Single Girdle Research Articles

Temperature- and strain-rate-dependent microfabric evolution in monomineralic mylonite: evidence from in situ deformation of norcamphor

Norcamphor (C 7H 10O) was subjected to plane strain simple shear in a see-through deformation rig at four different strain rate and temperature conditions. Two transient stages in the microfabric evolution to steady state are distinguished. The grain scale mechanisms associated with the microstructural and textural evolution vary with the applied temperature, strain rate and strain. In high-temperature-low-strain-rate experiments, computer integrated polarization microscopy reveals that the texture evolution is closely related to the crystallographic rotation paths and rotation rates of individual grains. High c-axis rotation rates at low to intermediate shear strains are related to the development of a symmetrical c-axis cross girdle by the end of the first transient stage (γ = 1.5 to 2). During the second transient stage (γ = 1.5 to 6), the cross girdle yields to an oblique c-axis single girdle as c-axis rotation rates decrease and the relative activity of grain boundary migration recrystallization increases. Steady state (γ > 8) is characterized by a stable end orientation of the sample texture and the cyclic growth, rotation and consumption of individual grains within the aggregate.

Quartz microstructures and c-axis preferred orientations in high-grade gneisses and mylonites around the Morin anorthosite (Grenville Province)

Quartz in deformed rocks from two large, high-grade shear zones around the Morin anorthosite (Morin terrane, Grenville Province) displays distinctive microstructures, as well as c-axis preferred orientations. In the west-dipping Morin shear zone, east of the Morin anorthosite, four distinct quartz microstructures (types 1–4) are identified, based on deformation features and grain size. The c-axis orientations are characterized by a single maximum near the stretching lineation and two maxima in type 1 microstructure, and by an asymmetrical, single girdle in type 2 microstructure. Quartz c axes show crossed-girdle pattern in type 3 microstructure. Both quartz microstructures and c-axis preferred orientations suggest that crystal–plastic slip and dynamic recrystallization are the dominant deformation mechanisms. The asymmetry of c-axis orientations with respect to the mylonitic foliation, as well as the substructures developed in quartz, indicates a dextral sense of shear in the Morin shear zone. Type 4 microstructure, which developed in some gneisses and granulites, is interpreted to record influence of postdeformation annealing by which quartz c-axis orientations were partially modified. In the north-northeast-trending, subvertical Labelle shear zone that separates the Morin terrane from the Mont-Laurier terrane, metamorphic assemblages and structural elements suggest that an early, sinistral strike-slip deformation occurred under granulite-facies conditions. This was overprinted by a late downdip movement of the Mont-Laurier terrane under retrogressive conditions. Quartz in felsic gneisses from this zone shows two types of microstructures: one is similar to type 4 from the Morin shear zone, the other is named type 5. Quartz c-axis orientations are complex and less systematic, due to overprinting by two episodes of deformation and possible annealing. These complexities limit the utility of quartz microstructures and c-axis data in the structural analysis of the Labelle shear zone.

Kinematic evolution of the Miller Range Shear Zone, Central Transantarctic Mountains, Antarctica, and implications for Neoproterozoic to Early Paleozoic tectonics of the East Antarctic Margin of Gondwana

High‐grade ductile tectonites of the Precambrian Nimrod Group in the central Transantarctic Mountains form the Miller Range shear zone (MRSZ). With no exposed boundaries, this zone has a minimum structural thickness of 12–15 km. Shear‐sense indicators record consistent top‐to‐the‐SE, or left‐lateral, shear within the NW striking, moderately SW dipping zone. Cylindrical folds with axes normal to elongation lineation (Le) are kinematically consistent with other shear indicators. They may represent early stages in the development of subordinate noncylindrical sheath folds, which indicate locally high bulk ductile strain and a moderate strain gradient. Pervasive, open to tight cylindrical folds with axes parallel to Le formed during shear and may reflect a component of constrictional strain. Quartz c axis fabrics from micaceous quartzites show asymmetric single girdles evident of dominantly rhombohedral slip, with limited basal‐plane slip, affirming both the consistency of shear sense and high‐grade syn‐kinematic conditions. Deformation did not persist during subamphibolite facies cooling, as shown by (1) a lack of basal‐plane slip in ductilely deformed quartz, (2) a lack of quartz subgrains and grain shape‐preferred orientation, and (3) the presence of oriented muscovite “fish” included within polygonal quartz grains, which show that quartz grain boundaries migrated and annealed under static conditions following ductile shear. From the uniform Le orientation and consistent shear sense, we interpret that ductile deformation resulted from a single, kinematically simple, left‐lateral (top‐to‐the‐SE) shear event. Together, the scale, high total strains (γ ≥ 5), fabric uniformity, and the widespread presence of asymmetric microstructures formed at high temperatures, all indicate that strain rates within the MRSZ were high and that it represents a major crustal structure. Orogen‐parallel displacements within this zone during the latest Neoproterozoic to Early Cambrian were at a high angle to penecontemporaneous orogen‐normal contraction in outboard supracrustal rocks, suggesting that the Neoproterozoic to early Paleozoic plate margin of Antarctica was characterized by left‐oblique convergence in which strain within the orogen was partitioned into deep‐level strike slip and shallow‐level contraction.

A simulation of the temperature dependence of quartz fabrics

Abstract This paper presents the predictions of a hybrid model of fabric development in quartzites that combines aspects of previous models based on lattice rotations and dynamic recrystallization. The input parameters for the model have been systematically varied in order to investigate changes in deformation fabrics due to increasing temperature, and predictions of crystallographic preferred orientations and grain shape fabrics are made. The resulting fabrics reflect the combination of both deformation processes over the range of the parameters used. The fabrics evolve continuously with progressive deformation, and produce crossed girdles, single girdles and point maxima for a single deformation geometry. The results suggest that the coupling of dynamic recrystallization with lattice rotations can actually produce a larger variation in crystallographic preferred orientations with temperature than is predicted by using a model that only considers lattice rotations. Using this model point maxima develop even though a multiple slip model has been assumed.

Simple shear deformation and quartz crystallographic fabrics: a possible natural example from the Torridon area of NW Scotland

In this paper the microstructures and crystallographic fabrics of a 30 cm wide deformed quartz vein from the Torridon area of NW Scotland are described. The vein, which lies along a joint separating two blocks of Proterozoic gneiss, was deformed by crystal-plastic processes when the rigid joint blocks moved in opposite directions parallel to the vein margins. Mylonitic foliation within this quartzose ductile shear zone locally displays the classic orientation variation expected for heterogeneous simple shear deformation. In thin section the vein is a mylonite composed of dynamically recrystallized, elongate quartz grains aligned oblique to the mylonitic foliation. Fabrics within the central part of the vein have been examined by optical and X-ray texture goniometry methods, together with Orientation Distribution Function (ODF) analysis. The a-axis fabric consists of a dominant single point maximum, orientated within the XZ plane at 9° to the lineation (X); this point maximum occupies a pole position to the corresponding single girdle c-axis fabric. For individual positions on the c-axis fabric skeleton, the orientations of potential crystallographic slip systems (slip plane and direction) relative to shear zone co-ordinates have been calculated by ODF analysis. The calculation of relative resolved shear stresses (Schmid factors) for the potential slip systems, assuming a simple shear kinematic framework, reveals an elegantly simple relationship between individual grain c-axis orientation and corresponding orientations of potential slip systems. These results are readily interpreted in terms of bulk simple shear deformation.

Quartz textures from the Simplon Fault Zone, southwest Switzerland and north Italy

Quartz textures measured from monomineralic veins within the Simplon Fault Zone show very similar patterns over a strike length of some 30 km. The c -axes form asymmetric, kinked single girdles, with variably developed vestiges of crossed girdle limbs. The kinking of the girdle may be controlled by the two c -axis orientations where double rhomb 〈 a 〉 slip is possible. A strong 〈 a 〉 maximum lies between 20° and 30° to the lineation in the XZ plane, which seems too large to be the angle to the bulk shear plane. Straight single girdle patterns may be possible for strict simple shear. In natural fault zones, where deformation is in general heterogeneous on all scales, kinked single girdles and Type I crossed girdle textures are much more common, perhaps because such textures provide more freedom to accommodate a general two-dimensional strain.

Heterogeneous deformation and quartz crystallographic fabric transitions: natural examples from the moine thrust zone at the stack of glencoul, northern assynt

Quartz crystallographic fabric transitions in well-exposed mylonites immediately beneath the Moine Thrust at the Stack of Glencoul (NW Scotland) have been investigated by optical microscopy, X-ray texture goniometry and Orientation Distribution Function analysis. A progressive change is observed from asymmetrical kinked single girdle c-axis fabrics at 0.5 cm beneath the Moine Thrust, through asymmetrical Type I cross-girdle fabrics to symmetrical Type I cross-girdle fabrics at 30 cm beneath the thrust. This c-axis fabric transition is accompanied by a transition from asymmetrical single a-axis maximum fabrics (0.5 cm beneath the thrust) through asymmetrical two maxima fabrics to essentially symmetrical two maxima a-axis fabrics. ODF analysis of these S > L and L - S tectonites indicates that c-axis positions on the ‘leading edge’ of the fabric skeleton are related by a common (a) direction oriented within the XZ plane at a moderate angle to the lineation ( X). In contrast, c-axis positions on the peripheral ‘trailing edge’ are related by a positive (r) rhomb pole oriented close to Z; (a) directions lying within this common rhomb plane progressively change through 180° in orientation traced around the c-axis fabric skeleton. Such contrasting ‘single crystal’ rhomb (a) preferred orientations on the ‘leading’ and ‘trailing’ edges of the fabric skeleton are interpreted as indicating localized (grain scale) plane strain and flattening deformation, respectively. They result in tectonites with essentially symmetrical c- and a-axis fabrics which display strongly asymmetrical positive (r) and negative (z) rhomb pole figures. The observed transition in quartz c- and a-axis fabrics is interpreted as indicating an increasing importance of non-coaxial plane-strain deformation as the Moine Thrust is approached. Even immediately (<1 cm) beneath the thrust, however, flow has still significantly departed from bulk simple shear and involved an important (heterogeneous) component of contemporaneous flattening deformation.

Atypical textures in quartz veins from the Simplon Fault Zone

Samples of monomineralic quartz veins from the Simplon Fault Zone in southwest Switzerland and north Italy generally have asymmetric, single girdle c-axis patterns similar to textures measured from many other regions. Several samples have characteristically different textures, however, with a strong single c-axis maximum near the intermediate specimen axis Y (the direction within the foliation perpendicular to the lineation X) and a tendency for the other crystal directions to be weakly constrained in their orientation about this dominant c-axis maximum. This results in ‘streaked’ pole figure patterns, with an axis of rotation parallel to the c-axis maximum. These atypical samples also have a distinctive optical microstructure, with advanced recrystallization and grain growth resulting in a strong shape fabric ( S B ) oblique to the dominant regional foliation ( S A ), whereas typical samples have a strong S A fabric outlined by very elongate, only partially recrystallized, ribbon grains. The recrystallized grains of the atypical samples are themselves deformed and show strong undulose extinction and a core-mantle recrystallization structure. The streaked texture is likely to be a direct consequence of lattice bending and kinking during heterogeneous slip on the favoured first-order prism (10 1 0) (a) system, the heterogeneity itself being due to problems in maintaining coherence across grain boundaries when insufficient independent easy-slip systems are available for homogeneous strain by dislocation glide. Such bending would be particularly prevalent in very elongate, thin ribbon grains, resulting in high internal strain energy and promoting recrystallization. Thus both the texture and the microstructure could be significantly modified by later strain increments affecting quartz grains with an already developed, nearly single-crystal texture.

The Himalayan Main Central Thrust pile and its quartz-rich tectonites in central Nepal

As a consequence of the India—Asia collision, a deeply rooted crustal slice has been moved southwards over the northern boundary of the Indian plate. At the present level of observation the Main Central Thrust pile, a rock-pile of several kilometers thickness, is affected by shear deformation which imprinted a flat foliation and a constant N—S trending lineation. These tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field. The M.C.T. shear zone is studied from the view-point of the quartz-rich rocks: over 100 samples helped to define a microstructural zonation, roughly parallel to the thermal one; 35 samples, particularly rich in quartz, have been used for the preferred orientation study of their c-axes, and a few of them for their 〈a〉 axes. Strong lattice orientations, when undisturbed by (mainly) secondary recrystallization, attest to large plastic strains in quartz. The c-axes are mostly distributed along unequally populated crossed-girdles which may evolve into an inclined single girdle. The 〈a〉 axes concentrate perpendicular to the more populated girdle. Concerning quartz the following is suggested: 1. (1) the deformation has a dominant shear component responsible for the dominant stretching lineation. It could be simple shear in case of inclined single girdles 2. (2) one set of 〈a〉 axes line up parallel to the shear direction. Slip on various planes in the 〈a〉 direction may be an important deformation mechanism 3. (3) asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the Himalayan thrusting in more than 80% of cases.

Oblique girdle orientation patterns of quartz C-axes from a shear zone in the basement core of the Maggia Nappe Ticino, Switzerland

In the granitic basement of the Maggia Nappe, a pre-existing quartz C-axis fabric has been modified by later shearing. The progressive changes in the preferred orientation patterns of quartz across one boundary of a shear zone are described. A single girdle fabric is developed and a reference framework to specify the ‘obliquity’ of such fabrics with respect to the major axes of the finite strain ellipsoid, is introduced.

Some effects of girdling, gibberellic acid sprays, bunch thinning and trimming on the sultana

Girdling, thinning and trimming bunches and gibberellic acid (GA) treatments were imposed on sultana vines over three years to study their effects on berry size, yield, skin strength, pedicel features and bunch characteristics. A double cut stem girdle (4.8 mm) combined with three GA sprays of 60 p.p.m. (applied at bloom, shatter and two weeks after shatter) and bunch thinning and trimming produced large berries weighing up to 5 g each. However this treatmen1 delayed fruit maturity and produced more compact bunches. Similar sized berries with no delay in maturity and looser bunches were obtained by using the same combination of treatments but reducing the GA sprays to two, one at 10 p.p.m. applied at 50 per cent bloom and the second at 60 p.p.m, at 3-4 mm berry diameter, which was two weeks after shatter. A double cut was more effective than a single cut girdle. Bunch thinning and trimming were necessary for best results but there were no differences in results between trimming to four or six laterals per bunch. Thinning to 15 bunches per vine produced more compact bunches and lower yield than 20 bunches per vine. Berry skin resistance to puncture was increased more effectively by bunch trimming and thinning than by girdling or applying GA. Pedicel length and thickness were increased substantially by applying GA.

The Relationship of Small Limb Movements During REM Sleep to Dreamed Limb Action

Ten adult subjects, selected for good dream recall and ability to produce spontaneously fine limb movements in the laboratory, reported their last dream experience after awakenings from REM sleep during various limb movement and no movement conditions. The reports were rated for amount and girdle location of dreamed actions by judges blind to the awakening condition. There was a significant correlation between the number of girdles moving before awakening and the amount of dreamed action. Also, when dreams from single girdle movement awakening conditions were examined, the girdle active before awakening was dreamed as moving to a significant degree.

The chloroplast nucleoid in Ochromonas danica. I. Three-dimensional morphology in light- and dark-grown cells.

ABSTRACTThe 3-dimensional structure of the plastid nucleoid was determined from serial sections of the plastid of dark-grown, greening, and light-grown cells of Ochromonas danica. In light-grown and greening cells, the chloroplast nucleoid forms a continuous cord or ring which closely follows the rim of each lateral lobe of the chloroplast and is continuous across the top and bottom of the bridge connecting the 2 chloroplast lobes. The nucleoid always lies just inside the chloroplast girdle bands where they loop around the rim of the plastid. It was demonstrated by electron-microscopic autoradiography of greening cells labelled with [3H]-thymidine that all the plastid DNA is localized in this peripheral ring-shaped nucleoid. In the proplastid of dark-grown cells, the nucleoid also forms a ring-shaped structure lying just inside the single girdle thylakoid, although frequent irregularities, such as gaps, are present. It is postulated that the girdle bands determine the shape of the chloroplast nucleoid, possibly by having specific attachment sites for the plastid DNA molecules. A survey of the literature shows that a peripheral ring-shaped chloroplast nucleoid is a characteristic feature of the 5 classes of algae whose chloroplasts possess girdle bands, namely the Raphidophyceae, Chrysophyceae, Bacillariophyceae, Xanthophyceae, and Phaeophyceae, and has never been observed in plants whose plastids lack girdle bands.