relative timing of heat and mass transfer processes in pluton‐wallrock systems.

Bedding parallel veins and their relationship to folding

Laminated bedding parallel veins hosted in turbiditic sandstone shale sequences from central Victoria, Australia, consist of stacked, millimetre thick, sub-parallel sheets of quartz separated by micaceous layers, wall rock slivers and pressure solution seams. They have very high length to thickness ratios, are laterally continuous over tens to hundreds of metres, and have relatively uniform thickness compared to other vein types. They are intimately associated with and folded by chevron folds, and the quartz grain shape elongation lineation is commonly orthogonal to mesoscopic and macroscopic fold hinge lines. The bedding parallel veins have two morphological forms. Type I are thin (commonly 5–10 cm) laminated veins which have complex microstructures dominated by phyllosilicate inclusion surfaces, related to oblique opening along bedding with varying rates of deposition (opening) relative to shear displacement (slip) along the bedding surfaces. More common are Type II, thicker (generally <20 cm), banded veins of alternating milky-white quartz with wall rock inclusion laminae (formerly fragments) bounded by stylolitic partings parallel to both bedding and the vein margins. The inclusion surfaces in Type I veins track the opening direction during vein formation. Vein opening-sense criteria suggest cyclical pore fluid pressure fluctuations which predate the amplification and propagation of the host chevron folds; i.e. prior to attainment of significant limb dip. Different layer parallel shortening and amplification rates for individual layers within the sedimentary sequence may lead to bedding parallel veins with an opening sense unrelated to the flexural slip folds which eventually follow.

Post-Caledonian tectonics along the Billefjorden fault zone, Svalbard, and implications for the Arctic region

The north-south Billefjorden fault zone of northern Spitsbergen in the Svalbard archipelago has been widely regarded as one of the classic areas for transpressional tectonics. The fault zone coincides with a late Caledonian shear zone and presently constitutes the eastern boundary of the main Devonian basin of northern Svalbard. Fold axial traces and thrust faults that developed in the Devonian to Permo-Carboniferous rocks parallel the strike of the fault zone. There is no evidence for the rotation of fold hinges, and nonaxial planar, transecting cleavages are lacking. Folds and thrusts with similar geometries affect the whole 60-km-wide Devonian basin fill. These observations, together with analyses of fault and fold patterns, indicate (A) pre-Devonian (Caledonian) left-lateral ductile shearing; (B) Late Silurian-Early Devonian extension; (C) Late Devonian-early Carboniferous inversion accompanied by folding of Devonian sedimentary rocks; (D) early Carboniferous extension followed by platform-wide subsidence into late Mesozoic time; (E) Late Cretaceous-Paleocene, east-west contraction, thrusting, and folding; and (E) post-Paleocene east-west extension. The Late Devonian deformation along the Billefjorden fault zone is compared with the Late Devonian to early Carboniferous Ellesmerian orogeny of the Canadian Arctic. The Late Devonian megashear model for the tectonic evolution of the North Atlantic region is not supported by the evidence from Svalbard.

Relationships between distributed and localized shear in the tectonic evolution of a Caledonian fold and thrust zone, northwest Ireland

AbstractBroad zones of distributed shear operating through mid-crustal regions of orogenic belts may incorporate narrow horizons of intense localized deformation culminating in discrete, large magnitude displacements. The relative importance and relationship between distributed and localized shear are influenced by a variety of factors including lithological variation, pre-existing structural anisotropy, strain rate and migration of fluids. Rigorous structural analysis of lower amphibolite facies Dalradian metasediments in northwestern Ireland demonstrates that an early (D1) discrete ductile detachment was subsequently reactivated during distributed non-coaxial D2 deformation operating in a broad zone through the structural pile. Regional shear was directed towards the southeast and resulted in the generation and translation of kilometre-scale, isoclinal, recumbent sheath folds which close and face towards the transport direction. The D1 detachment is clearly folded around the hinges of these major folds, whilst on fold limbs it was reactivated and acted as a local décollement within the zone of distributed shear. Shear criteria along the detachment indicate a southeast-directed translation of the major folds, in sympathy with regional shear. A broad zone of D3 translation operating through the nappe pile resulted in coaxial refolding of large scale F2 folds by the D3 Ballybofey Nappe producing a complex fold interference pattern. Non-coaxial D3 deformation resulted in continued reactivation of local decollements, together with the initiation of east-southeast directed oblique thrusts and partial dismemberment of D2 folds. Detailed structural investigation allows concepts of distributed and localized shear to be evaluated and models of crustal deformation to be assessed.

Exploration Opportunities in Inverted Rift Basins Defined by Syntectonic Strata and Axial Surface Mapping: ABSTRACT

Structural highs and syntectonic depositional sites in Central Sumatra rift basins are controlled by three-dimensional normal fault geometries and lateral fault slip distributions. The rift basins formed due to crustal extension accommodated by normal faults that flatten with depth, causing collapse of the hanging wall and formation of inclined rollover panels. The authors model this deformation by coulomb shear of the hanging wall localized along active fold hinges (axial surfaces) that are pinned at depth to fault bends. Three-dimensional fault geometries are predicted and mapped based on the shapes of rollover panels and fault segments imaged in seismic reflection (depth) profiles. Rollover panels widen during progressive fault slip and are bound by anticlinal and synclinal axial surfaces. Rollover panels also narrow upward into syntectonic strata, which record the kinematics and timing of basin formation. They map axial surfaces to define the positions of fault bends and widths of rollover panels, which are related to slip on underlying fault segments. Structural highs form where fault segments decrease in dip along strike and/or in regions of minimum fault slip. These areas are defined on fault maps by shallow slopes or fault segments and on axial surface maps by narrow rollover limb widths.more » Structural highs formed during rifting and later tectonic inversion can be defined on these maps prior to detailed contouring, which is often difficult in frontier areas and in lacustrine sequences. They present examples of producing hydrocarbon fields defined by axial surface maps from the Central Sumatra basin.« less

Structural evolution of the Grevet Zn-Cu massive sulfide deposit, Lebel-sur-Quevillon area, Abitibi Subprovince, Quebec

The Grevet Zn-Cu massive sulfide deposit is located within the major southeast-trending Cameron deformation zone in the central part of the Abitibi subprovince. This 5-km-wide deformation zone crosscuts the fabrics related to the earlier regional deformation and is characterized by a dextral horizontal component of movement. The Grevet deposit is composed of several sheetlike sulfide lenses hosted by a sequence of alternating units of mafic and felsic lava and volcaniclastic rocks, all of which are transected by several generations of mafic to felsic dikes.The Grevet area is characterized by: (1) mylonitic fabric axial planar to isoclinal folds with subhorizontal plunge, (2) subhorizontal stretching lineations, (3) boudinage with subvertical boudin axes, (4) transposition of earlier structures with Z-asymmetrical folds affecting originally oblique mafic dikes, (5) dextral shear-sense indicators, (6) late east-west crenulation cleavage associated with minor Z-asymmetrical folds, and (7) late shear fractures and kink bands. These features are all part of an evolutionary deformational sequence characterized by several increments of shearing and shortening components within the Cameron deformation zone.The 5-cm- to 12-m-thick sulfide lenses are parallel to the mylonitic foliation with lengths exceeding 100 m and occur at progressively greater depths from southwest to northeast. The major axes of the orebodies coincide well with the attitudes of fold axes and the stretching lineation. Three types of sulfide assemblages are distinguished in the mineralized lenses: (1) massive sphalerite and pyrite, (2) stringer and breccia sphalerite and pyrite, and (3) disseminated pyrite. These assemblages are irregularly distributed within each sulfide lens. The sulfide lenses have been folded, and the brecciated and disseminated sulfide assemblages can be interpreted in part as a result of tectonic dismemberment, especially in the hinges of folds. Local remobilization of sulfides has been observed.This study indicates that the Grevet deposit is probably a volcanogenic massive sulfide deposit modified during the progressive evolution of a dextral transpressive deformation zone.

Fold interference structures in thrust systems

Abstract The accommodation problems associated with the generation of interference folds in thrust belts are examined. There are several ways in which interference folds may grow in thrust systems, such as moulding on to non-planar ramps, or via the interaction of non-coaxial structures. The geometries of interference folds from thrust belts in Spain and Pakistan are described, and form the basis of an analog model. The model is used to quantify the volume of rock which must be accommodated during the growth of these structures. Mechanisms which might solve the inherent space problem are discussed. The simplest geometrical solution would involve a mechanism which caused thickening in the fold hinges—but this was not observed in the field. Field evidence is used to construct a linked fold-and-fault model which reconciles the absence of thickening in the interference fold hinge with the volume of material which must be accounted for.

Pre-lithification structures, deformation mechanisms, and fabric ellipsoids in slumped turbidites from the Pigeon Point Formation, California

Paterson, S.R. and Tobisch, O.T. 1993. Pre-lithification structures, deformation mechanisms, and fabric ellipsoids in slumped turbidites from the Pigeon Point Formation, California. Tectonophysics, 222: 135–149. Quantitative fabric, structural, and microstructural analyses of pre-lithification folds, foliations, and lineations formed by slumping of turbidite sequences in the Cretaceous Pigeon Point Formation, California, provide a useful comparison with strain and microstructures developed in lithified and tectonically deformed turbidites. Our results indicate the following: 1. (1) multiple generations of folds, cleavages, and lineations can develop prior to any post-lithification tectonic deformation 2. (2) individual grains in sandstones have variable axial ratios, but the ratios and orientations of large populations of grains define fabric ellipsoids with small axial ratios ( ave. = 1.25:1.13:1) 3. (3) phyllosilicate grains define moderate flattening fabrics (reflecting 20–40% shortening or volume loss), with the intensity of alignment partly controlled by the percent of quartz and feldspar grains 4. (4) the fabric ellipsoids in sand-rich layers largely reflect deposition and slumping: pre- and post-slump compactions did not occur, in sand-rich units but did align clay particles in mud-siltstone units, and 5. (5) intra-grain microstructures in quartz and feldspar (e.g., undulose extinction, subgrains) are inherited or recycled features rather than representing effects of post-lithification strains. Our data also suggest that prelithification slumping occurred by pervasive grain rotation and grain boundary sliding in saturated sands with some local movement of material along bedding horizons. A likely model for the folding and associated fabrics is that buckling and fold-hinge flattening drove fluid expulsion, which in turn caused local grain-scale realignment, transposition of bedding, and the development of an axial planar cleavage in the hinge zones. Continued fluid flow was channeled by the clay-silt layers in fold limbs and axial plane cleavege in fold hinges.

Implications of discrete strain compatibility in multilayer folding

Theoretical kinematic analysis of idealised folded multilayers shows that, throughout the fold, axial planar stretch of repeated points within the fold must be homogeneous. Layers are effectively pinned at fold limbs such that the rotation of competent layer limbs produces an axial planar stretch. To be strain compatible, this stretch (measured between discrete points) must be accommodated by an equivalent stretch in the fold hinges. Differing modes of accommodation of stretch in the hinge result in fold styles including saddle reef folds, Class 1b/3, Class 1c/3 and Class 2 multilayers. The process of accommodating stretch induced by limb rotation is termed compatible folding. Strain analysis of folds using the compatible folding model predicts significantly less shortening than for other fold flattening models. The model also predicts that Class 2 folds formed by compatible folding will redistribute a pre-existing lineation within a plane. Compatible folding is, therefore, one alternative to the shear folding model for explaining great-circle redistribution patterns of pre-existing linear elements.

Structural arcuation in the Berwyn Hills, North Wales

AbstractNew mapping has better defined the well known arc in Acadian (late Caledonian) structures centred in the Berwyn Hills. This arc, convex to the north‐west and about 20 km across, is defined by the strikes of bedding and cleavage and the trends of fold hinges. In contrast, a mineral lineation within the cleavage has a north‐north‐westerly plunge throughout the arc. It approximates to the long (X) axes of measured strain ellipsoids. A primary origin for the arcuation is implied by this consistency, by the absence of secondary cleavages and by opposing senses of fold transection across the arc. The new data favour the hypothesis of shortening of the Lower Palaeozoic cover against and over a pre‐existing basement high inferred from geophysical data and from metamorphic and sedimentary facies.

The influence of initial fold geometry on type 1 and type 2 interference patterns: an experimental approach

Experiments on superposed folding were performed in a plane strain-pure shear rig using paraffin wax as an analogue for rocks. A series of pre-formed cylindrical folds were refolded with the compression direction acting parallel to the initial fold hinge direction, the intermediate axis perpendicular to the first fold axial plane and the extension direction parallel to the first fold axial plane and perpendicular to its axis (Type 1 interference geometry). Roundness, tightness and amplitude were varied to investigate the influence of first-fold geometry on the interference patterns. Experimental folding of an initially planar layer oriented parallel to the first-fold envelope provides a reference geometry for comparison. The results suggest that hinge roundness has less influence on the interference pattern than other factors. Close initial folds refold into Type 2 interference patterns, which become even more pronounced as the tightness increases; open folds produce Type 1 interference patterns. Folds with the same interlimb angle and the same roundness produce different interference patterns depending on relative amplitude: folds with relatively large amplitude are refolded into Type 2 patterns, whereas folds with small amplitude give clear dome and basin structures. In a second set of experiments, the compression direction also acted parallel to the first-fold hinge, but the orientations of the intermediate and extension axes were interchanged. The interference patterns obtained are very similar to those in Type 1 interference geometry, suggesting that the major factor in determining whether Type 1 or Type 2 interference patterns develop is the initial fold geometry and not the kinematics of the second deformation.

Transected folds from the western part of the Bala lineament, Wales

Fold-cleavage relationships are described in the Cadair Idris area of the Lower Palaeozoic Welsh Basin, which includes the western part of the Bala Lineament. Many fold hinges are transected by the broadly contemporaneous Caledonian (Lower Devonian) cleavage. Transection angles are amongst the highest yet described from the Welsh Basin. There is a close relationship between fold strike and sense and amount of transection; N-S folds are clockwise transected, NE folds have an axial planar cleavage whilst ENE folds are anticlockwise transected. Similar relationships occur throughout the central Welsh Basin. A model of progressive deformation is described whereby weak regional transpression is modified by local factors. Viscosity contrasts are recognized as one of the most important controls on the timing of fold development and the amount of transection.