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

Transcription

1 1. nietxnorphic Ceol., 1996, 14, How useful are millipede and other similar porphyroblast microstructures for determining synmetamorphic deformation histories? S. E. JOHNSON AND T. H. BELL School of Earth Scienc-es, Macquarie University, Sydney, New South Wales, , Australia Department of Earth Sciences, James Cook University, Townsville, Queensland, , Australia 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, longstanding problem in structural geology. Key words: crenulation clcavage; dcformation history; millipede microstructure; oppositely concave microfolds; porphyroblast growth; porphyroblast rotation; spiral-shaped inclusion trails; strain states. INTRODUCTION A characteristic of some porphyroblast microstructures is that inclusion trails and/or their continuation as matrix foliations form microfolds that are outwardly concave in opposite directions (e.g. Fig. 1). Such oppositely concave microfolds (OCMs) have been identified in and adjacent to several different types of porphyroblasts from various orogenic settings (e.g. Rosenfeld. 1970; Schoneveld, 1979; Bell & Rubcnach, 1980; Bateman, 1985; Bell et al., 1986; Bell & Johnson, 1989; Vernon, 1988, 1989; Reinhardt & Rubenach, 1989; Hayward, 1992; Johnson, 1993a,b; Vernon et al., 1993; Passchier & Speck, 1994; Aerden, 1995), and have generated considerable discussion among metamorphic and structural geologists (e.g. Bell, 1981; Bell et id., ; Vernon, 1989; Passchier et al., 1992; Gray & Busa, 1994; Johnson & Moore, 1996). This paper examines the formation of OCMs within and adjacent to porphyroblasts, classifying them into five nongenetic types. Two of these types (1 & 2) are of particular interest because they regularly appear in the literature. Type 1 OCMs are found in sections parallel to spiral axes through porphyroblasts with spiral-shaped inclusion trails (e.g. Powell & Treagus, 1967; Rosenfeld, 1970; Schoneveld, 1979; Johnson, 1993a,b; Gray & Busa, 1994). They are commonly considered to form during single deformation events that are highly noncoaxial (e.g. Schoneveld, 1979; Brunel, 1986; Gray & Busa, 1994), and have played a fundamental role in developing specific models of porphyroblast growth during rotation relative to an 15

2 16 S. E. JOHNSON & T. H. BELL Given the use of OCMs as indicators for specific deformation histories and their role in developing porphyroblast growth and rotation models, they warrant further investigation. Thus, the aims of this paper are to: (1) describe and identify distinguishing characteristics of each of the five OCM types; (2) review proposed mechanisms of their formation; and (3) evaluate the extent to which they can constrain deformation histories. Fig. 1. Sketch of oppositely concave microfolds (OCMs) in and adjacent to a porphyroblast (stippled). Individual S1 foliation surfaces can be followed from the matrix, through the porphyroblast and out into the matrix on the other side. S1 on the top and bottom margins of the porphyroblast form symmetrical (Type 2) OCMs. The OCMs formed during the development of S2 as a result of heterogeneous extension parallel to S2. external reference frame (e.g. Rosenfeld, 1970; Schoneveld, 1979; Bell, 1985; Masuda & Mochizuki, 1989; Gray & Busa, 1994). However, equally valid alternative interpretations have been considered, involving porphyroblast growth, with or without rotation relative to an external reference frame, during successive overprinting foliation-forming events that can vary widely in degree of noncoaxiality (e.g. Ramsay, 1962; Bell & Johnson, 1989; Hayward, 1992; Johnson, a). Type 2 OCMs were referred to as millipede microstructure by Bell & Rubenach (1980), and described in three dimensions by Johnson & Moore ( 1996). They are characterized by symmetrical OCMs either side of the porphyroblast, and near-orthogonality between the folded and overprinting foliations (Fig. 1). Type 2 OCMs have played an important part in developing specific models of deformation partitioning and porphyroblast growth in regional metamorphic rocks, and are commonly interpreted as indicators of bulk heterogeneous shortening that is locally nearcoaxial (e.g. Bell & Rubenach, 1980; Bell, 1981; Bell et a/., 1986, 1992; Bell & Johnson, 1989). Recent critical discussions of millipede OCMs (e.g. Passchier et al., 1992; Johnson, 1993a; Gray & Busa, 1994) have emphasized that theoretically they can form in a range of deformation histories. THE FIVE OCM TYPES Below are descriptions and distinguishing characteristics of five types of OCMs that can occur in and adjacent to porphyroblasts. A new classification scheme is proposed because the term millipede insufficiently describes the five distinctly different OCM types, and is commonly used genetically, implying formation of OCMs by bulk coaxial shortening (e.g. Bell & Rubenach, 1980; Bell. 1981; Bell rt al., 1992). The limitations of this term are demonstrated by recent critical discussions by Passchier et al. ( 1992), Johnson (1993a) and Gray & Busa (1994), and the introduction of ambiguous terms such as millipede-like structure or asymmetrical equivalents of millipede structure to describe OCMs that clearly differ from those originally described by Bell & Rubenach (1980). A nongenetic classification is proposed because most or all of the five OCM types described here reflect only the strain state, and theoretically can form in a wide range of deformation histories (discussed below). To describe the five OCM types, a geometrical reference frame is required. In the following descriptions, a block diagram is used to show each type relative to typical structural fabric elements, including foliations, fold axes and mineral elongation lineations. Numbers used to identify particular foliation and fold generations (e.g. S1, S2, F2) are for convenience only, and will vary depending on the structural history of an area. Each OCM type has what we call an ideal section, in which it can be most clearly seen and identified. OCMs within and adjacent to porphyroblasts can generally be seen in other sections besides the ideal one, as a cut effect or resulting from heterogeneous strain (discussed below). An assumption in drawing the block diagrams is that the most recent structural fabrics shown (e.g. S2 and F2 in Type 2 OCMs) formed synchronously with the respective OCMs, except for Type 5, in which the porphyroblast post-dates both fold generations. If OCMs were preserved in the porphyroblasts during a deformation pre-dating these fabrics, the orientations of these fabrics would not necessarily be useful in determining the type of OCMs or the best orientation in which to observe them. In this situation, threedimensional thin sectioning (e.g. Johnson, 1993b, fig. 1) might be required to evaluate the OCMs, but the lack of synchronous fabrics in the surrounding rock might

3 'MILLIPEDE' AND OTHER SIMILAR PORPHYROBLAST MICROSTRUCTURES 17 make it difficult to determine the OCM type and to evaluate the deformation in which they formed. Type 1 : closed-loop OCMs Reference frame Figure 2 shows the ideal relationships between Type 1 OCMs, porphyroblast inclusion trails, one matrix foliation and a mineral elongation lineation. The lineation may or may not be present; so it is dashed. Total inclusion-trail curvature for the three porphyroblast intersections is approximately 160' (from core to rim), and the inclusion trails are continuous with the matrix foliation. Ideally, the axis of relative rotation should lie in the plane of the external foliation, perpendicular to the mineral elongation lineation. Under these circumstances, the best section for observing Type 1 OCMs is perpendicular to the matrix foliation and mineral elongation lineation (left-front face of Fig. 2). OCMs can also be seen in the plane parallel to the matrix foliation (top face of Fig. 2). However, the section perpendicular to the foliation is the most useful for distinguishing Type 1 OCMs from other types, because it can contain fully closed loops after only 90- of total inclusion-trail curvature. Distinguishing characteristics 1 The foliation-inclusion surfaces commonly form closed loops. Whether or not these loops close fully within a porphyroblast depends on the section orientation and amount of total inclusion-trail curvature (Fig. 3). In sections parallel to the left-front face of Fig. 2, closed loops should be seen within porphyroblasts after 90' of total inclusion trail curvature. In sections parallel to the top face of Fig. 2, closed loops should be seen within porphyroblasts after 180" of curvature. Closed loops outside porphyroblasts should not be observed in the left-front face of Fig. 2, whereas they should continue to be seen in sections parallel to the external foliation, regardless of the amount of inclusion-trail curvature. 2 In the section parallel to the spiral axis, nearest the centre of the porphyroblast (Fig. 3b), the closed loops show approximately mirror-image symmetry across the spiral axis. Sections either side of this central section (Fig. 3a,c) show asymmetrical closed loops, those on one side of the spiral axis being larger and better developed than those on the other. The side of the spiral axis on which the closed loops are larger and better developed changes from one side of the central section to the other (Fig. 3a-c). 3 In the region of closed loops, included foliation surfaces intersect the thin section at a very low angle. In porphyroblasts that are sufficiently large, elongate inclusions in the region of closed loops may define a mineral elongation lineation that formed prior to being overgrown by the porphyroblast (Fig. 3d). 4 Inclusion trails seen in sections perpendicular to the external foliation are concave (or closed) on the sides of the porphyroblast that contact the surrounding foliation, rather than the sides that lie against the strain or pressure shadows (Fig. 2). In sections parallel to the foliation, the OCMs are concave in a direction orthogonal to that seen in the section perpendicular to the foliation. Type 2: symmetrical OCMs Reference frame Figure 4 shows the ideal relationship between Type 2 OCMs, porphyroblast inclusion trails and two matrix foliations. The inclusion trails are S1, which continue outside the porphyroblast as a matrix foliation. The OCMs and S2 crenulation cleavage formed simultaneously, and S1 and S2 are approximately perpendicular. The ideal section for observing the OCMs is perpendicular to S 1, S2 and F2 microfold axes. because it provides a profile view of the F2 crenulations and the OCMs in central porphyroblast sections. Because the folds that define the OCMs within and adjacent to the porphyroblast are noncylindrical, OCMs can also be seen in sections parallel to S2, namely the top face of Fig. 4 (Johnson & Moore, 1996). Fig. 2. Block diagram showing the relationships between Type 1 OCMs, porphyroblast inclusion trails, one foliation and a mineral elongation lineation. The lineation is dashed because it may or may not be present. The total inclusion-trail curvature for each of the three porphyroblast intersections shown is approximately 160". Closed loops will appear inside the porphyroblast in the top-face section only after total inclusion-trail curvature exceeds 180'. Distinguishing characteristics 1 Type 2 OCMs are commonly symmetrical from section to section through a porphyroblast (Johnson & Moore, 1996), in contrast, for example, to Type 1 (Fig. 3). Minor asymmetry may occur, owing to variations in crystal shape and orientation of inclusion trails relative to the foliation. 2 The overprinting foliation lies at a high angle to the foliation being folded into OCMs (Fig. 4).

4 18 S. E. J O H N S O N & T. H. B E L L

5 MILLIPEDE AND OTHER SIMILAR PORPHYROBLAST MICROSTRUCTURES 19 Fig. 4. Block diagram showing the relationships between Type 2 OCMs, porphyroblast inclusion trails, two foliations and F2 fold axes. 3 In the ideal section (left-front face of Fig. 4), the OCMs are concave parallel to S2, and perpendicular to S1 and F2. In sections parallel to the syn-ocm foliation (top face of Fig. 4), they are concave in the same direction, perpendicular to S1 and F2. This relationship does not hold for Type 1 OCMs (compare Figs 2 & 4). Type 3: asymmetrical OCMs Reference frame Figure 5 shows the ideal relationship between Type 3 OCMs, porphyroblast inclusion trails, two matrix foliations and a mineral elongation lineation. The lineation may or may not be present; so it is dashed. The inclusion trails are S1, and S1 and S2 intersect at an oblique angle. The ideal section for observing the OCMs is perpendicular to S1, S2 and F2 microfold axes, as it provides a profile view of the OCMs in central porphyroblast sections. Because the folds that define the OCMs within and adjacent to the porphyroblast are noncylindrical, OCMs can also be seen in sections parallel to S2 (top face of Fig. 5). Fig. 5. Block diagram showing the relationships between Type 3 OCMs, porphyroblast inclusion trails, two foliations and a mineral elongation lineation. The lineation is dashed because it may or may not be present. If formed during bulk simple shearing, the top face of the diagram is parallel to the shear plane, and the shear sense is shown by arrows. perpendicular to S1 and F2. In sections parallel to the syn-ocm foliation (top face of Fig. 5), they are concave in the same direction, perpendicular to S1 and F2. This relationship holds for Type 2 OCMs, but not for Type 1 (compare Figs 2 & 5). Type 4: single-foliation OCMs Reference frame Figure 6 shows the ideal relationship between Type 4 OCMs, porphyroblast inclusion trails, one matrix foliation and a mineral elongation lineation. The inclusion trails and external foliation are continuous, and the OCMs form during rotation of the porphyroblast with respect to the foliation and the shear plane Distinguishing characteristics 1 Type 3 OCMs are consistently asymmetrical from section to section through a crystal. 2 The overprinting foliation lies at an oblique angle to the foliation being folded into OCMs (Fig. 5). 3 The crenulated foliation on two opposite margins of the porphyroblasts generally double back via a single fold (Fig. 5). 4 In the ideal section (left-front face of Fig. 5), the OCMs are generally concave parallel to S2, and Fig. 6. Block diagram showing the relationships between Type 4 OCMs, porphyroblast inclusion trails, one foliation and a mineral elongation lineation. The top face of the diagram is parallel to the shear plane, as is the foliation, and the shear sense is shown by arrows. Fig. 3. (a)-(c) Type I OCMs are illustrated by a set of three serial thin sections cut subparallel to the spiral axis through a single garnet porphyroblast with spiral-shaped inclusion trails. The section in (b) is approximately through the centre of the porphyroblast, and the closed-loop regions either side of the porphyroblasts are approximately the same size. The sections in (a) and (c) are approximately 1.25 mm either side of the central section in (b), and the closed-loop regions are not the same size in either of these sections. (d) Close-up of closed-loop area in (c) showing a mineral elongation lineation where the foliation trapped as inclusion trails is subparallel to the thin-section surface. Plane polarized light, long axis of (a)-(c) 17 mm, long axis of (d) 9 mm.

6 20 S. E. JOHNSON & T. H. BELL (fixed to the external reference frame). The ideal section for observing the OCMs is perpendicular to the foliation and parallel to the mineral elongation lineation (left-front face of Fig. 6), because it is likely to provide a profile view of OCMs in central porphyroblast sections. OCMs would probably occur in the top face of Fig. 6, as in Types 1-3, but their details are unknown and so we have not depicted them. Distinguishing characteristics 1 Only one foliation is present in the matrix, and it is continuous with the porphyroblast inclusion trails. 2 In the later stages of Type 4 OCM development, foliations on two opposite margins of the porphyroblasts always double back via a single fold that dies out at some distance from the porphyroblast. These folds occur on different margins of the porphyroblast and have opposite asymmetries for a given shear sense to those that develop as part of Type 3 OCMs (compare Figs 5 & 6). 3 Inclusion trails can be concave parallel or perpendicular to the shear plane, or anywhere between these positions, depending on the amount of porphyroblast rotation. This may be one way to distinguish Type 4 from Type 3, although porphyroblast rotation during formation of Type 3 may alter these relationships. Type 5: fold-interference OCMs Reference frame Figure 7 shows the ideal relationship between Type 5 OCMs, porphyroblast inclusion trails and two fold generations with orthogonal axial planes. The block represents a section through a porphyroblast. Thus, the folded surface is contained within the porphyroblast, and its intersection with the top of the block is what would be seen as inclusion trails in thin section. The ideal section for observing Type 5 OCMs is perpendicular to the axial planes of both F1 and F2 folds. Distinguishing characteristics 1 Type 5 OCMs should have an area in the centre of the pattern corresponding to a microfold culmination (Fig. 8), where the included foliation is near-parallel to the thin section. As well, there should be four microfolds that are outwardly concave (Fig. 8a,b). The two opposing outwardly concave microfolds that correspond to the overgrown doubly plunging microfold are likely to be larger and tighter than the other opposing pair. 2 Type 5 OCMs should be accompanied by other geometries resulting from porphyroblast growth over different fold interference patterns (e.g. Ramsay, 1967; Thiessen, 1986). A first-generation antiform overprinted by a second-generation synform can result in Type 5 OCMs (Fig. 8a,b). However, a different, but equally probable shape, can occur if a first-generation antiform is overprinted by a second-generation antiform, as shown in Fig. 8(c). 3 Because microfolds commonly are asymmetrical crenulations, a pattern likely to be preserved in a porphyroblast is shown in Fig. 8(d). REVIEW OF THEORETICAL, PHYSICAL AND COMPUTER MODELLING OF OCMs A principal aim of this paper is to determine whether or not any of the five OCM types described above are indicators of specific deformation histories, as discussed in the next section. To provide a basis for this determination, we review in this section the various theoretical, physical and computer models/simulations Fig. 7. Block diagram showing the relationships between Type 5 OCMs, porphyroblast inclusion trails and two fold generations with orthogonal axial planes. The block represents a section through a porphyroblast that has overgrown an F1 antiform that was overprinted by an F2 synform. Thus, the intersection of the folded surface with the top face of the block represents the trace of inclusions defining the OCMs. Fig. 8. Various geometries that might be expected as inclusion trails where porphyroblasts have grown in rocks that previously experienced microscale cross-folding. F1 and F2 have the same orientation in each example. (a) F1 antiform overprinted by an F2 synform, as shown in Fig. 7. Note that there are two orthogonal sets of oppositely concave microfolds. The pair aligned with F1 form the OCMs of interest. (b) Similar to (a), but with sheared-out FI limbs. (c) F1 antiform overprinted by an F2 antiform. (d) F1 fold pair or crenulation overprinted by a single F2 fold.

7 'MILLIPEDE' AND OTHER SIMILAR PORPHYROBLAST MICROSTRUCTURES 21 Q\, R close in the matrix outside the porphyroblast, assuming CD that no later deformation had destroyed them. Once the total inclusion-trail curvature exceeds 180", closed loops should be seen inside the porphyroblast in all s near-median sections parallel to the spiral axes. S Physical models Schoneveld (1979, fig. 18) presented five serial sections in each of eight different orientations through a physical model of a porphyroblast with spiral-shaped inclusion trails. Two of the serial sets were made parallel to the spiral axis and show Type 1 OCMs R with the general features depicted in Fig. 2. One set is Fig. 9. Three different cross-sections through the centre of a orientated perpendicular to the foliation (left-front face theoretical model porphyroblast with inclusion trails showing of Fig. 2) and the other is orientated parallel to the 90" of relative rotation. R-relative rotation axis; T-transport foliation (top face of Fig. 2). direction; S-pole to R-T plane. Modified from Powell & Treagus (1967, fig. 2). of OCMs in and adjacent to porphyroblasts or other rigid objects, which provide information about the possible range of deformation histories in which OCMs can form. Theoretical models Powell & Treagus (1967) conducted an analysis of the inclusion-trail geometries in various sections through a porphyroblast with 90" of total inclusion-trail curvature. Their theoretical model predicts that two types of OCMs should be seen in two different sections parallel to the axis of relative rotation (Fig. 9). In the R-S+45" section, the loops close like Type 1 OCMs typically seen in a section through spiral-shaped inclusion trails cut perpendicular to the foliation and parallel to the spiral axis (left-front face of Fig. 2). To see closed loops within the porphyroblast, a section must cross an individual inclusion surface three times. If the amount of inclusion-trail curvature is greater than 90", this can be accomplished in a section perpendicular to the foliation and lineation, as shown in the left-front face of Fig. 2. In the R-T section, OCMs in the porphyrohlast could be mistaken for Types 2-5. This section is analogous to the top face of Fig. 2 and, in general, the loops would be expected to Ghosh (1975) produced Type 2 and Type 3 OCMs during bulk simple shearing using rigid cylinders in a matrix of silicon putty, with marker lines initially at 135' to the shear plane (Fig. 10). The OCM pattern was very symmetrical when the marker lines were in the shortening field of the incremental strain ellipse, forming Type 2 OCMs. The pattern becomes progressively asymmetrical with increasing shear strain, as the marker lines rotate into the extensional field of the incremental strain ellipse, forming Type 3 OCMs. Ghosh & Ramberg (1976) also produced Type 2 and Type 3 OCMs during bulk simple shearing using rigid blocks in a matrix of silicon putty, with marker lines at 150' to the shear plane (Fig. 11). As with the experiments by Ghosh (1975), the OCMs are symmetrical (Type 2), until the marker lines rotate into the extensional field of the incremental strain ellipse, where the OCMs become asymmetrical (Type 3). Ghosh & Ramberg (1976) also showed that Type 2 OCMs can form around boudins during bulk coaxial shortening (Fig. 12). They explained that an especially high shear-strain rate develops at the contact between the boudin and surrounding material, decreasing both away from the boudin and towards its central plane. They also noted that the shear is of opposite sense either side of the central plane. Ghosh (1975) and Ghosh & Ramberg (1976) conducted bulk simple shear experiments that would have led to Type 4 OCMs, but the shear strains were Fig. 10. Types 2 & 3 OCMs formed around a wooden cylinder in silicon putty during bulk simple shearing. The shear plane is perpendicular to the page and parallel to the long axis of the figure. The shear sense is shown by arrows. Marker lines are initially at 135" to the shear plane. Shear strains are shown in (bj-(dj. Type 2 OCMs are well developed at stage (cj, whereas Type 3 are well developed at stage (d). Modified from Ghosh (1975, figs 11-13).

8 22 S. E. JOHNSON & T. H. BELL Fig. 12. Type 2 OCMs formed adjacent to a competent plastic inclusion enclosed in a viscous medium during bulk pure shearing. The lines were originally parallel, equally spaced and at right angles to the long axis of the inclusion. Modified from Ghosh & Ramberg (1976, fig. 53). Fig. 11. Types 2 & 3 OCMs formed around a rigid block in silicon putty during bulk simple shearing. The shear plane is perpendicular to the page and parallel to the short axis of the figure. The shear sense is shown by arrows. Marker lines are initially at 150' to the shear plane. Type 2 OCMs are well developed at stage (c), whereas Type 3 are well developed at stage (d). Modified from Ghosh & Ramberg (1976, figs 27 & 28). not high enough to produce them. Schoneveld (1979) produced Type 4 OCMs during bulk simple shearing using a cylinder of wood in a medium of wallpaper paste, with marker lines parallel to the shear plane (Fig. 13). Van Den Dreissche & Brun (1987) also produced Type 4 OCMs during bulk simple shearing using a rigid plasticine object in a matrix of silicon putty, with a marker grid of regularly spaced circles that deformed into strain ellipses. The 'curve envelopes' of these strain ellipses were traced to obtain a clearer picture of the behaviour of marker layers (Fig. 14). Computer simulations Computer simulations of spiral-shaped inclusion trails have been presented by Masuda & Mochizuki (1989), Bjornerud & Zhang (1994) and Gray & Busa (1994), but only Gray & Busa (1994) discussed sections parallel to the spiral axes. As with Schoneveld (1979), Gray & Busa (1994) showed Type 1 OCMs in sections parallel and perpendicular to the external foliation. Gray & Busa (1994) also showed that their model gave results consistent with the theoretical model of Powell & Treagus (1967) for a porphyroblast with 90" of total inclusion-trail curvature (Fig. 15; compare with Fig. 9). Type 2 OCMs have been simulated in bulk coaxial shortening here in Fig. 16 and by Gray & Busa (1994). Details of the method used to produce the simulation in Fig. 16 can be found in Bell et a/. (1989); the mesh for this simulation was identical to that in their fig. 2b. The marker lines in the mesh are connected at nodes, and the nodes along the two edges of the mesh perpendicular to the shortening direction are pinned, so that the edges remain parallel. The mesh has a random distribution of elements, strength and stiffness properties varying 10% either side of a mean, and the four central elements have strength and stiffness properties an order of magnitude greater than the mesh average. Thus, the four central elements simulate a predeformational porphyroblast, and the random variation of strength and stiffness throughout the mesh allows heterogeneous deformation. This simulation (Fig. 16) demonstrates that shapes very similar to natural OCMs can easily be formed in bulk coaxial shortening. The computer simulations of Gray & Busa (1994) were concerned mainly with spiral-shaped inclusion trails, and therefore Type 1 OCMs. However, they also attempted to simulate Type 2 OCMs in uniaxial pure shear (Fig. 17). The resulting geometry was considered by them to be a poor imitation of Type 2 OCMs (compare Figs 16 & 17), and they suggested that the

9 MILLIPEDE AND OTHER SIMILAR PORPHYROBLAST MICROSTRUCTURES 23 (a) y=1.2 -/ (C) y=6.2 I Y Fig. 14. Type 4 OCMs formed adjacent to a rigid plasticine block in a matrix of silicon putty during bulk simple shearing The shear plane is perpendicular to the page and parallel to arrows that show the shear sense. Marker lines are initially parallel to the shear plane. Shear strains shown. Modified from Van Den Driessche & Brun (1987, fig. 11). Fig. 13. Type 4 OCMs formed around a wooden cylinder in wallpaper paste during bulk simple shearing. The shear plane is perpendicular to the page and parallel to the short axis of the figure. The shear sense is shown by arrows. Marker lines arc initially parallel to the shear plane. Shear strains shown. Modified from Schoneveld ( 1979, fig. 24). R failure may be due to the homogeneous nature of their model. We agree and suggest that another possible reason is that they assumed slow, constant porphyroblast growth (112 = 3) during deformation, as opposed to the very rapid initial growth, or pre-existence, of the porphyroblasts envisaged by Bell & Rubenach (1980, p. T10) and Johnson & Moore (1996). Masuda & Ando (1988) simulated Types 2, 3 and 4 OCMs (Fig. 18). The left-hand side of Fig. 18 shows the formation of Type 4 OCMs, whereas the righthand side shows the formation of Types 2 and 3. DO OCMs INDICATE SPECIFIC DEFORMATION HISTORIES? OCMs are commonly used to infer specific deformation histories (e.g. Rosenfeld, 1970; Schoneveld, 1979; Bell & Rubenach, 1980; Bell, 1981; Bell et ul., 1986; Gray & Busa, 1994). In this section we evaluate the extent Fig. 15. Three different cross-sections through the centre of a simulated porphyroblast with inclusion trails showing 90 of relative rotation. R-relative rotation axis; T-transport direction; S-pole to R-T plane, which in this example is analogous to the shear plane. Similar to Fig. 9, but the closed loops in the upper-left section more closely resemble those seen in real porphyroblasts, and the right section clearly shows the loops closing in the matrix outside of the porphyroblast, similar to the top face of Fig. 2. Modified from Gray & Busa ( 1994, fig. 2).

10 24 S. E. JOHNSON & T. H. BELL (a) Initial Mesh Fig. 16. Finite element computer simulation showing Type 2 OCMs formed around a competent inclusion during bulk coaxial shortening. The original mesh is shown in (a), the direction of shortening is indicated by the large arrows in (b) and the percentage of shortening is shown at the bottom of (b)-(d). The four stippled squares in the centre of the mesh represent a porphyroblast.

11 MILLIPEDE AND OTHER SIMILAR PORPHYROBLAST MICROSTRUCTURES 25 - e- Type TVDeS2&3 y= (a) 1O:l 1OO:l Fig. 17. Simulated Type 2 OCMs in spherical porphyroblasts growing in a fluid matrix during bulk pure shearing. Bulk strain is shown below each simulation. Sections are perpendicular to the plane of flattening and the foliation. Plane of flattening is perpendicular to the page and the central inclusion line. Modified from Gray & Busa (1994, fig. 15). to which Types 1-4 can constrain deformation histories. Because Type 5 OCMs represent a fold interference pattern, they do not necessarily indicate any particular deformation history, and will not be further discussed. We emphasize that, unless otherwise stated, the discussion below is concerned with OCMs within and adjacent to porphyroblasts, or adjacent to other rigid objects. y= 1 y=2 y= 3 Type 1 OCMs Type 1 are the most commonly reported OCMs, owing principally to the occurrence of garnet porphyroblasts with spiral-shaped inclusion trails throughout many of the world s orogenic belts (see Johnson, 1993a, for reference list). The deformation history required to form spiral-shaped inclusion trails, and therefore Type 1 OCMs, is controversial. They are traditionally considered to form during highly noncoaxial deformation, involving porphyroblast rotation relative to the matrix foliation (generally considered to be parallel to the local shear plane) and the external reference frame (e.g. Rosenfeld, 1970; Schoneveld, 1979; Bell, 1985; Masuda & Mochizuki, 1989; Gray & Busa, 1994). However, equally valid alternative interpretations have been considered, involving porphyroblast growth, with or without rotation relative to an external reference frame, during successive overprinting foliation-forming events that can range widely in degree of noncoaxiality (e.g. Ramsay, 1962; Bell & Johnson, 1989; Hayward, 1992; Johnson, 1993a). Johnson (1993a) reviewed the formation of spiral-shaped inclusion trails. but was unable to determine which of the two models, rotational or nonrotational, provides the better explanation. However, he stated that a better understanding of the kinematics of crenulation cleavage development might provide evidence critical to solving the problem. Many porphyroblasts with spiral-shaped inclusion trails are found in rocks that have been strongly I Fig. 18. Types 2, 3 and 4 OCMs formed during a bulk simple shearing computer simulation. The shear plane IS perpendicular to the page and parallel to the short axis of the figure. The shear sense is shown by arrows. Marker lines are initially parallel to the shear plane for Type 4, and 135- to the shear plane for Types 2 & 3. Shear strains shown in left column. Different fold asymmetries are developed in Types 3 & 4, even though the shear sense is the same. Modified from Masuda & Ando (1988, fig. 5). deformed, and they have been used to infer a high degree of noncoaxiality during the deformation in which they grew (eg Brunel, 1986; Meier & Hiltner, 1993). However, some of these rocks show evidence for moderate to large components of bulk coaxial shortening, for example: (1) symmetrical quartz c- and u-axis preferred orientation fabrics in the Moine thrust zone at the Stack of Glencoul, NW Scotland (Law, 1990); (2) very low K values of flattened quartz grains in the Main Central Thrust zone, Nepal Himalaya (Bouchez & Pecher, 1981); and (3) straight quartz fibres around magnetite porphyroblasts in the Main Central Thrust zone, Nepal Himalaya (S. E. Johnson, unpublished data). Another problem is that we know of very few examples in which the growth timing of these porphyroblasts relative to the matrix foliation has been satisfactorily demonstrated on microstructural grounds

12 26 S. E. JOHNSON & T. H. BELL (Johnson, 1993a). Therefore, in the absence of appropriate microstructural relationships, even if it can be demonstrated that a schist was formed during highly noncoaxial deformation there is no guarantee that porphyroblast growth and the development of Type 1 OCMs was synchronous with this deformation. In summary: ( 1) rotational and nonrotational models can equally well explain the formation of Type 1 OCMs, (2) there is evidence for a moderate to large component of bulk coaxial shortening in some areas where they are found and (3) unambiguous microstructural criteria to infer the timing of OCM formation relative to particular deformation events are generally absent. Thus, until their formation is better understood, Type 1 OCMs should probably not be used as indicators of any particular deformation history. Type 2 OCMs Type 2 millipede OCMs have been interpreted as indicators of bulk heterogeneous shortening that is near-coaxial (e.g. Bell & Rubenach, 1980; Bell, 1981; Bell et nl., 1986). However. the physical models and computer simulations discussed above demonstrate that those occurring within and adjacent to porphyroblasts theoretically can form during bulk coaxial shortening and bulk simple shearing. These are simplified end-member deformations, and more realistic deformation histories would have to incorporate heterogeneous strain, heterogeneous distribution of the rotational component of the deformation (e.g. noncoaxial deformation in fold limbs during bulk heterogeneous shortening), volume loss along developing cleavage domains, or bulk volume loss or gain on the thin-section scale. However, given the right initial foliation configuration, Type 2 OCMs can form during any deformation history involving steady flow and many involving nonsteady flow, regardless of the degree of heterogeneity of the deformation, the relative components of bulk pure shear and bulk simple shear, or the degree of local or bulk volume changes. We conclude that the overall shape of Type 2 OCMs within and adjacent to porphyroblasts provides information about the strain state only, and not about any rotational component of the deformation in either the internal (i.e. the degree of noncoaxiality) or external reference frames; thus, they should not be used as indicators of specific deformation histories. Type 2 OCMs also occur independently of porphyroblasts and other rigid objects. These OCMs might indicate a deformation history of bulk coaxial shortening, but a lack of physical or computer modelling of such OCMs makes it difficult to draw conclusions about the extent to which they can be used to constrain deformation histories. Type 3 OCMs Type 3 OCMs theoretically can form during any deformation path between bulk coaxial shortening and bulk simple shearing. Figures 10, 11 & 18 show their formation during bulk simple shearing. To form them during bulk coaxial shortening, a situation similar to that of Fig. 16 is required, except that the direction of shortening must be oblique, rather than parallel, to the initial foliation. This causes asymmetrical deflection of the foliation around the porphyroblast, but simultaneous heterogeneous extension around the porphyroblasts results in Type 3 OCMs (e.g. Fig. 5; Passchier et al., 1992, fig. 3b). We conclude that the overall shape of Type 3 OCMs provides information about the strain state only, and not about any rotational component of the deformation. Type 4 OCMs If Type 4 OCMs could be identified, they might be good indicators of a highly noncoaxial deformation history. However, potential problems with differentiating between Types 3 and 4 OCMs may have important consequences for shear-sense determinations. The microfolds that form in Types 3 and 4 have opposite asymmetries. For example, compare stage (d) in the left (Type 4) and right (Type 3) simulations in Fig. 18. Although both types formed during bulk sinistral simple shearing, Type 4 OCMs have folds with dextral asymmetry, whereas Type 3 OCMs have folds with sinistral asymmetry. If a shear zone formed in rocks with sufficient variation in the initial foliation orientation, both Types 3 and 4 may develop. Although Types 3 and 4 have a slightly different appearance in Fig. 18, it may not be possible to tell them apart in rocks. Thus, it may not be possible to confidently determine the shear sense from such OCMs unless other indicators are present (cf. Marques & Cobbold, 1995). Discussion In this section we have concluded that Types 1-3 OCMs (and possibly Type 4) within and adjacent to porphyroblasts are not necessarily indicators of specific deformation histories. With regard to Types 2 and 3, these conclusions are based on overall OCM geometries formed in physical models and computer simulations that were concerned with the behaviour of marker lines that define the OCMs. None of these models or simulations consider possible variations in surrounding matrix microstructures, or the geometry of the overprinting syn-ocm foliation, that might help to constrain the deformation history responsible for the OCMs. Physical models or computer simulations specifically designed to test this may provide important insights into the problem, but perhaps it can be better tested if natural OCMs that formed during different deformation histories, in similar rocks and with similar finite strain values, can be identified and compared. Nevertheless, determining degrees of noncoaxiality from rock fabrics is a notoriously difficult problem of long standing in structural geology (Means, 1994).

13 MILLIPEDE AND OTHER SIMILAR PORPHYROBLAST MICROSTRUCTURES 27 SUMMARY AND CONCLUSIONS We recognize five types of oppositely concave microfolds (OCMs) in and adjacent to porphyroblasts in metamorphic rocks. Type 1 OCMs occur in sections parallel to the spiral axes through porphyroblasts with spiral-shaped inclusion trails. Type 2 OCMs are highly symmetrical, the foliation folded into OCMs being approximately perpendicular to the overprinting foliation. Type 3 OCMs 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. All OCMs described in rocks have been interpreted as Types 1-3. Type 1 OCMs are generally considered to form during highly noncoaxial deformation, and have been used to infer such deformation. However, theoretically they can form during a wide range of deformation histories, from successive bulk coaxial shortening deformations with variable degrees of noncoaxiality to a single simple shearing deformation (e.g. Ramsay, 1962; Johnson, 1993a). These deformations are generally considered to be steady flows, for simplicity, but Type 1 OCMs could also form during nonsteady flows. Although porphyroblasts containing Type 1 OCMs are commonly found in high-strain zones, there is local evidence in some of these zones of bulk near-coaxial deformation (e.g. Law, 1990; Bouchez & Pecher, 1981; S. E. Johnson, unpublished data). Furthermore, where Type 1 OCMs are found in zones interpreted as forming during highly noncoaxial deformation, there is a general lack of unequivocal microstructural evidence for syndeformational growth of the porphyroblasts (e.g. Johnson, 1993a,b). Type 1 OCMs seen within porphyroblasts in sections parallel to the syn-ocm foliation can closely resemble the other four types, particularly if the total inclusion-trail curvature is less than 180L, so that closed loops are absent within the porphyroblast. Type 2 OCMs, commonly referred to as millipede microstructure, have been interpreted as indicators of bulk coaxial shortening (e.g. Bell & Rubenach, 1980; Bell. 1981; Bell et a/., 1986, 1992). However, the overall OCM geometry provides information about the strain state only, and no information about the degree of noncoaxiality of the deformation responsible for their formation; this is true for Type 3 as well. Thus, Types 2 and 3 OCMs, in and adjacent to porphyroblasts, cannot be used on their own as indicators for specific deformation histories. Type 2 OCMs that occur independently of rigid objects might indicate bulk coaxial shortening. However, there is a lack of supporting physical or computer modelling required to reach a firm conclusion. It is possible that matrix microstructures around Type 2 OCMs, formed during highly coaxial and highly noncoaxial deformation histories, might have specific features characteristic of the degree of noncoaxiality. Important insights may be gained from physical models or computer simulations specifically designed to test this. However, determining degrees of noncoaxiality from rock fabrics has proven to be a difficult problem (e.g. Means, 1994). Type 4 OCMs differ from Type 3 in that they involve one foliation only, which is parallel to the local shear plane throughout the deformation responsible for the OCMs. Thus, if Type 4 OCMs could be unambiguously identified, they may indicate highly noncoaxial deformation and be useful as shear-sense indicators. However, at high strains it may be difficult to distinguish between Types 3 and 4, and so care must be taken when using such asymmetrical OCMs as shear-sense indicators, because Types 3 and 4, if formed during the same deformation, would indicate opposite shear senses. Although appropriate fold interference patterns are reasonably common in deformed metamorphic rocks, we are not aware of an instance in which Type 5 OCMs have been preserved in porphyroblasts. ACKNOWLEDGEMENTS S.E.J. acknowledges support for this project by the Australian Research Council in the form of a Queen Elizabeth I1 Research Fellowship, Grant Number A , and ARC Small Grants from Macquarie University. A. C. Duncan is acknowledged for his contribution to the computer modelling in Fig. 16. Support for this modelling was provided to T.H.B. by the Australian Research Council. We thank D. W. Durney, E. J. Hill and R. H. Vernon for helpful comments on the manuscript, and W. D. Means and M. L. 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