Parasitic folds with wrong vergence: How pre-existing geometrical asymmetries can be inherited during multilayer buckle folding

Parasitic folds are typical structures in geological multilayer folds; they are characterized by a small wavelength and are situated within folds with larger wavelength. Parasitic folds exhibit a characteristic asymmetry (or vergence) reflecting their structural relationship to the larger-scale fold. Here we investigate if a pre-existing geometrical asymmetry (e.g., from sedimentary structures or folds from a previous tectonic event) can be inherited during buckle folding to form parasitic folds with wrong vergence. We conduct 2D finite-element simulations of multilayer folding using Newtonian materials. The applied model setup comprises a thin layer exhibiting the pre-existing geometrical asymmetry sandwiched between two thicker layers, all intercalated with a lower-viscosity matrix and subjected to layer-parallel shortening. When the two outer thick layers buckle and amplify, two processes work against the asymmetry: layer-perpendicular flattening between the two thick layers and the rotational component of flexural flow folding. Both processes promote de-amplification and unfolding of the pre-existing asymmetry. We discuss how the efficiency of de-amplification is controlled by the larger-scale fold amplification and conclude that pre-existing asymmetries that are open and/or exhibit low amplitude are prone to de-amplification and may disappear during buckling of the multilayer system. Large-amplitude and/or tight to isoclinal folds may be inherited and develop type 3 fold interference patterns.     

Fold interference patterns in the Late Palaeozoic Anti-Atlas belt of Morocco

We document two phases of folding within the central part of the Late Palaeozoic Anti‐Atlas chain of Morocco. A first generation of SW–NE folds involve a horizontal shortening of 10–20%, accommodated by polyharmonic buckle folding of contrasting wavelengths in Ordovician Jbel Bani quartzites and Devonian Jbel Rich carbonates. A second generation of folds with similar style and wavelengths in an E–W direction lead to complex interference patterns. Dome and basins are developed within the Jbel Rich and within Lower Cambrian dolomites. Both folding phases are related to thick‐skinned uplift of Precambrian basement in a Laramide style. In contrast to the typical Rocky Mountain foreland style, however, cover deformation in the Anti‐Atlas is mostly decoupled from the undying basement along thick incompetent horizons such as the Lower Cambrian Lie‐de‐Vin and Silurian shales.     

The transition from upright to recumbent folding in the Variscan fold belt of southwest England: a model based on the kinematics of simple shear

A transition from upright folds, at high structural levels, to recumbent folds at depth is described from the Variscan fold belt in southwest England. The folds tighten and cleavage intensifies progressively as the axial plane dip decreases. A simple shear model is developed in which the shortening of a multilayer and its folding produces initially upright open folds which tighten as they rotate during increasing shear strain. The model predicts the observed relationship between interlimb angle and axial plane dip and is used to discuss the development of the structure of north Cornwall.     

Deformation history of the Hampden Synform in the Eastern Fold Belt of the Mt Isa terrane

The moderately metamorphosed and deformed rocks exposed in the Hampden Synform, Eastern Fold Belt, in the Mt Isa terrane, underwent complex multiple deformations during the early Mesoproterozoic Isan Orogeny (ca 1590–1500 Ma). The earliest deformation elements preserved in the Hampden Synform are first‐generation tight to isoclinal folds and an associated axial‐planar slaty cleavage. Preservation of recumbent first‐generation folds in the hinge zones of second‐generation folds, and the approximately northeast‐southwest orientation of restored L¹ ₀ intersection lineation suggest recumbent folding occurred during east‐west to northwest‐southeast shortening. First‐generation folds are refolded by north‐south‐oriented upright non‐cylindrical tight to isoclinal second‐generation folds. A differentiated axial‐planar cleavage to the second‐generation fold is the dominant fabric in the study area. This fabric crenulates an earlier fabric in the hinge zones of second‐generation folds, but forms a composite cleavage on the fold limbs. Two weakly developed steeply dipping crenulation cleavages overprint the dominant composite cleavage at a relatively high angle (>45°). These deformations appear to have had little regional effect. The composite cleavage is also overprinted by a subhorizontal crenulation cleavage inferred to have developed during vertical shortening associated with late‐orogenic pluton emplacement. We interpret the sequence of deformation events in the Hampden Synform to reflect the progression from thin‐skinned crustal shortening during the development of first‐generation structures to thick‐skinned crustal shortening during subsequent events. The Hampden Synform is interpreted to occur within a progressively deformed thrust slice located in the hangingwall of the Overhang Shear.     

Mapping folds and fractures in basement and cover rocks using UAV photogrammetry,Cape Liptrap and Cape Paterson,Victoria, Australia

Brittle and ductile deformation of alternating layers of Devonian sandstone and mudstone at Cape Liptrap, Victoria, Australia, resulted in upright folds with associated fold accommodation faults and multiple fracture sets. Structures were mapped at the Fold Stack locality at Cape Liptrap using high-resolution aerial photographs acquired by a digital camera mounted on an unmanned aerial vehicle (UAV). Subsequent photogrammetric modelling resulted in georeferenced spatial datasets (point cloud, digital elevation model and orthophotograph) with sub-cm resolution and cm accuracy, which were used to extract brittle and ductile structure orientation data. An extensive dataset of bedding measurements derived from the dense point cloud was used to compute a 3D implicit structural trend model to visualise along-strike changes of Devonian (Tabberabberan) folds at the Fold Stack locality and to estimate bulk shortening strain. This model and newly collected data indicate that first generation shallowly south-southwest plunging upright folds were gently refolded about a steeply plunging/subvertical fold axis during a Devonian low-strain north–south shortening event. This also led to the local tightening of first generation folds and possibly strike-slip movement along regional scale faults. In order to distinguish fractures associated with Devonian compression from those that formed during Cretaceous extension and later inversion, we compared the five fracture sets defined at Cape Liptrap to previously mapped joints and faults within the overlying sedimentary cover rocks of the Cretaceous Strzelecki Group (Gippsland Basin), which crop out nearby. An east-southeast trending fracture set that is not evident in the Strzelecki Group can be linked to the formation of Devonian folds. Additionally, hinge line traces extracted from the Fold Stack dataset are aligned parallel to a dominant fracture set within the overlying cover sediments. This suggests that basement structures (folds and coeval parallel faults) have an important influence on fault and joint orientations within Cretaceous cover rocks.     

Calcite fabrics around folds as indicators of deformation history

In the Morcles nappe (lowermost Helvetic nappe in western Switzerland) two phases of folding have been established. In this article the relationship between the calcite fabrics present in folded limestones and the folding history is analysed. Calcite fabrics around the first- and second-phase folds are related to the second phase of deformation. The following fabric patterns have been found. (1) The fabric geometry around a second-phase fold from the most internal part of the inverted limb of the nappe (locality Saillon) can be related to an overall simple shear deformation sequence, the sense of shear being related to the last advance of the Morcles nappe over the underlying autochthonous sedimentary cover of the Aiguilles Rouges massif. (2) In more external parts of the inverted limb (locality Petit Pré) the fabrics around a second-phase fold are interpreted as indicating a change in the deformation history from simple shear to a strain regime which can account for shortening along the first-phase cleavage by the formation of buckle folds. This change in the local strain regime could be related to a ‘locking’ of the frontal folds of the nappe during the last overthrust shear movements. (3) In the normal limb of the nappe the fabrics around a late second-phase kink fold (locality Neimia) are earlier than the folding event and appear to be rotated passively by the fold. (4) The fabrics around a first-phase fold (locality La Routia) are later than the folding event and overprint the fold.     

3D fold growth rates

Geological folds are inherently 3D structures; therefore, they also grow in three dimensions. Here, fold growth in all three dimensions is quantified by numerically simulating upright single‐layer folds in 3D Newtonian media. Horizontal uniaxial shortening leads to a buckling instability, which grows from a point‐like initial perturbation in all three dimensions by fold amplification (vertical), fold elongation (parallel to fold axis) and sequential fold growth (parallel to shortening direction) of secondary (and further) folds adjacent to the initial isolated fold. The two lateral directions exhibit similar averaged growth rates, leading to bulk fold structures with aspect ratios in map view close to 1. However, fold elongation is continuous with increasing bulk shortening, while sequential fold growth exhibits jumps whenever a new sequential fold appears and the bulk fold structure therefore suddenly occupies more space. Compared with the two lateral growth directions, fold amplification exhibits a slightly higher growth rate.     

Re-folded structure of syn-orogenic granitoids(Padrón dome,NW Iberia):Assessing rheological evolution of cooling continental crust in a collisional setting

This contribution discusses about the rheological, kinematic and dynamic frameworks necessary to produce recumbent and upright folds from syn-orogenic granitic massifs that were formed during an early stage of magma genesis related to the onset of a migmatitic dome. Syn-kinematic granitoids occurring within the high-grade infrastructure of the Padron migmatitic dome(NW Iberia) are deformed into largescale recumbent folds(D_2) that are later affected by upright folds(D_3). Petrostructural analysis of a selected area of this dome reveals that after a period of crustal thickening(D_1), NNW-directed extensional flow gave way to recumbent folds and penetrative axial plane foliation(S_2). Superimposed subhorizontal compression resulted in upright folds(D_3). A closer view into the dynamics of the dome allows exploring the factors that may condition the nucleation of folds with contrasting geometries during progressive deformation of molten continental crust. The formation of folds affecting syn-kinematic granitoids suggests a cooling metamorphic path in migmatitic domes. Active and passive folding mechanisms require a crystallizing(cooling) magma to nucleate folds. A more competent metamorphic host inhibits fold nucleation from much less competent magmas. As it crystallizes, magma becomes more rigid(competent),and approaches viscosity values of its host. Passive folding is favored when no significant competence contrast exists between magma and host, so this folding mechanism is more likely shortly after magma genesis and emplacement. In such conditions, and under dominant subhorizontal flow accompanied by flattening(D_2),passive folding would produce isoclinal recumbent geometries. Further magma cooling introduces a shift into the rheological behavior of partially molten crust. Thereon, crystallizing magma bodies would represent significant competence contrasts relative to their host. At this point, buckling is a more likely folding mechanism, and more regular, buckle folds re-fold previous structures after significant cooling. The geometry of resulting folds is upright due to dominant subhorizontal compression(D_3) at this stage.     

Early formed regional antiforms and synforms that fold younger matrix schistosities: their effect on sites of mineral growth

In the metamorphic cores of many orogenic belts, large macroscopic folds in compositional layering also appear to fold one or more pervasive matrix foliations. The latter geometry suggests the folds formed relatively late in the tectonic history, after foliation development. However, microstructural analysis of four examples of such folds suggests this is not the case. The folds formed relatively early in the orogenic history and are the end product of multiple, near orthogonal, overprinting bulk shortening events. Once large macroscopic folds initiate, they may tighten further during successive periods of sub-parallel shortening, folding or reactivation of foliations that develop during intervening periods of near orthogonal shortening. Reactivation of the compositional layering defining the fold limbs causes foliation to be rotated into parallelism with the limbs.Multiple periods of porphyroblast growth accompanied the multiple phases of deformation that postdated the initial development of these folds. Some of these phases of deformation were attended by the development of large numbers of same asymmetry spiral-shaped inclusion trails in porphyroblasts on one limb of the fold and not the other, or larger numbers of opposite asymmetry spirals on the other limb, or similar numbers of the same asymmetry spirals on both limbs. Significantly, the largest disparity in numbers from limb to limb occurred for the first of these cases. For all four regional folds examined, the structural relationships that accompanied these large disparities were identical. In each case the shear sense operating on steeply dipping foliations was opposite to that required to originally develop the fold. Reactivation of the folded compositional layering was not possible for this shear sense. This favoured the development of sites of approximately coaxial shortening early during the deformation history, enhancing microfracture and promoting the growth of porphyroblasts on this limb in comparision to the other. These distributions of inclusion trail geometries from limb to limb cannot be explained by porphyroblast rotation, or folding of pre-existing rotated porphyroblasts within a shear zone, but can be explained by development of the inclusion trails synchronous with successive sub-vertical and sub-horizontal foliations.     

From oblique accretion to transpression in the evolution of the Altaid collage: New insights from West Junggar,northwestern China

Along active margins, tectonic features that develop in response to plate convergence are strongly controlled by subduction zone geometry. In West Junggar, a segment of the giant Palaeozoic collage of Central Asia, the West Karamay Unit represents a Carboniferous accretionary complex composed of fore-arc sedimentary rocks and ophiolitic mélanges. The occurrence of quasi-synchronous upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the West Junggar. Latest Carboniferous (ca. 300 Ma) alkaline plutons postdate this early phase of folding, which was synchronous with accretion of the Carboniferous complex. The Permian Dalabute sinistral fault overprints Carboniferous ductile shearing and split the West Karamay Unit ca. 100 km apart. Oblique convergence may have been provoked by the buckling of the Kazakh orocline and relative rotations between its segments. Depending upon the shape of the convergence zone, either upright folds and fold with vertical axes, or alternatively, strike–slip brittle faults developed in response to strain partitioning. Sinistral brittle faulting may account for the lateral imbrication of units in the West Junggar accretionary complex.     

Thrust-breakthrough of asymmetric anticlines: Observational constraints from surveys in the Brooks Range,Alaska

To gain insights into the processes governing the thrust-truncation of anticlines, we conducted a field study of the thrust-truncated folds in the remote Brooks Range of northern Alaska, where there is a transition in fold style from symmetric detachment folds to thrust-truncated asymmetric folds. In order to document the detailed geometry of the km-scale folds exposed in cliff-forming, largely inaccessible outcrops, a new surveying technique was developed that combines data from a theodolite and laser range finder. The field observations, survey profiles, and cross section reconstructions, indicate that late-stage thrust breakthrough of the anticlines within the mechanically competent Lisburne Group carbonates accommodated continued shortening when other mechanisms became unfeasible, including fold tightening, forelimb rotation, and parasitic folding in the anticline forelimbs. These results provide constraints on the processes that govern the transition from buckle folding to thrust truncation in fold-and-thrust belts worldwide.     

A detachment fold model for fault zones in the Late Archean Abitibi greenstone belt

The Abitibi belt is one of the largest and most extensively studied Late Archean greenstone belts. The structural geology of the Abitibi belt consists of one generation of upright to slightly overturned, doubly plunging first-order folds with half-wavelengths of 20–60 km, and E–W-striking, steeply dipping fault zones that are parallel to the fold limbs. Two of the main fault zones are continuous for hundreds of kilometers. Previous tectonic models for the Abitibi belt interpret the fault zones to have formed as extensional growth faults bounding a volcanic-sedimentary basin, which were reactivated as thrusts during subsequent crustal shortening. Other models propose that the fault zones represent tectonic sutures, implying that the Abitibi belt is a collage of exotic terranes. However, distinct geological terranes have not been geologically demonstrated. We propose a new detachment fold model for the deformational history of the southern Abitibi belt, in Ontario, that explains the formation of the fault zones during the single, well-documented folding event that deformed the entire region. The internal structure of the fault zones, documented here with emphasis on the Porcupine–Destor fault zone, consists of isoclinally folded, strongly schistose, highly metamorphosed rock, cross-cut by numerous fault segments. We interpret that the upper crust (greenstones) was folded above a proposed detachment in the lower part of the volcanic stratigraphy. The fault zones would be, in essence, highly evolved detachment anticlines. Ultramafic metavolcanic rock that crops out within the fault zones would represent material from the detachment horizon that was emplaced in the cores of the detachment anticlines. The numerous segments that make up the mapped fault zones would be linked faults that formed within the isoclinal detachment anticlines to accommodate folding of the rheologically complex greenstones. The detachment fold model is compared to the results of analogue experiments designed to investigate crustal-scale folding, using viscous and frictional materials. Detachment folds are produced in the brittle upper crustal analogue on the limbs of folds formed in the ductile middle and lower crust analogues. The experimentally produced structures scale to the structures in the study area and indicate the detachment fold model for the southern Abitibi is mechanically viable.     

The Migif–Hafafit gneissic complex of the Egyptian Eastern Desert: fold interference patterns involving multiply deformed sheath folds

The Wadi Hafafit Complex (WHC) is an arcuate belt of orthogneisses, migmatites and other high-grade metamorphic rocks, which marks the boundary between the Central Eastern and the South Eastern Deserts of Egypt. In the WHC, gneissic meta-gabbro outlines macroscopic fold interference patterns characterized by elliptical to irregular culminations cored by gneissic meta-tonalite to meta-trondhjemite. The five main culminations of the WHC have previously been labeled A (most northerly), B, C, D and E (most southerly). A detailed structural investigation of B, C, D and E reveals that these structures are a result of the interference of four macroscopic fold phases, the first three of which may represent a single deformation event. The first folding involved sheath-like fold nappes, which were transported to the N or NW, assisted by translation on gently dipping mylonite zones. The regional gneissosity and mineral extension lineations formed during this folding event. The fold nappes were deformed by mainly open upright small macroscopic and mesocopic folds with approximately NE-trending hinges. As a probable continuation of the latter folding, the sheaths were buckled into large macroscopic folds and monoclines with the same NE-trends. The fourth macroscopic folding resulted from shortening along the NE–SW direction, producing mainly NW–SE-trending upright gently plunging folds. Gravitative uplift is disputed as a component of the deformation history of the WHC. The peculiarities of the fold interference pattern result from the interesting behaviour of sheath folds during their refolding.     

Tangled up in folds: tectonic significance of superimposed folding at the core of the Central Iberian curve (West Iberia)

The amalgamation of Pangea during the Carboniferous produced a winding mountain belt: the Variscan orogen of West Europe. In the Iberian Peninsula, this tortuous geometry is dominated by two major structures: the Cantabrian Orocline, to the north, and the Central Iberian curve (CIC) to the south. Here, we perform a detailed structural analysis of an area within the core of the CIC. This core was intensively deformed resulting in a corrugated superimposed folding pattern. We have identified three different phases of deformation that can be linked to regional Variscan deformation phases. The main collisional event produced upright to moderately inclined cylindrical folds with an associated axial planar cleavage. These folds were subsequently folded during extensional collapse, in which a second fold system with subhorizontal axes and an intense subhorizontal cleavage formed. Finally, during the formation of the Cantabrian Orocline, a third folding event refolded the two previous fold systems. This later phase formed upright open folds with fold axis trending 100° to 130°, a crenulation cleavage and brittle–ductile transcurrent conjugated shearing. Our results show that the first and last deformation phases are close to coaxial, which does not allow the CIC to be formed as a product of vertical axis rotations, i.e. an orocline. The origin of the curvature in Central Iberia, if a single process, had to be coeval or previous to the first deformation phase.     

纵弯褶皱叠加机制和类型的研究现状

叠加褶皱的研究是以叠加机制和叠加类型为基础的。从变质岩构造研究中形成的叠加褶皱理论是以剪切褶皱为基础,而沉积岩的纵弯褶皱叠加机制和类型均与之不同。国外有学者分别指出再褶皱时的斜纵弯褶皱机制和早期褶皱枢纽的迁移机制以及四种基本叠加类型。我国有人论述了早期褶皱的枢纽、拐线的迁移是正纵弯再褶皱的一种机制,依此提出了正纵弯叠加褶皱的三种基本类型。本文对这些成果的主要认识和依据予以介绍。     

Structural relations of charnockites of the Archaean Dharwar craton, southern India

Abstract Two varieties of charnockites are recognized in the Dharwar craton of southern India. The style and sequence of structures in one charnockite variety, and related intermediate to basic granulites, are similar to those in the supracrustal rocks of the Dharwar Supergroup and the adjacent Peninsular Gneiss. This style has isoclinal folds with long limbs and sharp hinges with an axial planar fabric in some instances. Additional evidence of flattening is provided by pinch-and-swell and boudinage structures, with basic granulites forming boudins in the more ductile charnockites/enderbites in the limbs of isoclinal folds. These folds are involved in near-coaxial upright folding resulting in the bending of the axial planes of the isoclinal folds and the associated boudins. All these structures are overprinted by non-coaxial upright folds with axial planes striking nearly N–S. The map pattern of charnockites suggests that this sequence of structures is present not only on a mesoscopic scale, but also on a macroscopic scale. Charnockites of this variety provide, in some instances, evidence of having been migmatized to give rise to hornblende–biotite gneiss and biotite gneiss, which form a part of the Peninsular Gneiss terrane.
The second variety comprises charnockite sensu stricto with an entirely different structural style. This type occurs in the tensional domains of the hinge zones of the later buckle folds, in the necks of foliation boudinage, in shear zones and in release joints parallel to the axial planes of the later folds in the Peninsular Gneiss. Because the non-coaxial later folds are associated with a strain pattern different from, and later than, that of the isoclinal folds of the first generation, it follows that charnockites of the Dharwar craton have evolved in at least two distinct phases, separate both in time and in process.     

Fault-related folding in the Ramshorn Peak area,Idaho-Wyoming thrust belt

The Ramshorn Peak area of the Idaho-Wyoming thrust belt lies in the toe of the Prospect thrust sheet along the eastern margin of the exposed part of the thrust belt. The terrain is folded with axes trending N-S and wavelengths ranging from 3 to 4.3 km. Thrusts occur exclusively along the eastern part of the map area where the toe of the Prospect thrust sheet is thinnest. The easternmost thrusts are backthrusts.Monoclinally folded rocks are thrust on less deformed rocks south of Ramshorn Peak. This fold and fault complex is interpreted to have formed by thrusting over a large oblique and small forward step. The oblique step is responsible for the formation of the monocline in the hanging wall of the thrust. All faults and associated folds are rotated by subsequent buckle folding.Second- and third-order folds (folds at the scale of the Ramshorn Peak fold and fault complex and smaller) appear to be isolated features associated with faults (fault-related folds rather than buckle folds) because they are not distributed throughout the map area. These folds were probably initiated by translation and adhesive drag. The early folding was terminated by large translation over a stepped thrust surface which caused additional folding as the hanging wall rocks conformed to the irregular shape of the footwall. The Rich model is utilized to explain the Ramshorn Peak complex because the fold is of monoclinal form and is an isolated feature rather than part of a buckle fold wave-train.     

Archaean tectonics in the Agnew supracrustal belt,Western Australia

The Agnew supracrustal belt consists of a greenstone sequence (interlayered metabasalt, differentiated gabbroic sills, ultramafic bodies, and black volcanogenic sediment) unconformably overlain by granitoid-clast conglomerate and meta-arkose. The base of the preserved sequence is intruded by grey tonalite with a crudely concordant upper contact, and by small discordant bodies of leucogranite.An early deformation (D1) produced isoclinal folds and a regional penetrative foliation. These structures were probably gently dipping when formed. D2 produced large-scale NNW-trending upright folds, a regional foliation, and a vertical N-trending ductile fault on the west side of the belt. D2 structures indicate a combination of ENE-WSW shortening, and right-lateral shear along the ductile fault. Both D1 and D2 were accompanied by metamorphism under upper greenschist to lower amphibolite facies conditions.The interpreted sequence of tectonic events is (1) deposition of the greenstone sequence on an unknown basement; (2) intrusion of large volumes of tonalite, separating the supracrustal rocks from their basement; (3) erosion of mafic rocks and tonalite to produce the clastic sedimentary sequence; (4) the first deformation; (5) intrusion of small volumes of leucogranite; (6) the second deformation.The bulk of the granitoid rocks were emplaced before the first recognisable deformation. Thus the granitoid magma cannot have been produced by partial melting of previously downbuckled ‘greenstone belt’ rocks, nor can the large-scale upright folds (D2) be a result of forceful emplacement of the magma — two common postulates for Archaean terrains. The D2 folds are closely related to the ductile fault bounding the zone: these structures, which give the present N-trending tectonic belt its form, are the youngest features in the terrain.     

Strain patterns and shortening in a folded thrust sheet: An example from the southern appalachians

Total strain patterns estimated across the Pulaski thrust sheet of the southwest Virginia Appalachians show an approximately homogeneous, plane strain deformation associated with folding and distortion above a subsurface décollement. Estimated strains are low (1.2 < < 2.0) with a subvertical extension. Chlorite fibers in pressure fringes on framboidal pyrite indicate that non-rotational deformation produced weak cleavage and pencil structure in mudrock. Variations in shape of pencils and fiber lengths in pressure fringes define highest strains in fold hinges and adjacent to contraction faults. Fabric transitions, delineated by distribution and intensity of cleavage, pencil structure and bedding fissility across the thrust sheet are strain dependent. Balanced cross-sections suggest 35% horizontal shortening due to regional folding and faulting within the Pulaski sheet. Strain integration techniques give 17–35% horizontal shortening associated with cleavage formation. Removal of this strain indicates that cleavage was superposed on open to tight, class-3 folds. Pre-existing thickness variations and anomalous low strains in tight folds require early folding accomodated by intergranular deformation (perhaps controlled grainboundary sliding). Suppression of cleavage formation and penetrative strain was possibly due to higher pore fluid pressure in the early stages of thrust sheet deformation. Observed variations in bedding-cleavage angle and low cleavage fans are compatible with this deformation sequence.     

Kinematics of large scale asymmetric folds and associated smaller scale brittle — Ductile structures in the proterozoic somnur formation, pranhita — Godavari Valley, South India

The development of structural elements and finite strain data are analysed to constrain kinematics of folds and faults at various scales within a Proterozoic fold-and-thrust belt in Pranhita-Godavari basin, south India. The first order structures in this belt are interpreted as large scale buckle folds above a subsurface decollement emphasizing the importance of detachment folding in thin skinned deformation of a sedimentary prism lying above a gneissic basement. That the folds have developed through fixed-hinge buckling is constrained by the nature of variation of mesoscopic fabric over large folds and finite strain data. Relatively low, irrotational flattening strain (X:Z-3.1-4.8, k<1) are associated with zones of near upright early mesoscopic folds and cleavage, whereas large flattening strain (X:Z-3.9-7.3, k<1) involving noncoaxiality are linked to domains of asymmetric, later inclined folds, faults and intense cleavage on the hanging wall of thrusts on the flanks of large folds. In the latter case, the bulk strain can be factorized to components of pure shear and simple shear with a maximum shearing strain of 3. The present work reiterates the importance of analysis of minor structures in conjunction with strain data to unravel the kinematic history of fold-and-thrust belts developed at shallow crustal level.