Orientation Of Foliation Research Articles

Lithospheric tectonic structures developed under high-grade metamorphism in the Southern part of Madagascar

This paper describes the tectono-metamorphic evolution of a segment of the Precambrian deep crust, in the southern Madagascar island. This crust corresponds to an Archaean basement reworked by a widespread, late panAfrican event (550–580 Ma) during the formation of the Mozambican belt. The finite geometry and associated metamorphism are depicted by satellite imaging, field mapping and P-T estimations using both conventional thermobarometric methods and TWEEQ software program with internally consistent thermodynamic data and uniform set of solution models. The structural pattern developed during high-grade metamorphism shows the juxtaposition of domains with complex fold geometries separated by a 15 km wide ductile shear zone. Within the folded domains, kilometre scale interference patterns associated with strongly dipping metamorphic stretching lineations can be described as superposed folding (F1 and F2 folds). The tight and upright F2 folds result from East-West horizontal shortening. The shear zone is defined by homogeneous orientations of steep foliations, sub-horizontal stretching lineations, and kilometre scale strain gradient. Within the shear zone, we observe dominant non-coaxial criteria at various scales that are consistent with a sinistral strike-slip system during D2 deformation stage. Nevertheless, we have also found in the shear zone, geometries typical of a horizontal shortening. Such a strain pattern is characteristic of transpression tectonics.The synkinematic metamorphic conditions are estimated on mafic garnetiferous metabasites. Results show that regional transpression tectonics has developed under very high and constant thermal regime (about 800°C). A pressure gap, of about 3 kbar between the domains separated by the shear zone is identified. This implies tectonic coupling of two different structural levels during tranpressive tectonic.

Invariants of contact structures and transversally oriented foliations

We exhibit new invariants of the contact structure E(α), the contact flow Fα and the transverse symplectic geometry of a contact manifold (M, α). The invariant of contact structures generalizes to transversally oriented foliations. We focus on the particular cases of orientations of smooth manifolds and transverse orientations of foliations. We define the transverse Calabi invariants and determine their kernels.

How useful are ‘millipede’ and other similar porphyroblast microstructures for determining synmetamorphic deformation histories?

ABSTRACT Oppositely concave microfolds (OCMs) in and adjacent to porphyroblasts can be classified into five nongenetic types. Type 1 OCMs are found in sections through porphyroblasts with spiral‐shaped inclusion trails cut parallel to the spiral axes, and commonly show closed foliation loops. Type 2 OCMs, commonly referred to as ‘millipede’ microstructure, are highly symmetrical, the foliation folded into OCMs being approximately perpendicular to the overprinting foliation. Type 3 OCMs are similar to Type 2, but are asymmetrical, the foliation folded into OCMs being variably oblique to the overprinting foliation. Type 4 OCMs are highly asymmetrical, only one foliation is present, and this foliation is parallel to the local shear plane. Type 5 OCMs result from porphyroblast growth over a microfold interference pattern.Types 1 and 2 are commonly interpreted as indicating highly noncoaxial and highly coaxial bulk deformation paths, respectively, during porphyroblast growth. However, theoretically they can form by any deformation path intermediate between bulk coaxial shortening and bulk simple shearing. Given particular initial foliation orientation and timing of porphyroblast growth, Type 3 OCMs can also form during these intermediate deformation paths, and are commonly found in the same rocks as Type 2 OCMs. Type 4 OCMs may indicate highly noncoaxial deformation during porphyroblast growth, but may be difficult to distinguish from Type 3 OCMs. Thus, Types 1–3 (and possibly 4) reflect the finite strain state, giving no information about the rotational component of the deformation(s) responsible for their formation. Furthermore, there is a lack of unequivocal independent evidence for the degree of noncoaxiality of deformation (s) during the growth of porphyroblasts containing OCMs. Type 2 OCMs that occur independently of porphyroblasts or other rigid objects might indicate highly coaxial bulk shortening, but there is a lack of supporting physical or computer modelling.It is possible that microstructures in the matrix around OCMs formed during highly noncoaxial and highly coaxial deformation histories might have specific characteristics that allow them to be distinguished from one another. However, determining degrees of noncoaxiality from rock fabrics is a major, longstanding problem in structural geology.

Kinematics of the Rockland Brook Fault, Nova Scotia: Implications for the interaction of the Meguma and Avalon terranes

The Cobequid-Chedabucto fault system of northern mainland Nova Scotia represents the surface expression of the Avalon-Meguma terrane boundary, but because it is exposed at high crustal levels in the Cobequid Highlands, the fault system provides little information as to the kinematic relationships of the two terranes in this area. In the eastern Cobequid Highlands, the Rockland Brook Fault (RBF) is exposed within the more deeply eroded highlands massif and juxtaposes units of widely varying ages and lithologies. Therefore, this fault is better suited to define the nature and timing of fault movement associated with Avalon-Meguma terrane interaction. In several large Carboniferous plutons along the length of the RBF, and in previously deformed Precambrian rocks, mylonitic foliation orientations are predominantly east-west trending and mineral lineations plunge southeast. Kinematic indicators such as minor fold vergence, porphyroclast systems, asymmetric boudins, shear-band fabrics, and preferred recrystallization orientations indicate dextral shear. These data are taken to infer that the central section of the RBF is dominated by dextral strike-slip motion. Transpression occurs locally where the RBF curves into restraining bends. Kinematic data in these bends indicate top to the northwest thrusting. At the easternmost extent of the RBF, high-level brittle normal faults predominate in the locally extensional environment. The timing of RBF movement is constrained only by the ca 360 Ma granite bodies which it deforms and by the Westphalian sedimentary rocks which are affected by only the latest stages of movement. These kinematic data are consistent with previously published kinematic models for the interaction of the southern margin of the Avalon Composite Terrane with the Meguma Terrane in mainland Nova Scotia. These models suggest that regional dextral shear was accompanied by localized components of transpressional thrusting, wrench tectonism, and small-scale sedimentary basin development during Devonian to Carboniferous terrane interaction.

Strain Transition and Microstructural-Fabric Analysis of Qaurtz-Mylonites Exposed within Pratapnagar Nappe Garhwal Himalaya, India

Abstract The Pratapnagar Thrust sheet in the Lesser Garhwal Himalaya consists of massive, white mylonitized Pratapnagar Quartzite and is delineated at its base by the Pratapnagar Thrust which brings this rock group to rest over the parautochthonous Argillocalc-arenite Group. The estimated three-dimensional finite strain (¯Ss) in the specimens located close to the Pratapnagar Thrust was 0.937 while away from the thrust it was 0.412. The quartzite exposed within the mylonitized zone of Pratapnagar Thrust underwent non-coaxial (= simple shear) deformation revealed by asymmetrical (unpopulated) C-axes girdles and rotation of mylonitic foliation (S3), lineation (Lm) and C-axes girdles, The variable orientation of C-axes girdles, mylonitic foliation, lineation and microstructural shear indicators (asymmetrical porphyroclasts and pressure shadows) noticed within the rock exposed along the thrust zone indicate sinistral (-ve) sense of shear movement (NE-SW) with respect to N. The quartzite away from the thrust were initialy subjected to coaxial (= pure shear) deformation as is indicated by scattered globular and tabular host quartz grains and symmetrical cross and small circle girdle patterns of C-axes. The microstructural transition and asymmetrical orientation of C-axes fabrics indicate that the mylonitized Pratapnagar Quartzite was formed by deformation under ductile flow and low grade metamorphic conditions.

Ductile deformation and the origin of layered gabbro in ophiolites

Layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3–6 km deep. Although large, steady state magma chambers provide a possible explanation for the limited diversity of mid‐ocean ridge basalts, their existence is not supported by geophysical observations. Sinton and Detrick (1992) recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small (1–4 km wide) magma chambers perched on large (∼8 km wide) zones of low‐velocity material, which are interpreted to consist of partially molten crystal cumulates. In this paper, we compare the geology of the gabbroic rocks of the Samail ophiolite, which is thought to have formed by fast spreading, with results of a finite element analysis of the Sinton‐Detrick model for fast spreading centers. Our results demonstrate that flow, similar to that postulated by Sleep [1975], within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the Samail gabbros. As a consequence of this deformation, (1) layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center, (2) the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime, let alone the magma chamber; (3) feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering, but those that invade farther from the ridge will be less deformed and retain crosscutting relationships; and (4) total deviatoric strain should increase down section toward the Moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros. Upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state.

Interference between pluton expansion and non-coaxial tectonic deformation: three-dimensional computer model and field implications

Pluton emplacement contributes to the complexity of structures in orogenic belts because the geometry of these structures is often controlled by interactions between regional tectonic-related and local pluton-related strain fields. Many questions remain about the genesis of these structures. For example: (1) to what extent does pluton-related strain contribute to the formation of these structures? (2) what is the three-dimensional geometry of these structures? and (3) how does this geometry change with progressive deformation? The present simulation models in three dimensions the position, type and kinematic evolution of structures around plutons that expand in a non-coaxial tectonic environment. The simulation generates patterns of four parameters: strain ellipsoid shapes and magnitudes, and orientation of foliation and mineral stretching lineations. Concave, lens-shaped, regions of high strain form on the sides of the pluton, whereas ellipsoidal regions of low strain form at the ends. In addition, foliation, stretching lineations, and constrictional regions form irregular three-dimensional rings around the pluton. Higher tectonic-related strains and strain rates favor rings that migrate towards the pluton, whereas higher expansion-related strains and strain rates favor ring migration away from the pluton. Furthermore, ring migration rates are much slower than tectonic deformation rates or expansion rates. Although analyses of natural strain fields should always be carried out in conjunction with radiometric age determinations, structural and microstructural analysis and strain measurements, the simulated strain patterns offer practical guidelines for field work. Specifically, these patterns can be used to evaluate: (1) the origin, geometry and evolution of structures around plutons; (2) the three-dimensional sense of movement and orientations of ductile shear zones; and (3) the contribution of pluton-related strains to deformation of country rocks in orogenic belts.

Calcite textures: examples from nappes with strain-path partitioning

Strain-path partitioning (partitioning of deformation into components of pure and simple shear) in two Alpine nappes is qualitatively indicated by a variation in orientation of foliation, microkinematic criteria, quartz textures, and a comparison with strain-path partitioning in theoretical nappe models. The degree of non-coaxial deformation relative to the total deformation is quantified by using the asymmetry of textures in calcite tectonites. The texture interpretation is based on both polycrystal plasticity models and experimental data. Calcite tectonites in the Graz-Paleozoic thrust sheet (Austria) and the Morcles nappe (Switzerland) were deformed in a strain path with both pure and simple shear components. Texture asymmetry supports the strain path derived from geological observations. As an exception, a high-strain ultramylonite deformed nearly exclusively in pure shear. It is suggested that this represents a softening instability since deformation in pure shear is energetically more favorable than in simple shear. The incompatibilities caused by heterogeneous pure shear are accommodated by detachments and heterogeneous simple shear adjacent to the pure shear zone.

An experimental steady-state foliation

The history of the intensity and orientation of a grain-shape foliation was investigated in octachloropropane deformed in simple shear at 80% of its absolute melting temperature and a shear strain-rate of 4 × 10−5S−1. Foliation orientation, developed from the beginning of the deformation, remains steady throughout the deformation. Foliation intensity becomes steady after a bulk shear strain of about 0.9 has been accumulated. The steadiness of foliation orientation and intensity is achieved by some balance between foliation-strengthening and foliation-weakening processes. The main foliation- -strengthening process is intragranular plastic deformation. Foliation-weakening processes include dynamic recrystallization by migration of straight or or slightly wavy grain boundaries, grain dissection, rotational recrystallization, grain amalgamation, relative rigidity of hard grains and grain boundary sliding. A small but definite angle, although not constant, is observed between the foliation and the long axis of the bulk total strain ellipse. The average aspect ratio of grains is lower than that of the bulk total strain ellipse by a factor of about 0.4 at total shear strain of 1.3. The average grain size also stabilizes, becoming steady from a shear strain of 0.5 onwards. A steady grain-shape foliation may be a possible paleostress or a paleostrain-rate indicator, but cannot be a paleostrain indicator.

Criteria helpful in recognizing shear-zone and diapiric mélanges: Examples from the Hoh accretionary complex, Olympic Peninsula, Washington

A study of the middle Miocene Hoh melanges exposed on the western Olympic Peninsula, Washington, provides criteria for differentiating between shear-zone and diapiric melanges. The shear-zone melange is characterized by a consistent foliation orientation throughout the outcrop, intense pervasive scaly foliation, uniform fold vergence directions, exotic clasts, predominantly phacoidal clasts, and consistent clast long-axis lineations. The diapiric melange is characterized by radial foliation in map view, well-developed scaly foliation at the contacts and poorly developed scaly foliation in the center of the melange, opposite fold vergence directions across the melange, and rare exotic clasts. Additionally, clasts vary in shape from phacoidal at the contacts to angular in the center. In the diapiric melange, large clasts are found only in the center of the melange, and clast long axes vary from tightly clustered at the contacts to near-random orientations in the center. The systematic variation in clast size in the diapir may be due to increased shear at the margins and/or clast migration to the center of the diapir caused by a velocity-viscosity contrast. Interpretation of field data indicates that some clasts rotate continuously in the matrix, yet clast long axes have a definite preferred orientation. This contradiction may be resolved by theoretical models indicating that clasts rotate fastest at high angles to the shear plane. Thus, although clasts above a critical aspect ratio asymptotically approach stability parallel to the shear plane, the long axes of rotating clasts spend a proportionally greater amount of time close to the shear plane. Parasitic folds in the matrix indicate that clast long axes align parallel to transport direction. Small-scale folds in the matrix, clast long-axis lineations, and the mean foliation indicate that the Hogsback section of the Hoh melange was emplaced by a high-angle oblique thrust directed to N10°E.

Strain and magnetic fabric in the Santa Catalina and Pinaleno Mountains Metamorphic Core Complex Mylonite Zones, Arizona

Anisotropy of magnetic susceptibility (AMS) is capable of recording finite strain in weakly magnetized rocks. AMS was measured for 228 samples from 20 sites in two mylonite zones with the same deformational history. AMS measurements were compared with finite strains determined from dike rotations and from foliation orientations. In one zone (the Santa Catalina Mountains) the orientations of susceptibility and finite strain ellipsoids are in excellent agreement, and there is a logarithmic relationship between susceptibility difference and finite strain magnitude. In the second zone (the Pinaleno Mountains) minimum susceptibility is perpendicular to the finite flattening plane, but the maximum susceptibility does not parallel the maximum extension direction, and there is no systematic relationship between susceptibility magnitude and strain magnitude. Oriented polished thin sections indicate that magnetite in the protolith of the Santa Catalina mylonite occurs as randomly oriented, elongate grains. With subsequent deformation, the long axes are rotated into the maximum extension direction. In the Pinaleno mylonites, both equant and elongate magnetite grains are present. With deformation, the elongate magnetite grains are rotated into the maximum flattening plane but show no preferred orientation within this plane. AMS in the two mylonite zones appears to be predominantly controlled by the orientation of elongate magnetite grains with respect to the megascopic fabric. The final orientation of the elongate grains is a function of their initial orientation as well as the finite strain. Therefore, despite similar deformational histories, the two zones display different AMS patterns due to the differences in occurrence, initial orientation, and shape of ferromagnetic grains.

The effect of foliation orientation on the inferred rotation axes and rotation angles of rotated porphyroblasts

The rotation axes and rotation angles inferred from the intersection of inclusion fabrics in rotated porphyroblasts and the external foliation are apparent axes and apparent angles whose orientation and magnitude are strongly influenced by the initial orientation of the foliation with respect to the geometry of the deformation. Spherical porphyroblasts may show apparent angles of rotation consistently smaller than the corresponding true rotation angles and apparent rotation axes oriented at a distinct angle with the true axis of rotation. For nonspherical porphyroblasts this effect may lead to a scatter of apparent rotation axes. Sections of a rock intersecting the foliation through the resulting sector of apparent axes will show apparent forward and apparent backward rotations, even when the foliation remains stationary throughout the deformation and all porphyroblasts rotate with the same sense of true rotation. This effect holds the key to one possible explanation of apparent reversals observed in some paracrystalline rotated porphyroblasts.

A statistical method to evaluate homogeneity of structural populations

For any specific rock unit in a discontinuous rock mass, the determination of structural domain boundaries is important because geologic and hydrologic properties vary from one domain to another. A domain can be considered as a structural population characterized by a distinct pattern of structure orientations (i.e., fracture, bedding, or foliation orientations).Boundaries are usually determined by comparing samples of structural data, each of which consists of a Schmidt plot that commonly displays at least 150 poles to fractures. When fracture orientations appear dispersed and random on the plots, visual comparisons are not sufficient to determine whether the samples were obtained from the same structural domain. An appropriate x2test has been adapted to this situation and is capable of evaluating the homogeneity of structural populations. The method uses a contingency table analysis based on the frequencies of fracture poles that occur in corresponding patches on the Schmidt plots being compared.

The geometry of folds in granitoid rocks of northeastern Alberta

Granitoid rocks which predominate in the Precambrian shield of northeastern Alberta show large-scale fold structures. A numerical procedure has been used to obtain modal foliation orientations. This procedure results in the smoothing of folded surfaces that show roughness on a detailed scale. Statistical tests are used to divide the study areas into cylindrical domains. Structural sections can be obtained for each domain, and horizontal and vertical sections are used to construct block diagrams. The projections are performed numerically and plotted by computer. This method permits blocks to be viewed from every possible angle. Both perspective and orthographic projections can be produced. The geometries of a dome in the Tulip Lake area and a synform in the Hooker Lake area have been obtained. The domal structure is compared with polyphase deformational interference patterns and with experimental diapiric structures obtained in a centrifuge system. The synform in the Hooker Lake area may be genetically related to the doming in the Tulip Lake area.

Experimentally developed folds in a material with a plannar mineral fabric

The experimental development of folds in a foliated wax/mica aggregate having an anisotropy value (( N 1/ Q 1) ∗ ∗ The coefficients N 1, and Q 1 are, respectively, the maximum and minimum principal values of the compressive stiffness of the material. of between 5 and 10 is compared with a theoretical model of finite fold development in statistically homogeneous anisotropic continua described by Cobbold (1976). A pure shear deformation with the compression parallel to the foliation of the model material resulted in the formation of a train of four folds which became chevron in form. The average foliation orientation during the development of these folds for the most part was in agreement with the theoretical predictions for chevron folds in a material with N 1/ Q 1 between 5 and ∞. Some of the mechanical effects of anisotropy predicted by the theoretical model were clearly demonstrated experimentally such as rapid fold amplification and the development of angular profiles at low and intermediate limb-dips.

Bubble Pattern of a Glacier near Tukche Peak in Hidden Valley, Mukut Himal

Depth of moraine, orientation of bubble elongation and bubble foliation were measured in the monsoon season, 1974 at about 100 points on the lower half of a glacier (lat. 28°47' N, long. 83°34' E, altitude about 5300 m) near Tukche Peak, Hidden Valley. Depth of moraine is larger near the terminus and near the margin of the glacier. Bare ice was exposed on the eastern side of the upper part of the glacier. Bubble foliation in the glacier is a three-dimensional layered structure with individual layers from 1 to 150 cm thick. Folia may be identified by the difference of bubble concentration : bubbly and clear. No discernible difference in the size of ice crystals was found between the layers. The pattern of the foliation is in the form of nested arcs convex down-glacier with the curvature of the arcs being smaller near the terminus. The direction of bubble elongation is almost orthogonal to the direction of bubble foliation. The elongated direction does not seem to coincide with flow direction. Circular bubbles were found near the terminus and at the part of the glacier where bare ice was exposed.

Effect of anisotropy on the coefficient of sliding friction in Schistose rocks

Foliated schist samples were sawcut at 45° to th cylinder axis and tested at 500 bars confining pressure, 25°C, and 10 −4sec −1 shortening rate, to examine the effect of anisotropy on the coefficient of sliding friction (μ). Cylinders were cored from sample blocks in a variety of orientations to produce sawcut-to-foliation (STF) angles ranging from 0° to 180° in 15° increments. The variation of μ with foliation orientation is sinusoidal. Maximum values of μ occur when the foliation is parallel to principal stress planes ( STF = 30°−45° and 105°−120°; τ = 0), and minimum values of μ correspond to planes of maximum resolved shear stress ( STF = 75° and 165°, τ = τ). Sliding-surface damage and gouge development vary inversely with μ. Surface damage results from the interaction of asperities with the sawcut surface, and failure and cataclasis of asperities produces gouge. Precursor events visible on force-time records, and partial loading tests terminated before megascopic sliding, indicate that surface damage and gouge are generated before megascopic sliding on the sawcut. Apparently, high resolved shear stress produces slip along the foliation prior to sliding on the sawcut. Foliation slip generates gouge at the sawcut interface; and the effect of the gouge is to lower μ.