The Experts below are selected from a list of 4350 Experts worldwide ranked by ideXlab platform

Scott E. Johnson - One of the best experts on this subject based on the ideXlab platform.

  • The effect of microstructural and rheological heterogeneity on Porphyroblast kinematics and bulk strength in Porphyroblastic schists
    Tectonophysics, 2013
    Co-Authors: Ben M. Frieman, Christopher Gerbi, Scott E. Johnson
    Abstract:

    Abstract The kinematic record and bulk viscous strength of polyphase rocks depend in part upon the relative strengths and distributions of rheologically distinct fabric elements. Here, we explore the effects of microstructural and rheological heterogeneity in Porphyroblastic schists. Electron backscatter diffraction and petrographic analyses reveal asymmetric microboudinage of staurolite, indicating relative rotation of staurolite Porphyroblasts synchronous with bulk non-coaxial strain. Boudinage and relative rotation both require Porphyroblast–matrix shear coupling. Based on 2D optical observations, the extent of the coupling appears related to the initial and boudinaged staurolite grain shape and orientation as well as the geometry of heterogeneities such as mica domains or shear bands. We designed 2D finite element numerical models to assess the role of microstructural variation and rheological heterogeneity on the degree of Porphyroblast–matrix shear coupling and bulk viscous strength. Model results indicate that the bulk strength of a three-phase system comprising inclusion, weak domain, and matrix is sensitive to the relative proximity of weak and strong domains, particularly at high viscosity contrasts (i.e. ηmatrix/ηweak > 10). The threshold for bulk weakening below the matrix strength occurs over a narrow range of weak domain viscosities (ηmatrix/ηweak = 2.6–5.5), regardless of the relative abundance and spatial distribution of weak domains. Kinematic decoupling of Porphyroblasts occurs at low viscosity contrasts when weak domains are proximal (ηmatrix/ηweak = 2–5), but for all other spatial distributions and modal abundances investigated, kinematic decoupling occurs at viscosity contrasts of ηmatrix/ηweak = 15–20. These data indicate that bulk weakening due to rheological heterogeneity is not necessarily coincident with kinematic decoupling.

  • The effects of Porphyroblast growth on the effective viscosity of metapelitic rocks: implications for the strength of the middle crust
    Journal of Metamorphic Geology, 2006
    Co-Authors: W. G. Groome, Scott E. Johnson, Peter O. Koons
    Abstract:

    Numerical models are used to examine the effects of Porphyroblast growth on the rheology of compositionally layered rocks (metapelites and metapsammites) and by extension the middle crust during prograde metamorphism. As Porphyroblast abundance increases during prograde metamorphism, metapelitic layers will strengthen relative to Porphyroblast-free metapelitic units, and potentially relative to quartzofeldspathic metapsammitic units. As metapelitic layers become stronger, the integrated strength of compositionally layered successions increases, potentially causing large volumes of mid- crustal rock to strengthen, altering the strain-rate distribution in the middle crust and affecting the geodynamic evolution of an orogenic belt. The growth of effectively rigid Porphyroblasts creates strength heterogeneities in the layer undergoing Porphyroblast growth, which leads to complex strain-rate distributions within the layer. At the orogen scale, the strengthening of large crustal volumes (on the order of thousands of cubic kilometres) changes the strain-rate distribution, which may change exhumation rates of high-grade metamorphic rocks, the geothermal structure and the topography of the orogen. The presence of a strong zone in the middle crust causes strain-rate partitioning around the zone, suppressed uplift rates within and above the zone and leads to the development of a basin on the surface.

  • How useful are ‘millipede’ and other similar Porphyroblast microstructures for determining synmetamorphic deformation histories?
    Journal of Metamorphic Geology, 2004
    Co-Authors: Scott E. Johnson, T. H. Bell
    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 asymmctrical, 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 intermcdiatc 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, long- standing problem in structural geology.

  • Porphyroblast microstructures; a review of current and future trends
    American Mineralogist, 1999
    Co-Authors: Scott E. Johnson
    Abstract:

    Many recent papers show how Porphyroblast microstructures play an important role in a wide range of structural and metamorphic studies. This paper reviews ten current applications of these microstructures: (1) Porphyroblast growth-timing criteria; (2) tracking progressive foliation development relative to changing metamorphic conditions; (3) timing of pluton emplacement relative to deformation and metamorphism; (4) finite longitudinal strain determinations; (5) kinematics and Porphyroblast rotation; (6) use of linear fabrics preserved in Porphyroblasts; (7) Porphyroblasts and folding mechanisms; (8) inclusion-trail orientations and orogenic processes; (9) inferring shear-strain rates from Porphyroblast growth rates; and (10) in-situ age determinations. Although there is still no concensus on the interpretation of some Porphyroblast microstructures, a bright future lies ahead as traditional and newly developed techniques of microstructural analysis are combined with modern chemical and microprobe techniques to provide an increased understanding of the relationships between deformation and metamorphism in a wide range of metamorphic settings.

  • Determining finite longitudinal strains from oppositely-concave microfolds in and around Porphyroblasts: a new quantitative method
    Journal of Structural Geology, 1998
    Co-Authors: Scott E. Johnson, Michael L. Williams
    Abstract:

    Abstract This paper describes a precise new method for determining finite longitudinal strains in Porphyroblastic metamorphic rocks, which makes use of oppositely-concave microfolds (OCMs) formed by heterogeneous strain of the matrix around Porphyroblasts. The initial spacing between two foliation surfaces is measured inside a Porphyroblast and compared to the spacing between the same two surfaces in the matrix, which results in a measure of extension ( e ) experienced by the rock during and/or after Porphyroblast nucleation. A natural example is provided by the well-known ‘millipede’ plagioclase Porphyroblasts from the Robertson River Metamorphics in Queensland, Australia. Twenty-four measurements were made from 22 serial thin sections cut parallel to both the X - Z and X - Y planes of finite strain, giving an average extension of 1.72 parallel to the X -direction of finite strain. The least-squares best-fit line to a plot of initial length vs change in length gives an R 2 value of 0.998. A minimum estimate of maximum shortening (negative e ) was also made by measuring the total lengths of S 1 folia that had been crenulated during OCM formation, giving a value of −0.54, which falls short of the −0.63 expected for constant-volume, plane-strain deformation. Because the OCM method is particularly suited to metapelites, results may provide new insight into mechanisms of folding and crenulation cleavage development, pressure-temperature-time-deformation histories, mass transport during deformation and metamorphism, and kinematic studies of Porphyroblast behavior (rotation vs non-rotation) during ductile deformation.

Ron H. Vernon - One of the best experts on this subject based on the ideXlab platform.

  • Inclusion trail patterns in Porphyroblasts from the Foothills Terrane, California: A record of orogenesis or local strain heterogeneity?
    Journal of Metamorphic Geology, 2001
    Co-Authors: Scott R. Paterson, Ron H. Vernon
    Abstract:

    A major problem with the current use of Porphyroblast–matrix microstructural relationships to infer orogenic histories, such as multiple orthogonal orogenic events, is that other evidence for these events is typically lacking. For example, a comparison of regional relationships and local structures formed in and adjacent to Porphyroblasts present in contact aureoles in the Foothills Terrane, Sierra Nevada, California, shows that: (1) except in shear zones, contact aureoles and local zones along lithological contacts, the Foothills Terrane has a single regional cleavage, although locally formed by multiple processes; (2) the regional cleavage and locally developed Porphyroblast inclusion trails have variable orientations, and neither dataset supports the formation of dominantly subhorizontal and subvertical cleavages in this orogen; (3) structural and metamorphic heterogeneities occur at all scales and can markedly affect inclusion trail patterns in Porphyroblasts; (4) complex Porphyroblast growth features and internal inclusion trail patterns can form in Porphyroblasts that grow during short time intervals in contact aureoles, indicating that local complexity in Porphyroblasts does not imply regional complexity. Because of these conclusions, multiple datasets, rather than data acquired only from Porphyroblasts, should be considered when attempting to understand the evolution of orogens. Furthermore, using microstructural information preserved only in Porphyroblasts to infer orogenic processes and plate motions is generally unjustified.

  • Stepping stones and pitfalls in the determination of an anticlockwise P‐T‐t‐deformation path: the low‐P, high‐T Cooma Complex, Australia
    Journal of Metamorphic Geology, 1995
    Co-Authors: Scott E. Johnson, Ron H. Vernon
    Abstract:

    Low-pressure/high-temperature (low-P/high-T) metamorphic rocks of the Cooma Complex, southeastern Australia, show evidence of an anticlockwise pressure-temperature-time-deformation (P-T-t-D) path, similar to those of some other low-P/high-T metamorphic areas of Australia. Prograde paths are reasonably well constrained in cordierite-andalusite schists, cordierite-K-feldspar gneisses and andalusite-K-feldspar gneisses. These paths are inferred to be convex to the temperature axis, involving increase in pressure with increase in temperature. Evidence of the retrograde path is inconclusive, but is consistent with approximately isobaric cooling, as are available isotopic data on the Cooma Granodiorite, which indicate initially rapid cooling following attainment of peak temperatures. The retrograde path is inconsistent with either a clockwise P-T-t-D path involving rapid or even moderate decompression immediately post-dating the peak of metamorphism, or a path in which the retrograde component simply reverses the prograde component, because both these paths should cross reactions forming cordierite from aluminosilicate, for which no evidence has been observed. Determination of the deformational-metamorphic history of the complex is not straightfoward and depends on careful examination of critical samples. Evidence necessary for successful elucidation of the prograde, and part of the retrograde, deformational-metamorphic history in the Cooma Complex includes: (1) sequentially grown Porphyroblasts that can be timed relative to surrounding foliations; (2) partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development; (3) a tectonic marker foliation (S4 at Cooma) that allows correlation of foliations from one location to another; and (4) single samples containing all of the foliations and all generations of Porphyroblast growth within a single metamorphic zone. The latest two or three foliations involve low strain accumulation, allowing relative timing relationships between foliations and Porphyroblasts to be more clearly determined. Sequential Porphyroblast growth and foliation development in the cordierite-andalusite schists is examined for situations involving rotation and non-rotation of Porphyroblasts relative to geographically fixed coordinates. Although the number of foliations developed varies in the rotational situation, depending on the deformation history proposed, the sequential order of Porphyroblast growths does not differ from the non-rotational situation. Thus, whether or not Porphyroblasts rotated in the Cooma rocks, the sequence of reactions, and therefore P-T-t paths inferred from the relative timing of Porphyroblast growths, remain the same, for the deformational histories evaluated.

  • Inferring the timing of Porphyroblast growth in the absence of continuity between inclusion trails and matrix foliations: can it be reliably done?
    Journal of Structural Geology, 1995
    Co-Authors: Scott E. Johnson, Ron H. Vernon
    Abstract:

    Abstract The timing of Porphyroblast inclusion trails can be confidently interpreted relative to surrounding external foliations only where there is continuity between the two. Where this continuity is broken, timing is ambiguous. Where single or multiple growths of two or more different Porphyroblastic minerals have occurred during a relatively complex deformation history, the risk of misinterpreting the relative timing of Porphyroblast growth is high, and can lead to wrong inferences about pressure-temperature-time-deformation ( P-T-t-d ) paths. Misinterpreting Porphyroblast timing can also have considerable consequences for determining rates of fabric evolution relative to changes in metamorphic conditions. The effects of Porphyroblast rotation vs non-rotation (relative to an externally fixed reference frame) on inferred P-T-t-d paths are poorly understood. However, the ‘ d ’ part of the path can differ considerably, depending on whether or not Porphyroblasts are inferred to have rotated. The effect on the P-T-t part of the path depends on what effect inferences about Porphyroblast rotation have on the inferred sequence of Porphyroblast growth.

  • Growth and deformation of Porphyroblasts in the Foothills terrane, central Sierra Nevada, California: negotiating a microstructural minefield
    Journal of Metamorphic Geology, 1993
    Co-Authors: Ron H. Vernon, Scott R. Paterson, D. Foster
    Abstract:

    The main Porphyroblastic minerals in schists and phyllites of the Foothills terrane, Western Metamorphic Belt, central Sierra Nevada, California, are cordierite and andalusite (mostly chiastolite). Less commonly, biotite, muscovite, chlorite, garnet or staurolite are also present as Porphyroblasts. The variety of Porphyroblast and matrix microstructures in these rocks makes them suitable for testing three modern hypotheses on growth and deformation of Porphyroblasts: (1) Porphyroblast growth is always syndeformational; (2) Porphyroblasts nucleate only in low-strain, largely coaxially deformed, quartz-rich (Q) domains of a crenulation foliation and are dissolved in active high-strain, non-coaxially deformed, mica-rich (M) domains, the spacing between which limits the size of the Porphyroblasts; and (3) Porphyroblasts generally do not rotate, with respect to geographical coordinates, during deformation, provided they do not deform internally, so that they may be used as reliable indicators of the orientation of former regional structural surfaces, even on the scale of orogenic belts. Some Porphyroblast–matrix relationships in the Foothills terrane are inconsistent with hypotheses 1 and 2, and others are equivocal. For example, in many rocks it cannot be determined whether the Porphyroblasts grew where the strain had already been partitioned into M and Q domains, whether the Porphyroblasts caused this partitioning, or both. Although most Porphyroblasts appear to be syndeformational, as predicted by hypothesis 1, observations that do not support the general application of hypotheses 1 and 2 to rocks of the Foothills terrane include: (a) lack of residual crenulations in many strain-shadows and alternative explanations where they are present; (b) absence of Porphyroblasts smaller than the distance between nearest mica-rich domains; (c) nucleation of crenulations on existing Porphyroblasts, rather than nucleation of Porphyroblasts between existing crenulations; (d) presence of micaceous ‘arcs’in an undifferentiated matrix against some Porphyroblasts, suggesting static growth; (e) absence of crenulations in Porphyroblastic rocks showing sedimentary bedding; and (f) Porphyroblasts with very small, random inclusions, which are probably pre-deformational. Similarly, Porphyroblasts that have overgrown sets of crenulations and Porphyroblasts with micaceous ‘arcs’are probably post-deformational, at least on the scale of a large thin section and probably over much larger areas, judging from mesoscopic structural evidence. Some Porphyroblasts in rocks of the Foothills terrane do not appear to have rotated, with respect to geographical coordinates, during matrix deformation, in accordance with hypothesis 3, at least on the scale of a large thin section. However, other Porphyroblasts evidently have rotated. In some instances, this appears to be due to mutual interference, but many apparently rotational Porphyroblasts are too far apart to have interfered with each other, which indicates that the rotation was associated with deformation of the matrix. The occurrence of planar bedding surfaces adjacent to Porphyroblasts about which bedding and/or foliation surfaces are folded suggests rotation of the Porphyroblasts during non-coaxial flow parallel to bedding, rather than crenulation of the matrix foliation around static Porphyroblasts. It appears that Porphyroblasts may rotate during deformation if the matrix is relatively homogeneous, so that the strain is effectively non-coaxial. This may occur after homogenization of a matrix in response to the strongest degree of crenulation folding, whereas the same Porphyroblasts may have been inhibited from rotating previously, when strain accumulation was partitioned in the matrix.

T. H. Bell - One of the best experts on this subject based on the ideXlab platform.

  • The control of deformation partitioning and strain localization on Porphyroblast behaviour in rocks and experiments
    Geosciences Journal, 2017
    Co-Authors: T. H. Bell, I. V. Sanislav, Jyotindra Sapkota
    Abstract:

    Multiple generations of sub-vertical and sub-horizontal foliations preserved as inclusion trails in garnet in mylonitic rocks from the hanging wall of the Main Central Thrust in the Himalayas indicate that these Porphyroblasts did not rotate during thrusting. This result is predicated by (i) a consistent succession of 5 changes in FIA trend (foliation inflection/intersection axes in Porphyroblasts) for samples where the orientation changes from Porphyroblast cores to rims; (ii) sub-vertical and sub-horizontal foliations occur as inclusion trails around each of the 5 FIAs in the succession, which would not be the case if the garnet Porphyroblasts rotated during subsequent phases of deformation as tectonism continued; (iii) a change in inclusion trail asymmetry immediately prior to the commencement of mylonitzation indicates top to the south thrusting only if the Porphyroblasts had not rotated as they grew; (iv) the latter asymmetry matches truncated crenulation relics preserved within the mylonitic matrix foliation that indicate top to the south thrusting as the latter foliation formed. Partitioning of deformation into shortening and shearing components stops rotation of Porphyroblasts during their growth and during following periods of ductile tectonism. This can be replicated via computer modelling by duplicating the crenulation-hinge-like coaxial environment in which Porphyroblasts nucleate and grow before the strain intensifies. This was done using Drucker-Prager constitutive models with temperature-dependent strain softening behaviour and resulted in no Porphyroblast rotation when followed by non-coaxial deformation no matter how intense. Furthermore, strain localization in the model containing competent objects of variable size, shape and orientation, produced no rotation during deformation involving components of shortening and shearing. These approaches to modelling mechanically resolve the sub-vertical/sub-horizontal foliations defined by inclusion trails and consistent FIA trend successions obtained from the Main Central Thrust rocks as well as in orogens elsewhere.

  • Ninety million years of orogenesis, 250 million years of quiescence and further orogenesis with no change in PT: Significance for the role of deformation in Porphyroblast growth
    Journal of Earth System Science, 2012
    Co-Authors: A A Shah, T. H. Bell
    Abstract:

    In situ dating of monazite grains preserved as inclusions within foliations defining FIAs (foliation inflection/intersection axes preserved within Porphyroblasts) contained within garnet, staurolite, andalusite and cordierite Porphyroblasts provides a chronology of ages that matches the FIA succession for the Big Thompson region of the northern Colorado Rocky Mountains. FIA sets 1, 2 and 3 trending NE–SW, E–W and SE–NW were formed at 1760.5 ± 9.7, 1719.7 ± 6.4 and 1674 ± 11 Ma, respectively. For three samples where garnet first grew during just one of each of these FIAs, the intersection of Ca, Mg, and Fe isopleths in their cores indicate that these rocks never got above 4 kbars throughout the Colorado Orogeny. Furthermore, they remained around approximately the same depth for ~250 million years to the onset of the younger Berthoud Orogeny at 1415 ± 16 Ma when the pressure decreased slightly as Porphyroblasts formed with inclusion trails preserving FIA set 4 trending NNE–SSW. No Porphyroblast growth occurred during the intervening ~250 million years of quiescence, even though the PT did not change over this period. This confirms microstructural evidence gathered over the past 25 years that crenulation deformation at the scale of a Porphyroblast is required for reactions to re-initiate and enable further growth.

  • Porphyroblast rotation versus nonrotation: conflict resolution
    Geology, 2008
    Co-Authors: C. Fay, T. H. Bell, Bruce E. Hobbs
    Abstract:

    Data on foliation intersection and/or inflection axes preserved in Porphyroblasts (FIAs) indicate that no Porphyroblast rotation occurs during ductile deformation relative to spatial coordinates. This contrasts with 99% of investigations of "rigid" objects in non-coaxially deforming media where the objects rotate. When anastomosing shear zone formation around relatively strong objects in a weaker matrix is modeled, no "Porphyroblast" rotation occurs. Formation of these anastomosing zones controls the development of this phenomenon, called gyrostasis. If such zones are absent, Porphyroblasts rotate. In weak materials the gyrostatic situation arises because the superposition of simple shearing deformation normal to initial coaxial shortening results in only small rotations of principal axes of stress. Since shear zones are controlled by the orientations of principal axes of stress, initial anastomosing zones retain their orientations and positions during subsequent non-coaxial deformation. The Porphyroblast is isolated from the embedding non-coaxially deforming matrix, but this material close to the Porphyroblast continues to deform coaxially; no local rotation occurs. This has major tectonic significance because, allowing for the effects of rotation due to brittle deformation, Porphyroblasts can now be routinely used to access lengthy structural and/or metamorphic histories destroyed in the matrix by reactivation such as movement directions, shear senses, and extended pressure-temperature-time paths.

  • progressive deformation partitioning and deformation history evidence from millipede structures
    Journal of Structural Geology, 2007
    Co-Authors: T. H. Bell, M.d. Bruce
    Abstract:

    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 inclusion trail geometries preserved within a first phase of Porphyroblast growth
    Journal of Structural Geology, 2006
    Co-Authors: T. H. Bell, M.d. Bruce
    Abstract:

    The inclusion trail geometry within a first phase of Porphyroblast growth can differ significantly from that preserved by further enlargement because the Porphyroblast forms a rigid mass up against which the rock preferentially strains during ensuing events. The geometry of the first overgrown inclusion trails is affected by their primary orientation, including any pre-existing curvature, combined with any heterogeneous rotation of this foliation about the developing stretching lineation. This can impact the apparent timing of foliation intersection/inflection axes preserved within Porphyroblasts (FIAs) that nucleated during the development of a sub-horizontal foliation, but is readily resolved. 3-D computer analysis of sigmoidal inclusion trails reveals that the asymmetry method for FIA determinations is unaffected by the cut location relative to the Porphyroblast core. Significantly, perfect spiral inclusion trail geometries can be produced from a sigmoidal shape in cuts up 30° away from the FIA. Therefore, since FIAs in most Porphyroblasts bear no relation to matrix structures, there is a 17% chance that thin-sections cut relative to the foliation lie within 30° of a FIA and could contain such an apparent spiral. FIAs maintain consistent trends for the first phase of Porphyroblast growth accompanying horizontal bulk shortening but may vary in plunge. FIAs have sub-horizontal plunges for Porphyroblasts nucleating during gravitational collapse, but may vary in trend. For all periods of Porphyroblast regrowth the data available indicates that FIAs remain consistently trending and sub-horizontal until the relative direction of plate motion causing orogenesis changes.

Roger L. Gibson - One of the best experts on this subject based on the ideXlab platform.

  • sequential syndeformational Porphyroblast growth during hercynian low pressure high temperature metamorphism in the canigou massif pyrenees
    Journal of Metamorphic Geology, 1992
    Co-Authors: Roger L. Gibson
    Abstract:

    The sequence of growth of garnet, staurolite and aluminosilicate in Fe-rich metapelitic rocks from the Canigou massif, Pyrenees, is established using evidence of inclusion, reaction and pseudomorphing textures between the different minerals, compositional zoning patterns in garnet and staurolite (that can be related to the KFMASH reaction grid), and the geometric relations between inclusion trails in the Porphyroblasts and the matrix microstructures. The evidence indicates that garnet and staurolite commenced growth before aluminosilicate in all cases, even where all three are in textural equilibrium. Interpretation of the reaction textures between the Porphyroblasts and of the compositional zoning in garnet and staurolite in terms of the KFMASH reaction grid indicates the importance of continuous reactions in the development of these phases. Some garnet and staurolite Porphyroblasts underwent renewed growth during breakdown, producing rims enriched in Mn and Zn respectively. The presence of aluminosilicate in these assemblages (i.e. the absence of a clear andalusite-absent zone in the field) is attributed to a strong pressure-dependence for the aluminosilicate-producing reactions. Porphyroblast-matrix microstructural relations indicate that Hercynian metamorphism in the massif was synchronous with the development of the regional subhorizontal foliation (S3).

  • Sequential, syndeformational Porphyroblast growth during Hercynian low-pressure/high-temperature metamorphism in the Canigou massif, Pyrenees
    Journal of Metamorphic Geology, 1992
    Co-Authors: Roger L. Gibson
    Abstract:

    The sequence of growth of garnet, staurolite and aluminosilicate in Fe-rich metapelitic rocks from the Canigou massif, Pyrenees, is established using evidence of inclusion, reaction and pseudomorphing textures between the different minerals, compositional zoning patterns in garnet and staurolite (that can be related to the KFMASH reaction grid), and the geometric relations between inclusion trails in the Porphyroblasts and the matrix microstructures. The evidence indicates that garnet and staurolite commenced growth before aluminosilicate in all cases, even where all three are in textural equilibrium. Interpretation of the reaction textures between the Porphyroblasts and of the compositional zoning in garnet and staurolite in terms of the KFMASH reaction grid indicates the importance of continuous reactions in the development of these phases. Some garnet and staurolite Porphyroblasts underwent renewed growth during breakdown, producing rims enriched in Mn and Zn respectively. The presence of aluminosilicate in these assemblages (i.e. the absence of a clear andalusite-absent zone in the field) is attributed to a strong pressure-dependence for the aluminosilicate-producing reactions. Porphyroblast-matrix microstructural relations indicate that Hercynian metamorphism in the massif was synchronous with the development of the regional subhorizontal foliation (S3).

Paul F. Williams - One of the best experts on this subject based on the ideXlab platform.

  • Reference frame, angular momentum, and Porphyroblast rotation
    Journal of Structural Geology, 2004
    Co-Authors: Dazhi Jiang, Paul F. Williams
    Abstract:

    Abstract Rotation of small rigid objects in a deforming ductile matrix can produce two different types of microstructure: a shape fabric due to alignment of the principal axes of a population of elongate objects and the inclusion trail microstructure preserved in syntectonic Porphyroblasts. We use numerical modeling to show that inclusion trails of elongate Porphyroblasts are expected to be extremely complex. In contrast, snowball garnets are readily interpretable. But misuse of reference frame and kinematic misconceptions have obfuscated the discussion on the formation of Porphyroblast inclusion trails in general and snowball garnet inclusion trails in particular. We clarify this point. Models for snowball garnet formation that are based on the notion of garnets being irrotational with respect to the earth can be rejected on a geometrical and kinematic basis. Further, the notion that rigid objects embedded in a deforming ductile matrix generally do not rotate is unsound—it violates the fundamental physical law of balance of angular momentum.

  • A new spin on ‘non-rotating’ Porphyroblasts: implications of cleavage refraction and reference frames
    Journal of Structural Geology, 2001
    Co-Authors: Jürgen Kraus, Paul F. Williams
    Abstract:

    Abstract It has been claimed that rigid Porphyroblasts which grow before or during folding and concurrent cleavage development do not rotate with respect to the geographical reference frame (GRF), even if the straining is non-coaxial ( Bell 1985; Bell and Johnson 1990 ). The explanation offered is based on strain partitioning. It is argued that the initial orientations of early fabrics included as internal foliations (S i ) in the Porphyroblasts have been preserved after polyphase deformation, and even after successive orogenies. According to the strain partitioning model, the Porphyroblasts are fixed in domains of coaxial straining (microlithons) and are isolated from the non-coaxial straining associated with the enveloping septa (S e ). This hypothesis, and also its discussions both pro and contra, suffer from insufficient attention to reference frames. We therefore attempt to demonstrate: (a) the need for rigorous treatment of reference frames in geological interpretations; (b) that, in a folding situation, grains that do not rotate with respect to their immediate matrix generally rotate with respect to the GRF; (c) that lack of Porphyroblast rotation with respect to the GRF demands a rare folding mechanism (slip fold model); and (d) that the non-rotation hypothesis is in conflict with heterogeneous deformation (cleavage refraction). Finally, we question the validity of the evidence in a study by Fyson (1980) , cited in support of non-rotation with respect to the GRF during folding. Fyson reported orientations of S i that are constant, after folding, over a large area; this scenario is a product of selective data acquisition. In summary, our investigation shows that the lack of Porphyroblast rotation with respect to a GRF during folding, while possible, is not universal. The development of microstructures (e.g. curved S i ) is only related to the local deformation path, the characterisation of which does not rely on the GRF.

  • Abstract: The role of conjugate crenulation cleavage and stepwise Porphyroblast growth in the development of “Millipede” microstructures
    1998
    Co-Authors: Chris J. Beaumont-smith, Paul F. Williams
    Abstract:

    This paper describes the results of a microstructural study into the development of 'Millipede' microstructures found in Archean metatwbiditic rocks sunoundingthe Back Rivervolcanic complex, eastern Slave structural province. Millipede microstructures are locally developed within andalusite Porphyroblasts which developed late in the deformation history of the study area in response to the intrusion oflate kinematic granitoids. The mechanism for the development of the millipedes involves the stepwise Porphyroblast growth with andalusite growth occurring after initial D 3 buckle folding. These initial andalusite Porphyroblasts represent a rheological heterogeneity resulting in the development of conjugate crenulation cleavage along the Porphyroblast margin. Subsequent Porphyroblast growth included the crenulation cleavage with the inclusion trail within _the Porphyroblasts defined by the trace of the quartz-rich crenulation cleavage microlithon. The classical reversal in inclusion trail vergence is the result of the Porphyroblasts overgrowing conjugate pairs formed along the Porphyroblast margin.

  • relationships between foliation development Porphyroblast growth and large scale folding in a metaturbidite suite snow lake canada
    Journal of Structural Geology, 1998
    Co-Authors: Jürgen Kraus, Paul F. Williams
    Abstract:

    AhstractXomplex relationships exist between cleavage development, metamorphism and large-scale folding in the well-bedded, polydeformed, staurolite-grade metaturbidite of the Burntwood Suite, internal Paleoproterozoic Trans-Hudson Orogen at Snow Lake, Manitoba, Canada. It is demonstrated: (a) that cleavage in anisotropic pelitic rock develops whenever microfolding is possible and that, commonly, initiation of a cleavage, which is pervasive on the scale of a fold, predates folding; (b) how a new axial planar fabric can develop on one fold limb of a symmetrical fold and not on the other; and (c) how two cleavages of different generations can be present in adjacent beds. It is further shown that Porphyroblasts rotate with respect to geographical coordinates during folding. Finally, dissolution of cleavage septa is suggested here as an alternative mechanism for the generation of schistosity. The Burntwood Suite is exposed on the dismembered limb of a macroscopic, isoclinal F2 structure and preserves a domainal cleavage (Sz), which locally grades into a schistosity. S, developed from crenulation of a generally bedding-parallel S, cleavage that is axial planar to F, isoclinal folds formed at 1.84 Ga. Porphyroblast growth coincided with crenulation of St early during F2 folding at 1.815-1.8 Ga. Early stages of & development are recorded by inclusion trails (SJ in the Porphyroblasts. During Fz flexural-flow folding, variations in magnitude of bedding-parallel shear in lithologies of different competency resulted in a strong S, refraction and thus heterogeneous strains between beds. Independent of shear magnitude and resulting &J/S, angle, S, and S2 remained sub-orthogonal everywhere, and thus Porphyroblasts and the enveloping S, rotated by equal amounts with respect to Ss. As the different magnitudes of Porphyroblast rotation in different beds could not be exactly balanced by the counteracting rotation of the fold limbs (same magnitude for all beds) during fold tightening, most Porphyroblasts also rotated with respect to geographical coordinates. S2 was crenulated prior to F3 large-scale folding, where favourably oriented. F3 crenulations were tightened on the eastern F3 limb and unfolded by sinistral layer-parallel shear on the western limb, where F2 and F3 layer-parallel shears were of opposite and the same sense, respectively. As a result, the initial developmental stages of an Ss are developed only on the eastern F3 limb, and there only in incompetent layers, whereas Sz is preserved in the competent layers, On the western limb, S2 is preserved and appears axial planar to the F3 structure. The S, domainal fabric was locally transformed into a schistosity by dissolution of the septa during widespread fluid activity, which endured until syn- or post-Fa. 5c 1998 Elsevier Science Ltd