Symmetry of pebble-deformation involving solution pits and slip-lineations in the northern Alpine Molasse Basin

Deformation features on pebbles of the Alpine Molasse Basin are most clearly developed in carbonate components. Ductile distortion is small; most of the pebbles moved against each other to produce solution pits and slip-lineations on the pebble surfaces. The complete lineation field has a triaxial geometry. From a compressional axis of divergence with maximum solution, fields of diverging lineations extend to meet at a plane of convergence. Their ends bend away from an intermediate axis towards an (extensional) axis of convergence. The strain-symmetry is pure shear for orthogonal lineation-field axes, uniaxial compression and extension representing special cases. The angle α between the divergent and the convergent axes decreases from 90 to 0° with the transition from pure to simple shear. For Molasse pebbles α angles between 90 and 60° were usually observed. Regional compression developed perpendicular to the Alpine structures and parallel to bedding, with increasing deviations near the Alpine border. Zones are recorded of vertical and horizontal extension and of compression perpendicular to bedding and horizontal extension perpendicular to the Rhinegraben existing at the northern border of the Molasse Basin.     

Quantitative finite strain and kinematic flow analyses along the Zagros transpression zone, Iran

The Ghouri area in southwest Iran exposes a cross section through the Zagros orogenic belt. The area provides an opportunity to investigate quantitative finite strain (Rs), kinematic vorticity number (W_k), proportions of pure shear and simple shear components, sense of shear indicators, steeply plunging lineations, and other moderate to steeply plunging stretching lineations in a transpressional zone. Based on a classical strain analysis of deformed microfossils with oblate strain ellipsoid shape, the Zagros orogenic belt is classified as a pure-shear dominated zone of transpression, but asymmetry of shear-sense indicators suggests that a significant component of simple shear was involved along the deformation zone boundaries. The long axes of the microfossils and stretched pebbles of a deformed conglomerate were used to indicate the stretching direction in this zone. The stretching lineations have a steep to moderate plunge but a constant strain magnitude. Characteristics of dextral inclined transpressional kinematics in the Zagros continental collision zone were quantified and indicate an estimated k-value < 1, an angle between the maximum horizontal axis of the instantaneous strain ellipsoid and the zone boundary (θ = 32°), asymmetrical dextral shear-sense indicators, and an angle of relative plate motion (α = 25°).     

云南晋宁梅树村下寒武统砾屑磷块岩的扫描电镜研究

通过扫描电镜观察.发现云南梅树村中谊村段砾屑磷块岩中的磷质砾屑主要由磷质鲕球、磷质微球和磷质雏晶堆积体组成.磷质微晶是磷酸盐的基本晶体.磷质雏晶是在扫描电镜下所能观察到的最小的磷酸盐颗粒,但它们晶形不完全.砾屑磷块岩的形成过程可以分为三个阶段:磷质鲕球、微晶和雏晶形成阶段,磷质砾屑形成阶段,和砾屑磷块岩形成阶段.磷质砾屑的形成环境为浅水近岸,水动力条件不稳定,沉积速率和通量不高.     

Exploring till bed kinematics using AMS magnetic fabrics and pebble fabrics: the Weedsport drumlin field,New York State,USA

Gentoso, M. J., Evenson, E. B., Kodama, K. P., Iverson, N. R., Alley, R. B., Berti, C. & Kozlowski, A. 2012 (January): Exploring till bed kinematics using AMS magnetic fabrics and pebble fabrics: the Weedsport drumlin field, New York State, USA. Boreas, Vol. 41, pp. 31–41. 10.1111/j.1502‐3885.2011.00221.x. ISSN 0300‐9483. Thick, relatively homogeneous basal tills exposed in the drumlins and flutes of the Weedsport drumlin and flute field in New York State exhibit anisotropy of magnetic susceptibility (AMS) and pebble fabrics that are consistently oriented parallel to the streamlined bedforms. The pebble fabrics and AMS fabrics are concordant. In this study, six drumlins and five flutes were sampled. Thermally induced, incremental reduction of isothermal remanent magnetization indicates that AMS is caused by primarily elongate maghaemite grains. The orientations of principal axes of maximum susceptibility (k₁) are generally parallel to pebble long‐axis orientations, and tend to plunge mildly up‐glacier. Fabric directions are generally parallel to drumlin long‐axis orientations, but deviate by 12°–23° from flute directions. Fabrics of the flutes are stronger and more unidirectional than those of the drumlins. These results support the use of AMS as a fast and objective method for characterizing fabrics in tills, and suggest hypotheses about basal processes linked to glacially streamlined landforms.     

Field experiments of aeolian dust accumulation on rock fragment substrata

The effect of rock fragments and rock fragment cover on the accumulation of airborne dust was examined in a long-term field experiment in the Negev desert of Israel. Four parameters were studied: pebble eccentricity, pebble size, pebble flattening and cover density. The effect of these parameters on the accumulation of dust on the pebbles, on the accumulation of dust between and underneath the pebbles, and on total dust accumulation (pebbles+interpebble space) was measured separately. Accumulation on the pebbles increased as the pebbles became larger, less flattened and more elongated, and as cover density increased. Accumulation between and underneath the pebbles increased as the pebbles became smaller, more flattened and more elongated, and as cover density increased (although the accumulation area available became smaller). Total dust accumulation increased as the pebbles became smaller, more flattened and more elongated, and as cover density increased. Rock fragments act as a dust trap as dust accumulation on pebble-covered surfaces is several tens of times larger than dust accumulation on similar but pebble-free surfaces. However, less than 20% of the initially settled dust is protected from further erosion. In the Negev desert, dust erosion by wind is of the order of 10 times larger than dust erosion by water. The rate of long-term dust accumulation in the Negev is of the order of 15–30 g.m^?2.yr^?1. This is considerably lower than the rates proposed elsewhere in the literature, which are based upon over optimistic dust retainment percentages.     

Quartz [0001]-axes preferred orientation, Bluff, New Zealand: Origin elucidated by grain-size measurements

Permian volcanic sediments at Bluff have been strained and thermally metamorphosed by Permian intrusives to metasediments of hornblende—hornfels facies. Quartz, which crystallised as a secondary mineral during metamorphism, has an unusual preferred orientation with c-axes either forming paired maxima in the plane containing the lineation (=maximum principal strain axis = direction of extension) and the perpendicular to schistosity (=minimum principal strain axis = shortening direction) or a broad maximum parallel to the lineation; the paired maxima are approximately 30° either side of the lineation. Some quartz grains are markedly elongate parallel to the lineation, and according to hypotheses of preferred orientation involving crystal plasticity, there should be some correlation between the shape of such grains and their c-axis orientations. Grain-size and shape analysis of Bluff quartz demonstrate that no such correlation exists; the analyses show that the preferred orientation results from oriented nucleation in the residual stress field immediately following the bulk straining of the rocks, with the distribution of c-axes as predicted by Kamb's hypothesis (1959). The time relationships of rock deformation, thermal metamorphism, and nucleation and growth of quartz are discussed.     

The strain history of a dalradian slide: Using pebbles with low fluctuations in axis orientation

The strain associated with the Horn Head Slide, a major tectonic break in the Dalradian rocks of NW Ireland, is recorded by pebbles in an adjacent quartzite horizon. Mean X/Y ratios of the deformed pebble shapes in excess of 8.0 are seen closest to the slide and the field of three-dimensional shapes lies along the K = 1 line. The usual methods of separating initial shape ratio (R_i) and tectonic strain ratio (R_s) from the deformed shape ratio (R_f) of ellipsoidal markers are based on measurements of variation in fluctuation (e.g., the (R_f/φ technique). However, due to the high X/Y strains in this situation and since the pebbles initially lay parallel to bedding and to a principal plane of the finite strain ellipsoid, fluctuation is generally very low. Thus, except for the least deformed X/Y data, the R_f/φ technique is inapplicable and other methods are used. For X/Y data with mean (R_f > 4.0: R_s is calculated as the harmonic mean of R_f; maximum R_i values only are obtained from the range of R_f data. For all Y/Z and X/Z data: R_i is calculated from the logarithmic range (ω_log) of R_f; R_s is simply obtained from the geometric mean of R_f modified by R_i. It is concluded from this that a varying prolate tectonic strain (K - 1.5) reaching X/Y values in excess of 8.00 was coaxially superimposed on an initial oblate shape fabric to produce the present field of deformed pebbles in the quartzite near the slide.     

剪切带运动学：重点评述三斜模式和区分韧性滑动擦痕与拉伸线理的重要性

详细评述了剪切带运动学和内部几何学研究的新进展。剪切带内的递进变形一般为三斜对称,单斜剪切带(包括简单剪切带)是特例情形。理论模拟表明,如同许多天然剪切带中所见到的,在三斜剪切带中,拉伸线理的方位可从近水平到平行于倾向连续变化。过去一般将拉伸线理的方向当作剪切运动方向的做法是以简单剪切模式为基础的,不适合于一般剪切带。在三斜剪切带中,拉伸线理和剪切方向之间不存在简单关系。然而,C面上的韧性滑动擦痕平行于剪切方向发育,是剪切方向的可靠标志。因此,在剪切带运动学解释中区分韧性滑动擦痕与拉伸线理非常重要。     

THE PROBLEM OF UNDERSTANDING THE NOMENCLATURE OF FACIES

Chert-plus-pyrite pebbles have been known for some time in a number of Witwatersrand conglomerates, but their distribution is highly variable. A wide variety of textures in chert-plus pyrite pebbles are documented here, and these textures place constraints on the origin of such pebbles. Replacement of chert pebbles by pyrite is indicated (pyrite grains and aggregates continue across many pebble boundaries), and both distributional and textural evidence favors a post-burial timing for this process. Significant mobility of sulfur after burial is indicated. Whether this replacement was diagenetic or metamorphic is not certain.     

The relation between microfabric and strain in a progressively deformed quartzite sequence from Central Australia

In the Ormiston Nappe Complex, west of Alice Springs, central Australia, a deformed zone up to 0.7 km thick is developed in the sedimentary Heavitree Quartzite. The deformed zone is adjacent to a major thrust fault and is defined by mylonitic foliation, which is parallel to the thrust plane and by isoclinal folds. Recognition of original detrital quartz grains allows strain ellipsoids to be measured across the zone. The strain generally plots in the flattening field and many specimens show pure flattening strain. The mylonitic foliation is an axial-plane structure to the folds and is parallel to the XY-plane of the strain ellipsoid. A quartz elongation lineation may be present within the foliation and is parallel to the principal extension direction (the X-axis) of the strain ellipsoid.Strain is accommodated principally by intracrystalline plastic deformation of the quartz grains. In detail the strain is not homogeneous and may vary even between adjacent grains of the same specimen. Quartz optic axis fabrics reflect this strain inhomogeneity. If the strain ellipsoid is an oblate spheroid, c-axes lie in small-circle girdles about the principal shortening axis (the Z-axis). With general triaxial strain, the c-axes lie in a great-circle girdle or girdles which intersect the foliation parallel to the intermediate strain axis (the Y-axis) and lie symmetrically about the Z-axis. A random population of grains from a specimen often shows a composite c-axis pattern between these two types.With approach to the thrust there is an increase in the amount of strain within the specimens. The increasing strain correlates with an increase in the degree of c-axis preferred orientation of the deformed detrital grains and in the amount of new strain-free grains present in the deformed quartzite. Adjacent to the thrust the quartzite is completely composed of polygonal new grains. The new grains probably formed under syntectonic conditions caused by movement along the thrust. The bulk of the new grains developed by increasing misorientation between the subareas of an initially polygonized old grain. There is no evidence of any marked host control on new-grain orientation, but new grain c-axis plots are generally similar to the corresponding old-grain plots from the same specimen.     

Strain analysis using deformed pebbles: The influence of initial pebble shape

Several methods exist for the determination of the finite strain ellipsoid from deformed pebble shapes. These methods are critically evaluated and others are proposed on the basis of calculations which predict both the sectional and three-dimensional shape of pebbles in simple deformed simulated conglomerates. In many cases it is found preferable to use an average pebble shape to estimate the tectonic strain and that the harmonic mean of the ratios of axial lengths yields an average pebble shape which is closest to the strain ellipsoid shape.     

Brittle deformation in pitted pebble conglomerates

The fracture patterns produced in pitted pebble conglomerates from the Alpine Molasse and the Carboniferous of northern Spain, have been studied in relation to the stress concentrations which were produced in the conglomerates during their deformation. The stress distributions which develop around pebble contacts at different stages of their pitting history have been determined from photoelastic experiments. The development of different types of fracture, having dominantly tensile or shear components, and their distribution within the pebbles, are shown to be related to the mineralogy of the pebbles, the strength of the matrix and the amount of deformation the conglomerate has suffered.     

Tectonically significant pebble types: Sheared,pocked and second-cycle examples

A history of repeated faulting, within an area, can be obtained from unconformity analysis or sediment-vector sequences (not treated in the present paper) or from studies of gravel tongues. The latter may include sheared pebbles, pocked pebbles, half-rounds, and second-cycle (or multi-cycle) pebbles. Sheared pebbles (and larger fragments) were cut by faulting after deposition, hence represent two episodes of deformation. Pocked pebbles (scarred pebbles) were ground against each other, after deposition, by a second episode. Sheared and pocked pebbles typically develop where sand matrix is not abundant; single pebbles “floating” in sandstone rarely show these features. Quartz sand schist is a fine-grained equivalent of the sheared-pebble gravel or conglomerate. Half-rounds represent transportation of sheared pebbles, after shearing; where half-rounds have been sheared, at least three episodes of deformation are indicated.A second-cycle conglomerate is one in which individual pebbles (or larger pieces) are themselves conglomerates. They likewise indicate at least two episodes of deformation.In some areas, details of displacement can be obtained best from unconformity analysis; in other areas, the special pebble types listed here may provide such details. In a few places, both types of information may be available, thus giving a fuller picture than could be obtained in other ways.The best example of a thick stratigraphic sequence containing these special pebble types (representing repeated deformation), described in the present paper, is located near Bucaramanga, Colombia. A second example from northern New Mexico is described less fully. Brief references to other occurrences, in Wyoming, Montana, Oklahoma and Alabama, are made. It is concluded that, although these pebble types may not be abundant at any one locality, they nevertheless occur at many places, and therefore can provide important information about deformational history.     

How do lineations reflect the strain history of transpressive shear zones? The example of the active Alpine Fault zone,New Zealand

Lineations within mylonites exhumed in the hanging wall of New Zealand's active Alpine Fault zone have a complicated relationship to contemporary plate kinematics. The shear zone is triclinic and macroscopic object lineations are not usually parallel to the simple shear direction, despite high total simple shear strains (γ ≥ 150). This is mostly because the lineations are inherited from pre-mylonitic fabrics, and have not been rotated into parallelism with the mylonitic stretching direction (which pitches c. 44° in the fault plane). Furthermore, some lineations have been variably rotated depending on whether they are present in shear bands or microlithons, which accommodated bulk strains with different vorticities. Total strains required to obtain parallelism between the finite maximum principal stretching direction calculated from transpression models and these mylonitic lineations, are pure shear stretch, S₁ ∼ 3.5; simple shear 11.7 < γ < 150. The observations and numerical models also show that linear features are not rotated much during simple shear because they initially lie within the shear plane, and that inherited fabric components may not be destroyed until very high simple shear strains have been attained.     

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 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.     

Non-coaxial experimental deformation of olivine

Dunite samples have been deformed non-coaxially at high temperatures and pressures. In samples which were deformed by translation glide on {Okl}[100], olivine [100] = Z-axes rotated toward the maximum extensile-strain axis. In samples which were partly or completely recrystallized syntectonically the new olivine grains attained preferred orientations with [010] = X and [100] = Z parallel to the maximum and least principal compressive stress axes, respectively. These results may be used for kinematic and dynamic analysis of naturally deformed dunites and peridotites.     

S–C fabrics developed in cataclastic rocks from the Nojima fault zone,Japan and their implications for tectonic history

S–C fabrics similar to those found in mylonites are observed in foliated cataclastic granitic rocks from the Nojima fault zone, southwest Japan. The foliated cataclastic rocks comprise cataclasite, fault breccia, gouge, and crushing-originated pseudotachylyte. The S–C fabrics observed in these cataclastic rocks involve S-surfaces defined by shape preferred orientation of biotite fragments or aggregates of quartz and feldspar fragments, and C-and C′-surfaces defined by microshears and shear bands, respectively, where fine-grained material is concentrated. Striations on the main fault plane are oriented parallel to the cataclasite lineations. A significant microstructural difference between the foliated cataclastic rocks and S–C mylonites is the absence of dynamically recrystallized grains in the foliated cataclasites. The striations, cataclastic lineations, and the S–C fabrics in the cataclastic rocks formed from the late Tertiary to the late Holocene indicate that the Nojima fault zone has moved as a dextral strike-slip fault, with a minor reverse component since it formed. S–C fabrics in cataclastic rocks provide important information on the tectonic history and are reliable kinematic indicators of the shear sense in brittle shear zones or faults.     

大别山北部糜棱岩中石英的高温应变

超高压变质岩的发现导致了人们对大别山构造格架的重新认识。最近的研究表明,超高压变质岩只产于大别山南部(Wang et al., 1992),而北部为古生代岛弧杂岩所占据,晓天-磨子潭断裂即为这些杂岩的北界(Wang et al., 1994)。晓天-磨子潭断裂作为左行走滑断裂已经得到公认,但笔者对该断裂南侧糜棱岩的研究表明,其并非左行走滑产物,而是早期变形的构造形迹。本文旨在分析这些糜棱岩中石英的显微构造,进而揭示其早期构造变形特征。     

The significance of stretching lineations in terms of progressive deformation and finite strain

Abstract

For a given increment of deformation, stretching lineations can be defined either by old grains or by new grains.

Progressive strain accumulated in a matrix causes reorientation of old grains. The velocity field acting within a grain depends on the degree of non-coaxiality of the progressive strain in the matrix, and on the competence contrast between grain and matrix (e.g. Lister & Williams, 1983). Deformed populations of old grains do not systematically track the principal directions of finite stretch; this depends on the mechanical behavior of the grains. On the other hand, these populations may be good markers of finite strain magnitude, provided that the initial distribution is known.

In contrast, the statistical shape orientation distribution of population of new grains cannot be related in a simple way to the finite strain magnitude, but their preferred orientation is considered to be the best indicator of the principal directions of finite stretch.

An example is considered from the schistes lustrés near Kenestrelle (Val Chisone, Western Alps), where the observed mineral lineation is attributed to a late E-W stretching event in the alpine orogenic history.     

Triaxial stress state deep in orogenic belts: an example from Turkey

A rare metachert pebble containing amphibole grains with microboudin structures in a wide range of orientations provides an opportunity to perform stress analysis in two orthogonal orientations on the foliation surface. The sample was analysed by the microboudin method to infer the triaxial stress state during microboudinage. Stress parameters proportional to the far-field differential stress were determined for sodic amphibole grains in the two orientations. The ratio of the stresses in the two orthogonal orientations (σ₁−σ₂)/(σ₁−σ₃) was calculated to be 0.64, indicating that σ₂ lies closer to the midpoint between σ₁ and σ₃ than to σ₃.