Coaxial Refolding Research Articles

Structural Evolution of the Rio das Velhas Greenstone Belt, Quadrilátero Ferrífero, Brazil: Influence of Proterozoic Orogenies on Its Western Archean Gold Deposits

The Quadrilátero Ferrífero region is located in the extreme southeast of the Brasiliano São Francisco craton, Minas Gerais state, Brazil. It is composed of (i) Archean TTG granite-gneaissic terranes; (ii) the Archean Rio das Velhas greenstone belt; (iii) the Proterozoic metasedimentary and metavolcano-sedimentary covers. The Rio das Velhas rocks were deposited in the synformal NW–SE-directed Nova Lima basin. The Archean deformation converted the Nova Lima basin into an ample synclinorium with an eastern inverted flank. Archean orogenic gold mineralization within the Rio das Velhas greenstone belt rocks is controlled by NNW–SSE-directed, Archean regional shear zones subparallel to the strata of the Nova Lima synclinorium borders. Transamazonian and Brasiliano orogenies are superposed onto the Archean structures that control gold mineralization. In the eastern domain, Brasiliano fold-and-fault belts prevail, whereas in the western domain Archean and Transamazonian structures abound. The present study focus mainly is the western domain where the Cuiabá, Morro Velho, Raposos, Lamego and Faria deposits are located. Gold orebodies plunge to the E–NE and are tectonically controlled by the Archean D1–D2 deformation. The D3 Transamazonian compression—Which had a SE–NW vector sub-parallel to the regional mineralized Archean foliation/bedding—Buckled these structures, resulting in commonly open, synformal and antiformal regional folds. These are well documented near the gold deposits, with NE–SW axial traces and fold axes plunging to E–NE. Such folds are normal to inverted, NW-verging, with an axial planar foliation dipping moderately to the SE. The Transamazonian compression has only been responsible for the reorientation of the mineralized Archean gold ores, due to coaxial refolding characterized by an opposite tectonic transport. It has therefore not caused any other significant changes. Thrust shear zones, sub-parallel to the strong Transamazonian foliation, have given rise to localized metric segmentation and to the dislocation of gold orebodies. Throughout the region, along the towns of Nova Lima to Sabará, structures pertaining to the Brasiliano Araçuaí orogeny are represented only by gentle folding and by a discrete, non-pervasive crenulation cleavage. Thrust-shear zones and small-scale normal faults have caused, at most, metric dislocations along N–S-oriented planes.

Systematic changes in orientation of linear mylonitic fabrics: An example of strain partitioning during transpressional deformation in north Golpaygan, Sanandaj–Sirjan zone, Iran

Kilometer-scale, shallowly dipping, NW-striking top-to-the NE reverse and dextral strike-slip shear zones occur in metamorphic rocks of north Golpaygan. These metamorphic rocks are exposed at the NE margin of the central part of the Sanandaj–Sirjan zone in the hinterland of the Zagros orogen. NW-striking top-to-the NE normal shear zones were also found in a small part of the study area. Structural evidence of three deformation stages were found. Pre-mylonitization metamorphic mineral growth happened during D1. The main mylonitization event was during the D2 deformational event, following coaxial refolding, synchronous to retrograde metamorphism of amphibolite to greenschist facies in the Late Cretaceous–Paleocene, and before D3 folding and related mylonitization. We documented the systematic changes in the orientations of D2 linear fabrics especially stretching lineations and superimposition relations of structures. It is concluded that the dextral strike-slip and dip-slip shear zones were coeval kinematic domains of partitioned dextral transpression. The shallowly dipping reverse and strike-slip shear zones are compatible with partitioning in a very inclined transpressional model. Fabric relations reflect that the top-to-the NE normal shear zones were not produced during deformation partitioning of inclined dextral transpression. The Late Cretaceous–Paleocene strain partitioning was followed by later N–S shortening and NE-extension in the north Golpaygan area.

Sinistral transpression along the Main Boundary Thrust in Amritpur area, Southeastern Kumaun Himalaya, India

Outcrop mapping and structural analyses in a 100m thick fault damage zone elucidate geometry and kinematics of the Main Boundary Thrust in the study area. The north-dipping fault damage zone is sandwiched between the Proterozoic Amritpur granite in the hangingwall and the Neogene Siwalik sediments in the footwall. The Amritpur granite is overlain by the volcanic-sedimentary sequence along a top-to-the-south thrust. The footwall sediments are deformed into gentle upright and steeply-plunging folds. By contrast, the hangingwall granite is generally massive. The km-scale folds in the Sub Himalayan sediments are geometrically discordant to the folds in the Lesser Himalayan volcanic-sedimentary sequence.The damage zone contains both the Sub Himalayan sediments and the Lesser Himalayan amphibolite. It consists of an imbricate structure, tectonic wedges and pressure solution bands. The imbricate structure is made up of seven fault-bounded lithotectonic domains that record the imprints of coaxial refolding and subsequent modification of the folds into sheath-like structures. The geometry and kinematics of mesoscopic structures and the results of paleostress analysis reveal a sinistral wrench accompanied N-S to NNE-SSW directed subhorizontal shortening in the damage zone. The aforesaid evidences also help distinguish between pure contractional, pure wrench, contraction-dominated and wrench-dominated deformation in the different domains and the tectonic wedges. The sinistral transpression in the damage zone is ascribed to the occurrence of an oblique/lateral ramp along the Main Boundary Thrust in the study area.

Evolution of Eastern Ghats granulite belt of India in a compressional tectonic regime and juxtaposition against Iron Ore Craton of Singhbhum by oblique collision — transpression

Three major episodes of folding are evident in the Eastern Ghats terrain. The first and second generation folds are the reclined type; coaxial refolding has produced hook-shaped folds, except in massif-type charnockites in which non-coaxial refolding has produced arrow head folds. The third generation folds are upright with a stretching lineation parallel to subhorizontal fold axes. The sequence of fold stylesreclinedF 1and coaxialF 2, clearly points to an early compressional regime and attendant progressive simple shear. Significant subhorizontal extension duringF 3folding is indicated by stretching lineation parallel to subhorizontal fold axes. In the massif-type charnockites low plunges ofF 2folds indicate a flattening type of deformation partitioning in the weakly foliated rocks (magmatic ?). The juxtaposition of EGMB against the Iron Ore Craton of Singhbhum by oblique collision is indicative of a transpressional regime.

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.

Deformation in portions of the distal continental margin to ancestral North America: An example from the westernmost internal zone, central and southern Appalachian Orogen, Virginia

Late Precambrian(?)–early Paleozoic metasedimentary and metavolcanic rocks of the Evington Group crop out in a belt along the eastern flank of the Blue Ridge, Virginia. Near Lynchburg, where the sequence is well preserved, the Evington Group is interpreted as a transitional rift/drift succession of the distal continental margin to ancestral North America, caught up in the internal zone of the southern Appalachian orogen. The initial deformation of the Evington Group involved recumbent folding and nappe thrusting, which emplaced distal facies over proximal facies rocks. A penetrative schistosity, axial planar to the early folds, developed under greenschist facies conditions. Nearly coaxial refolding about upright, NE trending, SE dipping axial surfaces followed, also under greenschist facies, and was contemporaneous with high‐angle reverse faulting vergent westward toward the craton. The age of metamorphism and tectonostratigraphic correlations along strike indicate that these deformations were coincident with arc convergence and collision during the Taconic Orogeny. Transpressional stresses related to NE striking Alleghanian (late Paleozoic) dextral strike‐slip faults outboard of the Evington Group rocks studied here are interpreted by others to have caused domes and basins to form 35 km along strike southwest of Lynchburg in the Roanoke River region. Integration of data collected along strike of the belt from the Roanoke River to Lynchburg indicates that dome formation produced F2 plunge variations NE in the belt of Evington Group rocks. Many orogens developed through continent‐arc collision which proceeded from internal to external zones. Comparison of the Evington Group with rock sequences from tectonostratigraphic settings similar to those in other orogens (e.g., Wopmay Orogen) indicates that they have roughly similar histories of deformation. Recumbent folding and thrust nappe emplacement in the internal zone is an early phase of orogenic evolution and proceeds toward the craton through time. Second‐stage coaxial refolding about upright axial planes may or may not occur or be recorded. Late stage strike‐slip faulting in the internal zone is a common feature of many orogens that may reflect oblique convergence or tectonic escape.

Experiments on folding in spreading-gliding nappes

A small scale model of spreading-gliding nappe was used to study the development of folds which are controlled by the geometry of external boundaries. Throughout in the model, the deformation combines pure shear and simple shear. Recumbent folds are produced above irregularities of the base. The cylindricity of folds is directly dependant on the cylindricity of basal irregularities. Models with homogeneous rheological properties produce only recumbent folds whereas models of contrasting rheological properties produce recumbent folds, and “en échelon” upright folds in the zones of lateral shear. The experiments demonstrate that: 1. (a) large scale recumbent folds in nappes can result from passive amplification processes in unsteady shear flow 2. (b) upright folds and coaxial refolding result from mechanical instability due to contrasting rheological properties between layers and develop during a unique progressive deformation. These laws can be directly applied to a spreading-gliding nappe but also on a larger scale to all nappes in which the shear component dominates the deformation.

Ductile-to-brittle transition in shear during thrust sheet emplacement, Southern Appalachian thrust belt

Mesoscopic structures in anchimetamorphic (T = 200–300°C) strata of the Pulaski thrust sheet, Southern Appalachian thrust belt, developed in progressive, heterogeneous simple shear near the ductile-to-brittle transition. Shear (γ≤3) was localized in weak, anisotropic pelitic rocks (Rome Formation) along the base of this 5–11 km thick thrust sheet. Folds, which vary from upright and open to isoclinal and NW-facing, developed during ductile shearing and display a correlation between tightness and axial-surface dip. Movement along brecciated thrust zones, which evolved progressively from zones of greatest ductile strain, resulted in low-angle truncation of fold axis trends, coaxial refolding of earlier structures, and imbrication of the thrust sheet.Transient variations in fluid pressure (Pf) controlled the mechanical behavior of the thrust sheet. Systematic veins imply Pf >σ3 + T (T = tensile strength) during ductile deformation, whereas later non-systematic vein arrays in high strain zones record periods of nearly hydrostatic stress. Elevated Pf, which led to fracturing, dilation, and fault initiation, appears confined to pelitic zones within the Rome Formation. This, coupled with decreasing temperature, resulted in the transition from ductile folding to brittle thrusting. Changing physical conditions probably reflect erosional unroofing during uplift and late Paleozoic thrust sheet emplacement.

Takaka River Recumbent Fold Complex, Nelson, New Zealand

Abstract The Ordovician-Silurian Wangapeka and Hailes Formations in the region of the Takaka River, southwest of Upper Takaka, have been folded and refolded, more or less coaxially, about axes lying generally in the N-NW/S-SE sector, into a complex series of recumbent folds with associated flat-lying cleavage. Coaxial refolding has resulted in anticlinal closures facing both westwards and eastwards, but several observations suggest that the movement direction during deformation was from WSW to ENE relative to basement. The recumbent structures and associated cleavages in the region of Takaka Bridge have been bent into a steeper orientation in response to reverse movement on west-dipping shears. The entire complex is here named the Takaka River Recumbent Fold Complex, and, to illustrate the style of deformation, the Sam Creek Fold and the Takaka Bridge Coaxial Fold are separately described. The significance of these folds is discussed in terms of previously proposed models for the Paleozoic tectonics of Nelson. A riebeckite microgranite intrusion is noted.

Geologia da regiao de pouso alegre machado: analise geometrica de dobramentos superpostos

The present paper is a result of geological mapping of an area of about 3 000 km 2 situated in the South of Minas Gerais. Geologically the area is underlain by the Pinhal Group, Amparo Group and the Silvianopolis Complex of Brasiliano, Trans-Amazonic and Archaean ages respectively. On the basis of structural analysis the effects of superposed folding in the area assignable to four phases of deformation were identified. The oldest of these is designated Fn followed by Fn+1, Fn+2 and Fn+3. The Fn phase represents the culmination of a pre-Trans-Amazonic event involving migmatization and formation of granulites and is recognisable solely in the rocks of the Silvianopolis Complex. The Fn+1 phase characterized by isoclinal folding, which affected both the Amparo Group and the Silvianopolis Complex, took place in the Trans-Amazonic orogeny. The subsequent Fn+2 phase caused isoclinal folding of the Fn+1 folds with fold axes and axial planes in a north-westerly direction and vergence to the northeast. Crescent shaped interference figures of type 2 were formed in this phase as a result of interference consequent on the refolding of the Fn+1 folds. This phase is provisionally correlated with the Uruacuano Cycle. Finally, the Fn+ folding affected the Fn+1 and Fn+2 structures resulting in flexural or open folds with northeast fold axes and axial surfaces without any definable vergence. Type 3 interference figures originated by the superposition of the Fn+3 on Fn+1 folds and are characterized by approximate coaxial refolding. The Fn+3 deformation is tentatively related to the Brasiliano Cycle.

Superposed folding and metamorphism in the Seve nappe around Åreskutan in the Swedish Caledonides

Abstract The earliest recognisable folds (F1) in the Seve nappe in the Åre area in the central Swedish Caledonides include a phase of co-axial refolding (F1A and F1B). These isoclinal, tight reclined or recumbent folds are parallel to the principal mineral lineation and mylonitic striation. There is a set of tight, reclined folds (F2) which refold the mineral lineation and F1 hinges at large angles. The open F3 folds refold the F2 hinges at small angles. In addition to the above structures, there is also a set of open, upright folds (F1) with axes parallel to the mineral lineation (and to F1) which is younger than F1. The time-relation of these F1 folds to the F2 folds is not clear. The field evidence indicates that F1B and later folds have developed during different stages of thrusting. The period of migmatization and high-grade metamorphism within the nappe has overlapped with the period of development of the F1 and F2 structures.

Early Precambrian stratigraphy of central and southern Rajasthan, India

Abstract Structural, stratigraphic and petrologic studies between Amet and Sembal in the Udaipur district of southcentral Rajasthan indicate that all the rocks belonging to the Banded Gneissic Complex, the Aravalli Group and the Raialo Formation have been involved in isoclinal folding on a westerly trend, co-axial refolding, and upright folding on a north to north-northeast trend. There is neither an unconformity nor an overlap between the Aravallis and the Raialos. The conglomerates supposed to mark the erosional unconformity above the Banded Gneissic Complex near Rajnagar is a tectonic melange of folded and torn quartz veins in mica schist within the Aravalli Group. The Aravalli—Raialo metasediments have been migmatized synkinematically with the first folding to give rise to the Banded Gneissic Complex; the gneissic complex does not have any separate stratigraphic entity. By contrast, there is an undoubted erosional unconformity between the type Aravalli rocks and the underlying Sarara granite to the south. These relations, coupled with the continuity of the Aravalli rocks of Udaipur northward to the metasedimentary rocks of the Sembal—Amet area along the strike, and a comparable structural history, point to granitic rocks of at least two generations in the Early Precambrian of central and southern Rajasthan. Preliminary radiometric dating of rocks of known stratigraphic—structural relationship seems to confirm the presence of granitic rocks of two ages in the Early Precambrian, and of a considerable interval between the deposition of the Aravalli—Raialo rocks and the Delhi rocks. The Udaipur granite, post-dating the first deformation but preceding the upright folding on the northerly trend, provides evidence for granitic activity of a third phase before the deposition of rocks of the Delhi Group.

Structure of the Rajnagar marble band and its bearing on the early Precambrian stratigraphy of central Rajasthan, Western India

The marble band of supposedly Raialo (Precambrian) age around Rajnagar in the Udaipur district of Rajasthan represents a large isoclinal fold with the two limbs coalesced and with the terminations marking the sharp hinges. The arcuate map pattern is due to coaxial refolding of the axial surface of the isoclinal fold, on which open, NNE-trending upright folds have been overprinted. The isoclinal folds of small scale are reclined or inclined in style and plunge gently westward where not involved in the upright folding. The axes of the isoclinal folds have been scattered by superposed folding, so that some of the isoclinal folds now plunge even to the south. The plunge of the upright folds is gentler and steeper in the limb zones of reclined and inclined isoclinal folds respectively. Subvertical upright folds occur only in the hinges of the westerly trending reclined folds. Interference patterns of dome-and-basin type and mirror-image type have formed in zones where both hinges and limbs of the early folds have been affected by upright folding. The structure of the marble band, the stratigraphic relations that the marble bears with the supposedly older Aravalli mica schists and migmatites of the Banded Gneissic Complex engulfing it, and the stratigraphic findings in adjacent terrains indicate that the Banded Gneissic Complex represents migmatized portions of the metasediments of the Aravalli and the Raialo groups, and is not the basement rock in central Rajasthan. Evidence for granitic rocks of at least two generations in the Early Precambrians of central Rajasthan is adduced.

Reclined folds and coaxial refolding in the paleozoic rocks of North-West Nelson

Abstract Complex folding in Ordovician shales and quartzites adjacent to a large granite intrusion is clearly visible on high dtitude vertical air photographs of the Gouland Downs in North-West Nelson. An almost complete absence of sympathetic mesoscopic folding and the partial obliteration of fabric elements associated with the deformation by post-tectonic contact metamorphism hinders the interpretation of the structure. Most of the folds, however, appear to be isoclinal with axes that plunge steeply to the east or north-east, almost at right angles to the trend of the folds. The emplacement of granite caused further folding in one area, apparently coaxial with the earlier deformation. Almost all the recognisable fold noses close to the south and are synforms, and it appears likely that they are small nappe-type folds separated by thrust planes. The Slate Range Fault, a large late Tertiary feature probably related to the Wakamarama Fault, crosses the area and a sinistral component of 2,800 ft is estimated.

Some aspects of coaxial refolding

The author has observed many examples of coaxial refolding in the Moinian rocks of the North-West Highlands of Scotland. This coaxial nature of refolding is not the result of the continuation of a single movement phase, but developed where a later fold refolded both limbs of an earlier fold. It is concluded that the coaxiality results from a strong flexural component in the later folding.