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Computer animations ('movies') showing progressive serial scctioning in three mutually orthogonal directions of the 3-D image in Fig. lO(a). In these movies the plagioclase porphyroblasts are green, and the five inclusion-foliation surfaces are variably coloured depending on their orientations. Numbers along the margins indicate true dimensions (mm). These movies can be accessed
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ABSTRACT Seventy-five spatially orientated, serial thin sections cut from a single rock containing ‘millipede’ porphyroblast microstructure from the Robertson River Metamorphics, Australia, reveal the three-dimensional (3-D) geometry of oppositely concave microfolds (OCMs) that define the microstructure. Electronic animations showing progressive se...
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Debabrata Chattopadhyay is Adjunct Professor at the University of Queensland, Brisbane, Australia. He currently works at the World Bank Energy and Extractives Practice. He has previously worked for Deloitte Australia and Charles River Associates.
This paper investigated the textural character of surface sediments across a range of inset-flood plain surfaces on the Barwon Darling River, Australia. Surface sediments ranged in size from clay to coarse sand (-1Φ) - <4.75Φ) but varied in composition between different inset-flood plain surfaces. Multivariate entropy analysis detected five sedimen...
A new assemblage of bradoriids is reported from the Giles Creek Dolostone (Cambrian Series 3, Stage 5; Templetonian) in the Ross River Gorge area, NE Amadeus Basin, central Australia. One new genus and four new species are described: Annge iperte gen. et sp. nov., Flemingopsis anteriospinata sp. nov., Monasterium ferox sp. nov. and Mongolitubulus t...
http://web.natur.cuni.cz/hydroeco2011/download/presentations/poster/A/45_sessionA_PAPER%28not_poster%29_D_Berhane_W_Vervoort_P_Serov_Thermograph_analysis_etc.pdf
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... In these situations bedding/S 1 lies in the shortening field during D 3 . Thus, the layering/foliation embedded by the conjugate shears drags and shortens to form oppositely concave microfolds (Johnson and Moore, 1996) that resemble the typical millipede structures described by Bell and Rubenach (1980) and Bell (1981), such in the example shown in Fig. 5. The angle 2q between dextral and sinistral shears is bisected by the regional shortening axis and is always larger than 100 , even for incipient lozenges. ...
Tectonic lozenges are elongate bodies bounded by relatively more deformed rocks. The focus of this study is on the 2-D structure of tectonic lozenges developed during ductile shear in rocks with a pre-existing mechanical anisotropy. On the basis of a detailed analysis of shear zones in foliated rocks from the Cap de Creus area (Variscan of the eastern Pyrenees), five mechanisms to explain the development of different types of lozenges in foliated rocks are suggested. These mechanisms are explained on the basis of the orientation of the previous foliation relative to the bulk shearing direction. It is shown that the prevailing mechanism does not majorly depend on the bulk kinematics but on the angular relationship between the pre-existing foliation and the bulk kinematic axes, and on shear zone interaction. This has implications on the use of lozenge shapes in tectonic interpretations. The fact that there is a wide range of initial orientations, propagation modes and coalescence types implies that the final lozenge geometry is not univocally related neither to the type of strain nor to the kinematic regime.
... A hand sample containing millipede microstructure , better described as oppositely concave microfolds (OCMs), was serially thin sectioned at ~1.5 mm intervals and described in Johnson and Moore (1996). The sample contained the hinge and one limb of a small fold (Johnson and Moore, 1996, their fi gure 8). ...
... A hand sample containing millipede microstructure , better described as oppositely concave microfolds (OCMs), was serially thin sectioned at ~1.5 mm intervals and described in Johnson and Moore (1996). The sample contained the hinge and one limb of a small fold (Johnson and Moore, 1996, their fi gure 8). We prepared seven thin-section blocks from this sample and 75 serial sections were made from these blocks. ...
... All porphyroblasts examined are composed of a core in which inclusion trails are invariably straight, surrounded by a narrow rim in which inclusion trails curve into the matrix, forming the characteristic OCMs (Fig. 1). Of critical importance, the porphyroblast inclusion trails are continuous with matrix S 2 (Fig. 1), which allowed us (Johnson and Moore, 1996) to reconstruct the OCM geometry in three dimensions. We (Johnson and Williams, 1998) developed a new strain tool by comparing the spacing between S 2 surfaces inside and outside of the porphyroblasts, calculating an elongation of 172% during the development of S 3 . ...
This contribution shows unequivocally that porphyroblasts rotate relative to one another during ductile deformation. The porphyroblasts described here have special significance because they are from the original "millipede" rocks that led to the nonrotation hypothesis. Thus, the debate that has lasted for more than 20 years is settled. Despite this finding, porphyroblast microstructures continue to provide important evidence for deformation and metamorphic histories. Although porphyroblasts clearly rotate relative to one another during ductile deformation, there are several factors that contribute to relatively minor rotation in many instances, including (1) low strain during and after porphyroblast growth in comparison, for example, to mylonitic shear zones; (2) small axial ratios combined with relatively low internal vorticity during growth and post-growth deformation; and (3) strain localization at the porphyroblast-matrix interface. Thus, given the right circumstances, porphyroblasts may preserve the approximate orientations of deformation fabrics present at the time of their growth, but each case must be individually assessed.
... grains but becomes curved towards the outermost grain edges where it becomes continuous with the external foliation ( Figure 8b). In some plagioclase porphyroblasts the internal foliation on opposite sides is curved in opposite directions forming a millipede-type microtexture (Bell and Rubenach 1980;Johnson and Moore 1996;Passchier and Trouw 1996). Grt 3 (Figure 8a) commonly occurs as 100 mm diameter polygonal grains with rational contacts with other mineral phases. ...
Phengite-bearing schists of the northern Adula Nappe experienced a polymetamorphic and polycyclic evolution that was associated with five deformation episodes. Evidence of a pre-Alpine metamorphic event is preserved within garnet cores of some amphibole-bearing schists. The D1 and D2 deformation episodes are recorded by S1 and S2 foliations preserved only within metre-scale domains of low-D3 strain. S1 is a relict foliation. Blueschist-facies conditions at 565 ± 10°C and 11.5 ± 1.5 kbar were attained during D2 and were associated with the development of isoclinal folding and an S2 foliation. The D3 episode took place at 665 ± 50°C and 11.5 ± 2.1 kbar and was responsible for the development of a transpositive S3 foliation. The D4 episode took place at T < 550 ± 10°C and was associated with the development of a discrete S4 foliation and S-C structures. The D5 episode is recorded by sub-vertical metre-scale open folds or centimetre-scale kinks.
The structural and metamorphic evolution described here indicates that the northern and central parts of the Adula Nappe were distinct continental crustal fragments and were brought together under amphibolite-facies conditions. Copyright
... Johnson and Moore (1996) used the computer program Mathematica Ò to produce a three-dimensional representation of a millipede porphyroblast microstructure from one of our samples of Robertson River Metamorphics, Australia collected from the same location as that shown in Figs. 5ce11. ...
... The 12 traced sections (from Fig. 9 in Johnson and Moore, 1996) were digitized and imported into 3ds Max, where they were scaled and positioned using a vertical spacing of 1.5 mm (Fig. 12a). Once in position, the foliation and porphyroblast surfaces were reconstructed by stretching ''U-loft'' surfaces between the traced curves. ...
... Reconstruction of millipede structure afterJohnson and Moore (1996). (a) Digitized and scaled tracings of 12 thin sections positioned with a spacing of 1.5 mm. ...
The progressive development and migration of patterns of deformation partitioning at all scales through the rock matrix commonly destroys any record of the ductile history associated with previous events making the problem of similar structures developing through multiple pathways generally intractable. However, records of the small-scale geometries that form as deformation commences and begins to partition through a rock are routinely trapped and protected by porphyroblasts because these large crystals nucleate and/or grow at this time. This allows examination of the geometry of microstructures formed at the start of deformation partitioning that were destroyed by the same event in the matrix, or which formed during an event prior to any preserved in the matrix. Porphyroblasts locally preserve oppositely concave microfolds (``millipedes''), which, in all examples that we have found, exclusively indicate a deformation history of bulk inhomogeneous shortening. Very similar structures have been formed experimentally during inhomogeneous simple shear but can readily be distinguished from those trapped in porphyroblasts that form during progressive bulk inhomogeneous shortening. Oppositely concave microfolds in some porphyroblasts reveal that deformation near orthogonal to a previously developed foliation occurred by axial plane shear driven rotation that led to rapid reactivational ``card-deck-like'' collapse of the pre-existing foliation. Differentiated crenulation cleavages may result from the same process providing yet another reason for the cessation of porphyroblast growth at the start of differentiation.
... The internal geometry of individual large porphyroblasts can be resolved in great detail by combining 2-D images of a series of closely spaced thin-sections in a 3-D reconstruction using various image-analysis techniques (Schoneveld, 1979;Johnson, 1993;Johnson and Moore, 1996;Jung et al., 1999). Another promising imaging technique recently applied to metamorphic textures (after earlier use in the medical-and materials sciences) is high-resolution computed X-ray tomography (Denison et al., 1997). ...
The ‘FitPitch’ computer program allows characterisation of preferred orientation of planar microstructures in a rock from the orientations of their intersection-lines on different sections. Measured pitch or strike angles of such intersection-lines are fitted to the theoretical intersection-lines of a single model-plane, or a combination of two or three model-planes. The degree of fit is quantified in terms of the deviation between natural and theoretical intersection-lines, normalised by uniform data. A first application is described concerning porphyroblast inclusion trails whose orientation in 28 samples from the Variscan Iberian Massif was measured. Analysis of these data illustrates how the method can be used to quantify preferred orientation of relict foliations contained in a group of porphyroblasts and how this complements existing techniques for determining foliation intersection axes (FIA) in porphyroblasts. A second described application deals with previously published orientation data for microfracture trace lines in quartz grains of a Variscan granite in the Rhine Graben. Qualitatively established best-fit planes derived originally from this data are compared with the numerically established planes using the FitPitch program, and discussed in terms of the advantages and potential limitations of the method.
... Inclusion trails are a typical feature of many natural synkinematic porphyroblasts (Fig. 1d ), which commonly record minute details of the deformation tory. Consequently, the study of porphyroblast systems has been in vogue over several decades (Rast, 1958; Zwart, 1960; Spry, 1969; Rosenfeld, 1970; Schoneveld, 1977; Powell and Vernon, 1979; Bell and Rubenach, 1986; Bell, 1985; Bell and Johnson, 1989; Passchier et al., 1992; Johnson and Bell, 1996; Johnson and Moore, 1996 ). All the studies on the development of inclusion trails in porphyroblasts are based on two distinctly different models: rotating porphyroblast model and non-rotating porphyroblast model. ...
Rocks that are mechanically heterogeneous due to the presence of stiff or rigid inclusions floating in a ductile matrix, commonly show a variety of micro- to macro-scale structures developing under the influence of heterogeneous flow field in the neighbourhood of the inclusions. It is of fundamental importance to apprehend the nature of strain heterogeneity around inclusions to understand progressive development of structures associated with rigid inclusions such as strain shadow, foliation drag, porphyroclast mantle, porphyroblast inclusion trails, intragranular fractures, etc. The development of these diverse types of structures can be analyzed with the help of a suitable hydrodynamic theory. In this paper, we review different continuum models that have been proposed to characterize the heterogeneous flow field around rigid inclusions, focusing on recent developments. Recent studies reveal that Jeffery's [Proc. R. Soc. Lond. A 120 (1922) 161.] theory dealing with the motion of ellipsoidal rigid bodies in an infinitely extended viscous medium is more general in nature, and applicable for modeling the heterogeneous flow around both equant and inequant shapes of inclusions and ideal or non-ideal shear deformation of the matrix. The application of this theory, therefore, has advantages over other models, based on Lamb's [Lamb, H., 1932. Hydrodynamics. Cambridge University Press, Cambridge.] theory dealing with spherical inclusions. The review finally illustrates numerical simulations based on hydrodynamic theories, highlighting the controls of physical and kinematic factors on the progressive development of the structures mentioned above.
... Inclusion trails are a typical feature of many natural synkinematic porphyroblasts, which frequently record minute details of deformation history. Consequently, the study of porphyroblast systems has been in vogue over several decades (Rast, 1958;Zwart, 1960;Spry, 1969;Rosenfeld, 1970;Schoneveld, 1977;Powell and Vernon, 1979;Bell and Rubenach, 1980;Bell, 1985;Bell and Johnson, 1989;Passchier et al., 1992;Passchier and Speck, 1994;Johnson and Bell, 1996;Johnson and Moore, 1996). ...
The paper presents two-dimensional numerical simulations of inclusion trail patterns of spherical and non-spherical synkinematic porphyroblasts rotating and growing over a pre-existing, passive foliation during a single progressive deformation taking into account the effects of heterogeneous strain field around the rigid porphyroblast. The analysis is based on the velocity functions around a rigid object hosted in a Newtonian viscous medium. Using Visual Basic software, the trail patterns have been simulated in a PC assuming that the porphyroblast grows by increments of constant radius, over a pre-existing foliation, which simultaneously gets distorted due to heterogeneous strain field around the porphyroblast and included within the porphyroblast. The numerical simulations show that the trail patterns in spherical porphyroblasts are controlled by two factors: (1) the initial orientation of the foliation marker relative to the shear direction (θ) and (2) the ratio of pure shear and simple shear rates in the bulk deformation (Sr). Complex trail structures form when θ<0° and Sr⩽0.75. The ratio of growth rates along axial directions of non-spherical porphyroblasts is an additional factor that controls the trail patterns.
... Bell and Rubenach (1980) described oppositely concave microfolds in and around plagioclase porphyroblasts, and their discovery played a fundamental role in the development of a popular model for how deformation partitions in deforming rocks (Bell, 1981). Because this microstructure has played such an important role in deformation partitioning models, and because it represents a fold geometry that is particularly difficult to visualise in three dimensions, we reconstructed an example from the now classic rocks originally described by Bell and Rubenach (1980) and re-evaluated by Johnson and Moore (1996). ...
... By shifting the plane, animations were created that allow the full geometry to be examined. In Johnson and Moore (1996), animations are presented for slicings perpendicular to the X-, Yand Z-axis of finite strain. Fig. 9 is a single frame from one of these movies (available at http://www.es.mq. ...
... edu.au/jmg/1996/1996.html#ji). Johnson and Moore (1996). View at right shows section parallel to X2Z plane of finite strain, in which straight layers within central plagioclase crystal can be seen, with oppositely concave microfolds around it; it shows just upper half, viewed from below, of full reconstruction at left. ...
In this paper we provide the following three examples of how the software system Mathematica can be used to reconstruct or model the three-dimensional shapes of folded surfaces. (1) First, we revisit the reconstruction of the central inclusion surface within a garnet porphyroblast that contains spiral-shaped inclusion trails. (2) Next, we revisit the reconstruction of five foliation surfaces that define oppositely concave folds within and surrounding a plagioclase porphyroblast. (3) For the main part of this paper we model superposed folds, and the many interference patterns that can be found in two-dimensional sections through these folds. Because this special issue is accompanied by a compact disk, we have included a series of reconstructions, models and animations to illustrate these three examples. Our reconstructions and models have, in some instances, provided important constraints on the interpretations of complex or controversial microstructures, and in all instances have provided useful teaching aids.
... cubic voxels. Serial thin sectioning (Johnson and Moore, 1996; Johnson, 1993) and computed X-ray tomography (CT) (Denison et al., 1997 ) creates a voxelized representation that can be used for various kinds of visualization such as perspective shaded views of 3D objects, plane through cuts, and block cuts (Marschallinger, 1996Marschallinger, , 1998), or at the largest scales of mantle imagery (Kennett et al., 1998; Megnin et al., 1997). However, additional numerical procedures are required to obtain quantitative information about individual objects such as volume (number of voxels), surface area (Coker and Torquato, 1995), interface surface area, interconnectivity, coordination number, object shape (aspect ratio), orientation in 3D, or vector translation of objects (Cohenor and Kaufman, 1995). ...
3D voxelized images can be manipulated if their component parts can be identified, cataloged, and measured. To accomplish this, it is necessary to separate individual convex objects from the complex structures that result from digital observation techniques such as X-ray tomography. Toward this end, we have developed schemes that peel away sequential layers of voxels from complex structures until narrow waists that connect individual objects disappear, and each component object can be identified. These peeling schemes provide the most uniform possible cumulative thickness of removed layers regardless of the orientation of the voxel grid pattern. Consequently, they lead to the most accurate application regarding inter-object interfaces, medial axis analysis, and individual object statistics such as volumes, orientations and interconnectivity. Peeling schemes can be categorized by the number of steps involved in each peeling iteration. Each step removes voxels according to three possible criteria for defining the exterior of a voxel: exposed faces, edges, or corners. Each of these ultimately causes an initial sphere, for example, to evolve into a cube, dodecahedron, or octahedron, respectively. Combinations of steps can be used to create more complex polyhedra (tetrahexadra, trisoctahedra, trapezohedra, and hexoctahedra). The resulting polyhedron that most closely resembles a starting sphere depends on the appropriate definition of “sphericity”. Using a metric based on the standard deviation of the polyhedral surface from that of a concentric sphere of equal volume, the optimal scheme is peeling by faces 7 times, by edges 3 times, and by corners 4 times. This leads to a hexoctahedron with Miller indices (14 7 4) and a standard deviation of 0.025. Using a metric based on minimizing surface area, the optimal scheme is peeling by faces 9 times, by edges 6 times, and by corners 5 times, leading to a hexoctahedron with Miller indices (20 11 5). In the past, only 1-step peeling has been used (by faces or corners). If computational or conceptual constraints limit peeling to 1-step, the criterion of edges should be used, as the dodecahedron that results deviates from a sphere by only half the amount of either the cube or octahedron resulting from 1-step peeling of faces or corners, respectively. We also determined the best criteria for 2-step and 3-step peeling. The peeling schemes we identify can be used to separate objects from complex structures for application to a number of geological and other problems. Information that emerges from the analysis includes object volumes, which can be used for determining grain- or bubble-size distributions in volcanologic, petrologic, and sedimentary applications, among others.
... Similar structures are known to develop around rotating rigid objects at low strains in laboratory experiments (Ghosh, 1975(Ghosh, , 1977Ghosh and Ramberg, 1976). Johnson and Moore (1996) and Johnson and Bell (1996) have stated that the presence of millipedes indicates a state of low strain during their genesis. Since the microfolds that make up the millipedes within the biotite are open compared with those in the matrix, the biotite porphyroblast is interpreted to have grown under a low strain state during D 2 . ...
The southern margin of the Aravalli Mountain Belt (AMB) is known to have undergone polyphase deformation during the Mesoproterozoic. The Lunavada Group of rocks, which is an important constituent of the southern parts of AMB, reveals three episodes of deformation; D1, D2 and D3. In this paper, interpretations based on petrographic studies of schists and quartzites of the region are presented and the relationship between metamorphic and deformational events is discussed. It is established that from north to south, there is a marked zonation from chlorite to garnet–biotite schists. Metamorphism (M1) accompanied D1 and was progressive. M2-1 metamorphism associated with major part of D2 was also progressive. However, M2-2 that synchronized with the waning phases of D2 and early-D3 deformation was retrogressive. Porphyroblast–matrix relationships in the garnet–biotite schists of the region have been useful in establishing these facts. The metamorphic rocks studied were intruded by Godhra Granite during the late-D3/post-D3 event. The heat supplied by this granite resulted in static recrystallization and formation of annealing microstructures in rocks close to the granite. It is established that Grain Boundary Migration Recrystallization associated with dislocation creep and Grain Boundary Area Reduction were the two deformation mechanisms dominant in rocks lying far and close from the Godhra Granite, respectively.
