Development Of Lineation Research Articles

Indosinian high‐strain deformation for the Yunkaidashan tectonic belt, south China: Kinematics and40Ar/39Ar geochronological constraints

Structural and40Ar/39Ar data from the Yunkaidashan Belt document kinematic and tectonothermal characteristics of early Mesozoic Indosinian orogenesis in the southern part of the South China Block. The Yunkaidashan Belt is tectonically divided from east to west into the Wuchuang‐Sihui shear zone, Xinyi‐Gaozhou block, and the Fengshan‐Qinxi shear zone. Indosinian structural elements ascribed to the Indosinian orogeny include D2and D3deformation. The early D2phase is characterized by folding and thrusting with associated foliation and lineation development, related to NW‐SE transpression under amphibolite‐ to greenschist‐facies conditions. This event is heterogeneously overprinted by D3deformation characterized by a gentle‐dipping S3foliation, subhorizontally to shallowly plunging L3lineation, some reactived‐D2folds and low‐angle normal faults. The D3fabrics suggest a sinistral transtensional regime under greenschist‐facies metamorphism. The timing of the D2and D3events have been constrained to the early to middle Triassic (∼248–220 Ma) and late Triassic (∼220–200 Ma) respectively on the basis of40Ar/39Ar geochronology and regional geological relations. The change from oblique thrusting (D2) to sinistral transtension (D3) may reflect oblique convergence and crustal thickening followed by relaxation of the overthickened crust. In combination with the regional relations from Xuefengshan to Yunkaidashan and on to Wuyishan, the early phase of the Indosinian orogeny constituted a large‐scale positive flower structure and is related to the intracontinental convergence during the assembly of Pangea in which the less competent South China Orogen was squeezed between the more competent North China and Indosinian Blocks.

Deformation in a complex crustal-scale shear zone: Errabiddy Shear Zone, Western Australia

Abstract Detailed mapping of four areas representing different geological units with varying formation histories within the crustal-scale Errabiddy Shear Zone shows an apparently simple temporal progression from foliation and mineral lineation development to folding and then to brittle deformation across the shear zone. However, in detail the structural evolution of the shear zone shows considerable complexity. The dominant foliation throughout the shear zone was formed in the upper greenschist to amphibolite facies during the 2000–1960 Ma Glenburgh Orogeny, which involved the accretion of the Archaean to Palaeoproterozoic Glenburgh Terrane onto the Archaean Yilgarn Craton and the subsequent formation of the Errabiddy Shear Zone. Orthorhombic kinematic indicators formed during the Glenburgh Orogeny as did the widespread mineral lineation. These fabrics were overprinted by a greenschist facies deformation and metamorphic event during the 1830–1780 Ma Capricorn Orogeny. During the Capricorn Orogeny mineral lineation development was rare, and mostly took place in high-Capricorn strain zones in areas where a pre-existing Glenburgh-aged mineral lineation was present. Such mineral lineations trend parallel to Capricorn-aged fold hinges. Regardless of the presence or absence of Capricornaged mineral lineations, dextral strike-slip kinematics and simple shear indicated by delta and sigma porphyroclasts, and displacement along detachment faults, are prevalent close to discrete shear zone boundaries, within the Errabiddy Shear Zone. However, between shear zone boundaries flattening and coaxial strain dominated during the Capricorn Orogeny. This difference in Capricorn Orogeny kinematics throughout the shear zone is caused by strain partitioning — although progressive deformation throughout the shear zone with dextral strike-slip faults overprinting older structures formed by pure shear also took place. These results suggest that analyses of small parts of shear zones may not give the complete history of an evolving transpressional shear zone because of the presence of strain partitioning and strain localization over time.

Controls on lineation development in low to medium grade shear zones: a study from the Cap de Creus peninsula, NE Spain

Lineations composed of similarly oriented elongate mineral aggregates or grains are a common feature in deformed rocks, but it is unclear which factors control the development of such lineations. Field observations and microstructural analysis of samples, which were taken from discrete greenschist to lower amphibolite facies shear zones of the easternmost Variscan Pyrenees, show that strain is only one of several factors that control the strength and type of a lineation. Dynamic recrystallization, metamorphic reactions and rigid body rotation are also important controlling factors for the development of lineations. The most important of these is dynamic recrystallization. The way in which dynamic recrystallization influences lineation development is largely a function of the initial fabric in a specific rock type. Different lithologies with different initial fabrics produce distinctly different types and strengths of lineations even if deformed to the same finite strain in the same shear zone. An initial fine grain size and a monomineralic composition of the parent rock type commonly hinder the development of lineations. In contrast, in initially coarse-grained polymineralic rocks, well-developed lineations commonly develop. Therefore, the ratio of initial to dynamically recrystallized grain size determines to a large extend the development of such lineations.

Late orogenic extension in the Bohemian Massif: petrostructural evidence in the Hlinsko region

The tectonic contact between low-grade metase-dimentary series and high-grade rocks in the Hlinsko region (Bohemian Massif) is commonly interpreted as a thrust of the Barrandian sediments over the upper Moldanubian nappe.The sediments occur in an E-facing synform that contains a tonalitic laccolith on its eastern boundary with the Moldanubian, and is truncated by a granodiorite pluton to the west. The synform represents a late deformational folding event related to the granodiorite intrusion. NW-oriented normal shear in the tonalite is indicated by S-C microstructures. Kinematic criteria associated with the major foliation and lineation development in the metasediments also indicate a north-westward, normal shear. In addition, Moldanubian gneiss display late shear bands due to north-westward, normal shear. Consequently, the presumed thrust is a low-angle, normal shear zone.Low-pressure type metamorphism (3 < P < 4 x 102 MPa) coeval with the major deformational phase in pelites of the Hlinsko synform is attributed to both the tonalite aureole and the extensive HT metamorphism (under P > 6 x 102 MPa) that has affected the underlying Moldanubian.The possibly polyphase normal fault is consistent with the meta-morphic pressure jump between the metasediments and the Moldanubian.We suggest that the tonalite intruded syntectonically within the normal ductile shear zone active during waning stages of the Variscan orogeny.

Progressive evolution from uplift to orogen‐parallel transport in a late‐acadian, upper amphibolite‐ to granulite‐facies shear zone, south‐central Massachusetts

Fabrics within a 1‐km‐wide shear zone along the eastern margin of the Bronson Hill anticlinorium in central Massachusetts provide important information on the regional tectonic development during the Devonian Acadian orogeny. Two distinct mineral lineations observed within the zone are each defined by alignment of coarse sillimanite needles and elongated quartz and feldspar grains and aggregates. Both lineations are associated with shearing and development of mylonitic fabrics. The character of the two lineations and related fabrics differs mainly in orientation and in the relationship of each lineation to the local orientation of quartz c axis fabrics. The interpreted earlier lineation has a steep west plunge and the later lineation has a shallow plunge and north‐south to southwest trend, parallel to the regional trend of the orogen. Extensive dynamic recrystallization of feldspar, the possible activity of prism slip in quartz, and the mineral assemblages in apparent equilibrium during deformation suggest that both lineations probably formed under upper amphibolite facies conditions, just after the thermal peak of Acadian metamorphism. A progressive change between the two deformation fabrics is suggested by evidence of overlap between the two, by their striking similarity in style and metamorphic conditions of formation, and by the similarity in orientation of their bulk strain axes, with a change only in the transport direction between them, kinematic indicators associated with each phase of lineation development are consistent throughout the shear zone. The earlier phase has a west‐side‐up sense of shear and the later phase indicates west‐side‐north movement. These phases are correlated with the regional backfold and dome stages of Acadian deformation. The kinematics and relationships between them suggest a revised tectonic model for the later part of the Acadian orogeny and for development of orogen‐parallel lineations in this region. In this model, uplift and thickening of basement gneisses in the Bronson Hill anticlinorium during the backfold stage was impeded by gravitational instability in a thermally weakened crust. This led to a change from subvertical to subhorizontal extension. East‐west Acadian shortening was still in progress, constraining this extension to a north‐south direction, parallel to the trend of the orogen.

The Cold Spring Melange and a possible model for Dunnage–Gander zone interaction in central Newfoundland

Structural relationships at Cold Spring Pond and the recognition of ophiolitic melange bear on the important questions of timing and style of structural superpositioning of Dunnage Zone rocks above Gander Zone rocks in central Newfoundland. The latest models emphasize ductile shear boundaries and orogen-parallel movements. Previous models proposed west-to-east or head-on obduction of Dunnage ophiolitic rocks across the Gander Zone.At the Dunnage (Exploits Subzone) – Gander (Meelpaeg Subzone) boundary at Cold Spring Pond, discrete, outcrop-size ultramafic blocks and smaller quartzite blocks are randomly distributed, and they are surrounded by, or are embedded in, homogeneous black graphitic shale or phyllite. The ultramafic blocks are typical of nearby Early Ordovician Dunnage ophiolite suites, the quartzite blocks are typical of adjacent Early Ordovician or earlier Gander clastic rocks, and the matrix black shales are similar to those of Middle or Early Ordovician age that occur throughout central Newfoundland. This chaotic mixture of almost coeval lithologies at Cold Spring Pond is interpreted as an olistostromal melange; the Cold Spring Melange. It resembles melanges that are dated as Ordovician elsewhere in Newfoundland.The Cold Spring Melange is overprinted by the full range of structures and metamorphic effects evident in adjacent rocks of the Exploits (Dunnage) and Meelpaeg (Gander) subzones. These include the development of lineations, cleavages, schistosities, zones of ductile shearing, regional metamorphism, and contact metamorphism. The oldest of these effects are interpreted as Silurian, based on isotopic dating in southern Newfoundland.The formation of olistostromal, ophiolitic melange implies disruption of the oceanic tract (Exploits Subzone of the Dunnage Zone), and in the case of the Cold Spring example, juxtapositioning or transport of Exploits Subzone ophiolite suites against or across the supracrustal rocks of the Meelpaeg Subzone (Gander Zone). The age and provenance of Cold Spring components, lack of post-Ordovician components, overprinting structural relationships, and comparison with other Newfoundland melanges all support an Ordovician age of formation. Overprinting relationships indicate that major ductile shears at other Dunnage–Gander zone boundaries postdate initial Dunnage–Gander superpositioning.

Development of Lineation in Complex Fold Systems

Development of Lineation in Complex Fold Systems

Development of Lineation in Complex Fold Systems

Critical discussion of the concepts, evidence, and presentation of data relating to simple, multiple, and anomalous lineation in complex fold structures contained in a paper by P. Clifford and others.

The Development of Lineation in Complex Fold Systems

AbstractThe paper refers to a number of areas in which the lineations are related to major folds. It shows that the shapes of individual folds, the interference of neighbouring folds, and the imposition of a second generation of folds all influence the arrangement of linear structures and that lineation maps may only become explicable when the major structure has been determined.