Bell, T.H. and Johnson, S.E., 1989a. Porphyroblast inclusion trails: the key to orogenesis

/. metamorphic Ceol., 1989, 7, 279-310

Porphyroblast inclusion trails: the key to orogenesis

T. H. BELL AND S. E. JOHNSON Department of Geology, James Cook University, Townsville, Queensland 487 1, Australia

ABSTRACT Detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral-shaped inclusion trails are common indicates that spiral-shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates. They formed instead by progressive growth by porphyroblasts over several sets of near-orthogonal foliations that successively overprint one another. The orientations of these near-orthogonal foliations are alternately near-vertical and near-horizontal in all porphyroblasts examined. This provides very strong evidence for lack of porphyroblast rotation.

The deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two-stage cycle of : (1) crustal shortening and thickening, with the development of a near-vertical foliation with a steep stretching lineation; followed by (2) gravitational instability and collapse of this uplifted pile with the development of a near-horizontal foliation, gravitational spreading, nearcoaxial vertical shortening and consequent thrusting on the orogen margins. Correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen. This information, combined with information about chemical zoning in porphyroblasts, provides details about the structural/metamorphic (P-T-t) paths the rocks have followed.

The ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical, ductile tear/transcurrent faults across which there is no component of bulk shortening or transpression.

Key w& Coaxial shortening; crustal thickening; deformation history; foliation development; inclusion trails; orogenesis; orogenic model; porphyroblast growth; progressive deformation.

INTRODUCTION

This paper discusses the methodology, results and wide-ranging implications of a detailed microstructural analysis of rocks with garnet porphyroblasts that commonly contain spiral-shaped inclusion trails. Samples of schist were collected from around the Chester Dome area, Vermont in order to obtain porphyroblasts similar to those in fig. 14-2 of Rosenfeld (1968), which show welldeveloped spiral-shaped inclusion trails from the porphyroblast core to the rim/matrix boundary. Many samples containing classic 'snowball' garnet porphyroblasts were collected. The specimen described in detail in this paper was located 1 km, on a bearing of W , west of the bridge at Brockways Mills-Saxtons River Quadrangle.

Samples of rock with classic spiral-shaped inclusion trails in garnet porphyroblasts were also obtained from schists near the Main Central Thrust in the Modi Khola Valley of central Nepal (courtesy of M. J. Rubenach). The specimen described in detail in this paper was located 0.65 km south of Landrung, north-east of Pokhara. Samples were also obtained from the Karakoram Schist, Pakistan (courtesy of

C. McA. Powell and R. H. Vernon). The specimen described in detail in this paper comes from a roadside outcrop 4 km east of Hini, in the H u m Valley (Powell & Vernon, 1979).

Garnet porphyroblasts with spiral-shaped inclusion trails occur in several orogenic belts around the world (e.g., Rosenfeld, 1%8, Cooper, 1974; Schoeneveld, 1979; Powell & Vernon, 1979; Bell & Brothers, 1985; Brunel, 1986; Meneilly & Storey, 1986). These porphyroblasts are generally interpreted as having rotated during growth, and are used as sense of shear indicators for the deformation event in which they grew (e.g., Simpson & Schmid, 1983). Although Schoneveld (1979) built a mechanical model to explain spiral-shaped inclusion trails as a product of porphyroblast rotation during growth, a detailed micrw structural analysis demonstrating that rotation is necessary to explain the microstructures observed has never been published. The spiral shaped geometry of these inclusion trails seems such an obvious product of rotation of the porphyroblast that this probably was not considered to be necessary. In some areas where porphyroblasts grew very early

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210 T. H. BELL & S. E. JOHNSON

during the deformation, it has been possible to dem- onstrate that there was no rotation of the porphyroblasts during that or subsequent deformation events, because the inclusion trails remain constant in orientation across subsequently formed folds, sometimes over large areas (e.g., Fyson, 1975, 1980; Johnson, 1989; Steinhardt, 1989). However, this type of study has not been done in areas where spiral-shaped and other relatively complex inclusion-trail geometries are common, because, until recently, possibly no one considered that the porphyroblasts may not have rotated.

If spiral-shaped inclusion trails are a product of rotation of porphyroblasts as they grew, these porphyroblasts would provide a rare criterion for determining a homogeneous simple shear deformation history during the part of the deformation in which they grew (Bell, 1985; Bell, Duncan & Simmons, 1989). If the spiral-shaped inclusion trails are not a product of porphyroblast rotation, understanding how they are generated may provide important information about orogenic processes.

Our detailed microstructural analysis of the rocks from Nepal, Pakistan and Vermont shows that correlated foliations delined by inclusion trails remain constant in orientation in a single rock sample (Nepal and Pakistan) and across macroscopic folds (Vermont; Bell, unpublished data), even in porphyroblasts where the overall inclusion- trail geometry is spiral-shaped. This demonstrates that the porphyroblasts have not rotated and that the deformation history was dominated by progressive bulk inhomogeneous shortening, which may have been coaxial or non-coaxial on the bulk scale (Bell, 1981, 1985), as explained later. On the basis of this detailed microstructural analysis, we present a model for the formation of spiral-shaped inclusion trails that requires no rotation of the porphyroblast relative to geographical coordinates, and is consistent with observations recorded in the literature by other workers.

The microstructural information obtained in this study has wide ranging implications for orogenic processes and sense of shear criteria. We present therefore a model of orogenesis that is totally consistent with the micro-, meso- and macroscale observations in3hese and other rocks. It explains how thickening (‘wedging’, cf. Platt & Behrman, 1986) will occur in the heel of a thrust belt, and how high- grade metamorphic rocks can be exposed at the earth’s surface. It also shows how gravity spreading may play a major role in orogenesis, providing an explanation for nearcoaxial vertical shortening in shallowly dipping foliations (e.g., Law, Knipe & Dayan, 1984; Erskine & Wenk, 1985; Reymer & Oertel, 1985; Law. Casey, & Knipe, 1986; Law, 1987).

The terminology used in this paper for thin sections oriented relative to the mineral elongation lineation within a foliation in the matrix is N for sections perpendicular to lineation, P for sections parallel to lineation but perpendicular to foliation, and S for sections parallel to foliation. The method of inclusion-trail analysis and correlation used in the following text is described in the Appendix.

OBSERVATIONS N O T CONSISTENT W I T H PORPHYROBLAST R O T A T I O N

Truncation zones

In most garnet porphyroblasts examined, ‘truncation’ zones that separate two apparently different-generation sets of inclusion trails are common. These can be abrupt truncations, with the two inclusion-trail sets meeting at a high angle (Figs lb, c & d). Alternatively, they can be relatively continuous, resembling the differentiated zones of crenulation cleavages where the ‘truncated’ inclusion trails, or crenulation hinges, merge continuously with the ‘truncating’ inclusion trails or differentiation zones (core of Figs lb & c). ‘Truncations’ commonly change from the abrupt type to the continuous type along the length of the zone (Figs lb, c, d, e & f ) . The two inclusion-trail sets eventually (‘merge’) into one continuous set (Figs lb & d). For convenience, they are all referred to as truncation zones or truncations in the remainder of this paper.

In porphyroblasts with spiral-shaped inclusion trails, truncation zones commonly cut the spiral-shaped traces of foliations and strain-shadowlpressure-fringe quartz at a high angle. Their shapes, as they cut through the spiral trace, do not always conform with the folded shape of that trace (Fig. 1). In our experience, these truncations are the rule in porphyroblasts with complex inclusion trail geometries (whether or not they are spiral-shaped). This is demonstrated for N sections in Figs 2-7 for several porphyroblasts with the same orientation (see Appendix) from Nepal (Figs 2 & 3), Pakistan (Figs 4 & 5 ) and Vermont (Figs 6 & 7). It is also demonstrated for P and S-sections from the same rocks in Figs 8 and 9.

’Millipede’ microstructure

Many garnet porphyroblasts from Nepal, Pakistan and Vermont contain an inclusion trail geometry (Figs 10-13) that we call a ‘millipede’ microstructure. after descriptions of it by Bell & Rubenach (1980); see also Bell (1981). This microstructure, when found in garnet porphyroblasts associated with other garnet porphyroblasts that contain spiral-shaped inclusion trails, has previously been interpreted as representing a section of a rotated porphyroblast with spiral-shaped inclusion trails cut parallel to the rotation axis (e.g., Cox, 1%; Powell & Treagus. 1967, and 1970 Fig. 5a and 5b; Powell & Vernon, 1979; Rosenfeld, 1968; Schoneveld, 1978, p. 18; 1979, p. 41).

Several microstructural characteristics associated with the occurrence of ‘millipedes’ that are inconsistent with the above interpretation are described below.

‘Millipedes’ most commonly have shallowly dipping axial planes (Figs 10-13), which is very difficult to explain in terms of rotation during thrusting because the axis of apparent rotation is nearly vertical. This problem is compounded when multiple ‘millipede’ microstructures have formed (Figs 12 & 13).

2. ‘Millipede’ geometries occur in garnet porphyroblasts that superficially show spiral-shaped inclusion trails (Figs 2e,4a,4b,4c,4e,6a,6c,6f,6h,8a,&and 14).

PORPHYROBLAST INCLUSION TRAILS 2Ul

3. Inclusion trails defining ‘millipedes’ are commonly truncated, as discussed above, in a manner that is inconsistent with a section through a porphyroblast that rotated during growth (Figs 2e, 4a, 4b, 4c, 4e, 6a, 6c, 6f, 6h, &, &, 10-13). 4. The presence of ‘millipede’ microstructures in rocks

where spiral-shaped inclusion trails are common is inconsistent with rotational growth because ‘millipede’ geometries suggest progressive bulk inhomogeneous shortening, not progressive homogeneous simple shear, as discussed later (d. Bell, 1981, 1985; Bell et al., 1989).

5. ‘Millipede’ and spiral-shaped inclusion trails occur at the Same stage of inclusion-trail development, in the same or identically oriented thin sections (Fig. 3, stage 3; Fig. 5, stages 3 & 5; Fig. 7, stage 3).

Consistency in the orientation of correlated trails

As shown in Figs 2-9, the orientations of the correlated inclusion trails are extremely consistent from porphyroblast to porphyroblast, even though we included porphyroblasts showing maximum variation in these orientations (see Appendix for discussion). If the garnet porphyroblasts in Figs 2-9 had rotated during growth, we would not expect this consistency in the orientation of correlated inclusion trails. The small variation in the Nepal, Pakistan and Vermont garnet porphyroblasts can readily be explained in terms of the anastomosing nature of foliations, especially in heterogeneous zones of progressive shortening (Bell & Rubenach, 1980; Bell, 1981).

Figure 15 shows examples of ‘millipedes’ formed in plagioclase porphyroblasts where the central straight portions of the inclusion trail orientations in each porphyroblast vary from one another by 45”. However, in the porphyroblast with the diagonal trails in its centre, the inclusion trails on the porphyroblast rims are asymmetric, such that the anastomosing truncation zone is oriented parallel to the foliation in the matrix. Hence, the oblique orientation of the internal inclusion trails is not a product of porphyroblast rotation. These porphyroblasts grew over an earlier foliation that had already been rotated by the effects of folding about axial planes defined by the ‘millipede’ rims and the external differentiated crenulation cleavage.

Abrupt chemical discontinuities at truncations and along the trace of a spiral

Microprobe pointcheck analyses for MnO, MgO, and FeO were made for several of the garnet porphyroblasts in Figs 2-9 to determine whether or not chemical discontinuities OCCUI across truncations shown in the accompanying line diagrams. As sedn in Fig. 16, chemical discontinuities do OCCUT across these structural truncations. The largest discontinuities are in the Nepal garnets, at the S, truncation. The smallest are in the Pakistan garnets. Microprobe data were also collected along one side of a spiral-shaped trace, as shown in Fig. 17a, because the phyllosilicates that are concentrated in this zone are

replaced by garnet, whereas the quartz grains in the s@-shaped centre are commonly not replaced by garnet. The microprobe sites are shown in Fig. 17b, which illustrates the same garnet as that studied in detail microstructurally and shown in Fig. 1. The data for these sites (Fig. 18) show that chemical discontinuities occur across the structural truncations that cut the spiral trace at a high angle. These chemical discontinuities across truncation zones are inconsistent with rotation during porphyroblast growth.

Some of the trends in Figs 16 and 18 may have wide implications for metamorphic histories, as well as for compositional zoning in garnet in general (e.g., Powell & Vernon, 1979; Spear & Selverstone, 1983a & b; Karabinos, 1984; Selverstone & Munoz, 1987). For instance, the large change in weight percent FeO and MnO across the D3 core boundary in ‘the Nepal garnet porphyroblasts; or the Occurrence of both sympathetic and antipathetic changes in weight percent of FeO to MgO across truncation boundaries. However, detailed treatment of these trends is beyond the scope of this paper.

Spiral-shaped inclusion trails in N, P and S sections

Spiral-shaped inclusion trails are found in nearly equal proportions in the N, P and S sections from Nepal, Pakistan and Vermont (Figs 2-9). Two unsatisfactory explanations for this Occurrence in a rotational growth model are:

1. The axis of rotation is precessing or changing markedly from garnet to garnet.

2. Porphyroblasts in one section that show spiral-shaped inclusion trails are of ditrerent generations than those in other sections that show this geometry, and therefore have rotated about merent axes.

These two explanations are not consistent with the observation that, when the thin section plane cuts the garnet centre, the same correlative stages of growth are always preserved between the core and rim in all three section orientations, whether the inclusion trails are spiral-shaped or not (Figs 3b-f, Sa, 5c, 5e to 5g, 7a, 7c, 7f, 7l1, 9a & 9e).

Microfold asymmetry changes

The asymmetry of microfolds in garnet porphyroblasts with spiral-shaped inclusion trails can vary from one porphyroblast to another, and within individual porphyroblasts. This can lead to porphyroblasts that appear to have rotated in opposite directions and to complex geometries within a single garnet. This can either be because of the occurrence of ‘millipede’ geometry in a porphyroblast, as in Fig. 19, or because of a microfold asymmetry change not related to ‘millipede’ formation, as in Fig. 9d.

282 T. H . BELL 81 S . E. J O H N S O N

Q. 1. Series of enlargements of a garnet porphyroblast from Vermont in partially crossed polars, in order to illustrate ‘truncation’ geomehies in what appears to be welldeveloped spiral-shaped inclusion trails. (a) Most of the porphyroblast and the location of the enlargements in (9. (h) and (j). The straight line from core to rim show the position of a microprobe traverse. Section cut perpendicular to stretching lineation (N-section). Base 5.28 nun. (b) Enlargement of the centre of (a) to better show the inclusion trails other than the quartz spiral. Base 2.10 mm. (c) Sketch of the locations of inclusion trails in (b). (d) Enlargement of the centre of (b) to show the S, truncations of the crenulated S,, and the S, truncations of the crenulated S,. Base 0.82 mm. (e) Sketch of the locations of inclusion trails in (d). ( f ) Enlargement of rectangle marked ‘T in (a). It show truncation of S, by S?, and S, by S5. Base 0.82 mm. (g) Sketch of the locations of inclusion trals in (9. (h) Enlargement of the rectangle marked ‘h’ in (a). Shows truncation of S, by S,, and of S, by S,. Note that S4 is crenulated by S, prior to truncation. Base 0.82 mm. (i) Sketch of the locations of inclusion trails in (h). (j) Enlargement of the rectangle ‘j’ marked in (a). Truncation by S, of Ss and relics of S, which have been crenulated by Ss. Base 0.82 mm. (k) Sketch of the locations of inclusion trails in (j).

P O R P H Y R O B L A S T I N C L U S I O N TRAILS 213

---__----- - _ _ -/-----

T. H. BELL & S. E. JOHNSON

Fig. 2. Eight N-section garnet porphyroblasts from Nepal. Each porphyroblast has the same orientation. Tbey are aligned such that a smke line 00 the thin section would be approximately vertical. Sec Appendix for discusion of tbe methods uscd to photograph, analyse and cornlate inclusion tmils in these porphyroblasts. The porphyroblasls range in diameter from 0.67-1.36mm across their largest dimension. Sketches of the foliation relationships dcfhed by inclusion trails are shown in Fig. 3. Partially crosscd polars.

~ ~ ~ _ ~ ~ ~ _ _ ~ ~ ~ _ _ ~ ~ ~ _ ~ ~ ~ _ _ ~ ~ ~ _

PORPHYROBLAST INCLUSION TRAILS 216

pb. 3. Sketches of the foliations preserved as inclusion trails in the Fig. 2 porphyroblasts. Note the similarity of orientation of foliations S, to S6 from porphyroblast to porpbpblast.

T . H. BELL & S. E. JOHNSON

m. 4. Eight N-section garnet porphyroblasts from Pakistan. Each porphyroblast has the same orientation. The porphyroblasts range in diameter from 2.75-4.03 mm across their largest dimension. They are aligned such that a strike Line drawn on the thin section would be approximately vertical. Partially crossed polars. See Appendix for discussion of the methods used to analyse and correlate inclusion trails in the porphyroblasts.


a. 5. Sketches of the foliations preserved as inclusions in the Fig. 4 porphyroblasts. Note the similarity in orientation of foliations S, to S, from porphyroblast to porphyroblast.

29 T. H . BELL 81 S. E. JOHNSON

Fig. 6. Eight N-section garnet porphyroblasts from Vermont. The porphyroblasts range in diameter from 2.30-3.80 mm across their largest dimension. They are aligned such that a strike Line drawn on the thin scction is approximately northcast on the page. Partially crossed polars. See Appendix for discussion of the methods used to analyse and correlate inclusion trails in these porphyroblasts. Figure 6c is the same garnet as that featured in detail in Fig. 1.

P O R P H Y R O B L A S T INCLUSION TRAILS

p9r. 7. Sketches of the foliations preserved as inclusions in the Fig. 6 porphyroblasts. Note the similarity in orientation of foliations S, to S, from porphyroblast to porphyroblast.

2w T. H. BELL 81 S. E . JOHNSON

Fig. 8. Shows one P- and Ssection garnet, respectively, from Nepal (a and b), Pakistan (c and d) and Vermont (e and f). The porphyroblasts range in size from 0.78 - 4 . 0 mm across their largest dimension. Partially crosscd polars. See Appendix for discussion of thc methods used to analyse and correlate inclusion trails in these porphyroblasts.

P O R P H Y R O B L A S T I N C L U S I O N T R A I L S 29l

Fig. 9. Sketches of the foliations preserved as inclusion trails in the Fig. 8 porphyroblasts.

2M T. H. BELL & S. E. JOHNSON

Fig. 10. (a) Inclusion trails with ‘millipede’ microstructure preserved in thccore of a garnet porphyroblast from Nepal. The ‘millipede’ microstructure is the change in inclusion-trail shape from convex to concave through a straight portion of inclusion trails. Note the truncation of the ‘millipede’ microstructure, especially on the top, by a subsequently formed foliation, also preserved as inclusion trails within the porphyroblast, and the accompanying change in composition and concentration of these trails. This garnet porphyroblast has an inner core in which a mnulation is preserved. The straight line from core to rim shows the position of a microprobe traverse. Section cut perpendicular to stretching lineation (N-section). Base of figure corresponds approximately to the horizontal. Plane polarized light. Width of base 2.1 mm (b) Sketch of the locations of inclusion trails in (a).

Flg. ll. ‘Millipede’ microstructure in a garnet porphyroblast from Pakistan. The ‘millipede’ microstructure is truncated by a weakly preserved foliation. Some pressure-fringe quartz formed during ‘millipede’ formation is abruptly truncated by a foliation that postdates the ‘millipede’-forming event. Section cut perpendicular to stretching lineation (N-section). Base of the figure corresponds to approximately to the horizontal. Partially crossed polars. Base 3.98 mm. (b) Sketch of the locations of inclusion trails in (a).

Location of pressure-fringe/strilin-shadow quark

Pressure-fringe/strain-shadow quartz has commonly only formed in conjunction with some of the foliations preserved as inclusion trails in garnet porphyroblasts. This, plus their consistent location from porphyroblast to porphyroblast, is inconsistent with rotation of porphyroblasts during growth (Figs 4a, 4c-f, 6a-h, & 8f).

Multiple crenulations

Close examination of porphyroblasts that contain spiral- shaped inclusion trails sometimes reveals that several parallel crenulations are preserved at locations within the porphyroblast that were not in strain shadows at the time the crenulations formed (Figs lh-k, 12a,b, 13a,c, 14a.b). This cannot be explained by rotation of the porphyroblast as it grew.

REINTERPRETATION

Introduction

The above microstructural and microprobe observations do not support the interpretation that porphyroblasts with spiral-shaped inclusion trails rotated as they grew. Though some of them could possibly be incorporated into a model of rotation during and/or after growth, the model would be complex, unable to explain all the microstructural relationships, and would have many special requirements that would be unacceptable.

Any interpretation of how spiral-shaped inclusion trails form must be totally consistent with all observations in these and other rocks in which spiral-shaped and other complex inclusion trail geometries occur. Our interpretation, discussed in detail below, is consistent with:

1. The microstructural details preserved in porphyroblasts that contain spiral-shaped inclusion trails.

P O R P H Y R O B L A S T INCLUSION TRAILS 293

2. The microstructural details preserved in porphyre blasts containing inclusion trails that are not spiral-shaped, which appear to have been previously ignored or regarded as too complex to explain. 3. The effect of partitioning during deformation and

metamorphism on porphyroblast nucleation, growth and dissolution (e.g., Bell er al., 1986). 4. Growth stages in the gamet porphyroblasts that are

easily recogmzd by growth-zones containing different- composition inclusion trails, and that are divided by boundaries mimicking tbe crystal outlines of the garnet.

5. The field relationships found in orogenic belts where porphyroblasts with spiral-shaped and other complex inclusion trails are found.

6. Studies in which inclusion-trail orientations have been found to be consistently oriented over large areas (Fyson, 1975, 1980; Johnson, 1989; Steinhardt, 1989; T. H. Bell, unpublished data).

Spird-shaped indusion trails

The model is shown in Fig. 20. At stage 1, a garnet porphyroblast has overgrown a crenulation at a relatively early stage in the development of the crenulation cleavage (labelled S, for convenience), as discussed by Bell & Rubenach (1983). Early in the stage 1 garnet growth (dotted outline), the developing foliation begins to wrap around the garnet. Thus, the final stage 1 garnet (dashed outline) grows over a crenulation that has an anastomosing axial-plane trace (Fig. 21). At stage 2 (Fig. 20b). the crenulation cleavage (S,) has developed further, but the stage of crenulation cleavage development at which the porphyroblast grew is preserved as inclusion trails within the porphyroblast.

At stage 3 (Fig. 2&), the initial crenulation cleavage (SJ, at a relatively advanced stage of its development, is folded by another foliation/crenulation cleavage (S,) which overprints the earlier foliation at a high angle, and has the same asymmetry. This high-angle/sarne-asymmetry overprinting has the effect of rotating the well developed S, cleavage into alignment with the developing zones of S, Merentiation (Fig. 2Oc). At this point there may be another garnet growth stage as indicated by the dashed garnet outline.

Once the stage 3 garnet growth has occurred and S, becomes well developed, the resulting stage 4 geometry (Fig. 2Od) is identical to the spiral-shaped inclusion trail geometry so commonly interpreted as indicating a history of rotation of the garnet porphyroblast during growth (e.g., Rosenfeld, 1968; Schoneveld, 1977; Powell & Vernon, 1979). The requirement that the garnet grow at this time relative to the development of the S, crenulation cleavage, and the requirement that the garnet not grow into the developing zone of S, Merentiation, fits well with the timing and location of porphyroblast growth suggested by Bell, Fleming & Rubenach (1986).

It is potentially not neceSSary for garnet growth to OCCUT at stage 1, because the same geometry preserved in the garnet at stage 3 could also form by overprinting

foliations/crenulations without initial garnet being present. However, the core of a garnet porphyroblast formed in this manner would probably have a tighter spiral shape than with stage 1 garnet growth, because the initial crenulations being folded would be much tighter because their progressive development (Bell & Rubenach, 1983) would not be stalled by overgrowth by a garnet porphyroblast. Alternatively, the crenulated foliation within the core would be variably oriented (Figs 1 & 2) due to the effects of progressive shortening (with commonly a slight non-coaxial component) of the crenulation hinge (e.g., strain-field diagram in Bell, 1985.

Stages 3 and 4 of spiral-shaped inclusion trail development (Figs 2Oc and 2Od) may repeat once, or several more times, resulting in the geometries shown in F ig 20e-h (stages 5-8). Figure 20h corresponds to an apparent rotation of 270” (stage 8). It requires a minimum of four foliations/crenulation cleavages (plus the earlier S,) overprinting one another at a high angle, with the same asymmetry, as well as a minimum of three or four garnet growth stages, depending on whether or not growth occurred at stage 1. Figure 1 shows a garnet porphyroblast from Vermont in which all the microstructural details described by the above model are readily visible.

During any foliation-overprinting event, crenulations may form in the strain shadow regions associated with the newly-forming foliation. If these crenulations form close enough to the porphyroblast, they may be overgrown and preserved in the next porphyroblast growth stage (Figs 22, lc, 5a, 5e-h, 7f, 9d & 14).

fig. 1). I

‘Millipede’ microstructure

Bell & Rubenach (1980), and Bell (1981) showed that this microstructure can form by progressive, bulk, inhomogeneous shortening where a preexisting foliation is overprinted by a n e d y orthogonal foliation/crenulation cleavage. ‘Millipede’ microstructures commonly occur in staurolite, chloritoid, andalusite and plagioclase (Fig. 15) porphyroblasts where no evidence for spiral-shaped inclusion trails is present in any thin-section orientation (Bell & Rubenach, 1980; Bell, 1985; Bell, 1986; Bell et d. , 1986). ‘Millipedes’ also occur in matrix foliations where porphyroblasts are absent (fig. 2 in Bell & Rubenach, 1980), indicating that a ‘solid block’ about which the foliation diverges is not neceSSary to form the ‘millipede’ microstructure.

We suggest that these geometries, as preserved in porphyroblasts associated with other porphyroblasts that contain spiral-shaped inclusion trails. are formed by the same process as spiral-shaped trails, namely by nucleation and growth of the porphyroblast in a zone of progressive shortening deformation, where the preexisting foliation is at a nearathogonal angle to the subsequently formed foliation. This is shown progressively in Fig. 23 for both single and double ‘millipedes’, which can be compared with the natural examples from Nepal, Pakistan and Vermon shown in Figs 10-13.

Kg. W. (b) Sketch of thc locations of inclusion trails in (a).

Fig. 14. (b) Sketch of the locations of inclusion trails in (a).

296 T. H. BELL & S . E . JOHNSON

FQ. 15. (a) ‘Millipede’-shaped inclusion trails in plagioclase prphyroblasts from the Robertson River Metamorphics, North Queensland highlighted by trails marked in white or black. The variation in orientation of the ‘millipedes’ is due to nucleation of the porphyroblasts over primary heterogeneities generated during progressive bulk inhomogeneous shortening. Note the increased asymmetry of the convex-concave geometry when the internal inclusion trails lie at a lower angle to the foliation (oriented E-W) that formed during the deformation that generated the ‘millipede’ geometry. Section cut perpendicular to stretching lineation (N-section). Base of the figure corresponds to the horizontal. Crossed polan. Base 21.3 mm.

At stage 1 of ‘millipede’ formation, a preexisting foliation (S, for convenience) is orthogonally deformed by progressive bulk inhomogeneous shortening (Fig. 23a). The final position of the stage 1 porphyroblast growth is shown by the dashed garnet outline in Fig. 23a. The porphyroblast does not grow any larger during this cleavage forming event because, as discussed by Bell et al. (1986), it cannot grow across active zones of progressive shearing which, in this diagram, develop along the limbs of the microfolds shown in Fig. 23a.

The final stage 1 garnet porphyroblast is shown in Fig. 23b, and S, has become well differentiated. In this diagram, the sharp angles between garnet crystal faces have been smoothed, presumably because of dissolution of the garnet along the well differentiated &. The garnet now preserves ‘millipede’ microstructures. Occasionally, a garnet porphyroblast may overgrow the microfolds associated with formation of a ‘millipede’ microstructure, preserving its developmental geometry (Plate 3, Spry, 1%3).

Figures 23c and 23d show how an early ‘millipede’ can be overprinted by a second ‘millipede’. In Fig. 23c the foliation formed during the lint ‘millipede’-forming event (Fig. 23b) is overprinted at a high angle by a non-coaxial crenulation cleavage. In Fig. 23d the second ‘millipede’ is formed by repeating the processes shown in Figs 23a and 23b. These overprinting ‘millipedes’ are quite common in the Nepal, Pakistan and Vermont rocks (Figs 12 & 13). The geometry shown in Fig. 23d requires the formation of at least four foliations at high angles to one another.

If ‘millipedes’ do form as above, which the available evidence strongly suggests, their generally shallowly dipping axial planes (Figs 10-13, 15) indicate common and

repeated, near-coaxial vertical shortening during orogenesis. This is discussed in more detail later.

SIGNIFICANCE AND DISCUSSION Implications for tectonic development of orogenic belts The garnet porphyroblasts in these rocks from Nepal, Pakistan and Vermont preserve evidence of processes operating during orogenesis that have previously never been fully conceptualized. They indicate a cyclic formation of near-vertical and near-horizontal foliations (Figs 20 & 23). A review of recent literature on rocks associated with thrusting and nappe formation indicates that multiple overprinting of near-orthogonal foliations in orogenic belts is commonly observed (e.g., Carreras, Estrada, & White, 1977; Hutton, 1983; Platt, van den Eechout, Janzen, Konert, Simon & Weijermars, 1983; Hanmer, 1984; Verhoef, Vissers & Zwart, 1984; O’Brien, 1985; Brunel, 1986; Meneilly & Storey, 1986; Platt & Behrman, 1986; Lacassin, 1987). However, the orientations of foliations mapped in the field are not always near-vertical and/or near-horizontal. Reasons for this are discussed below.

The generation of multiple sets of ‘millipede’ microstructures associated with alternate sets of axial-plane foliations with a shallowly dipping orientation (Figs 12, 13 & 23), indicate that one stage of each cycle involved nearcoaxial vertical shortening (on the bulk scale) of the previously formed, steeplydipping foliation. Other structural geologists have also inferred a coaxial character for shortening during formation of shallowlydipping foliations in orogenic belts (e.g., Law et al., 1984; Erskine & Wenk, 1985; Reymer & Oertel, 1985; Law et al . , 1986; Law, 1987).

This indicates that orogenesis is a process of repeating cycles, where each cycle consists of two stages: horizontal shortening and crustal thickening associated with the formation of a steeply dipping foliation, followed by gravitational instability and spreading resulting in a flat-lying to shallowly dipping foliation and zones of nearcoaxial vertical shortening separated by zones of shearing (Figs 24 & 25). This uplift-collapse cycle can be repeated many times. For example, the garnet porphyroblasts in Vermont and Pakistan record at least four repeats of the cycle. Those in Nepal record at least three repeats.

We expect the first foliation forming during orogenesis at moderate to deep levels in the metamorphic pile to be vertical, as show6 in Figs 24a & 25a. The foliation labelled S, in Figs 1-8, was not necessarily the first foliation formed. Only detailed study of the garnet porphyroblasts in many rocks from the same area would determine this. S, varies in orientation in Figs 1 & 2, whereas &-S, do not. This suggests that the garnet core grew later than very early during D1. The foliations labelled S, and S3 in Figs 1-8 have respectively steep and shallowly dipping orientations. Therefore we suspect that at least one earlier foliation than that labelled S, exists in some porphyroblasts.

Rim Core

4 -

3-

2 -

Rim Core

I I I o o 0 7

I IlO I I l l I

NEPAL GARNET C

PAKISTAN GARNET C

Rim Core

VERMONT GARNET C

Fig. 16. Microprobe traverses across garnet porphyroblasts from Nepal, Pakistan and Vermont. Note that abrupt changes in compositim OQNI at the truncation boundaries marked by vertical dashed lines. Microprobe traverse lines shown in Figs 10.12 and la.

Fig. 17. (a) Schoneveld's (19n) model for the development of a spiral-shaped inclusion trails in garnet porphyroblasts by rotation of thc porphyroblast during growth and the geometry of inclusion trails that his model would predict. Ideal probe sites to detect changes io composition of the garnet along the spiral trace are indicated by circles. The ideal locations for truncating foliations, such as those shown in Fig. 1, are shown by the solid black lines. Where the probe trace (circles) crosses these truncations we expect chemical discontinuities. (b) Probe sites for the garnet composition data in Fig. 18 within a garnet porphyroblast with spiral-shaped inclusion trails from Vermont. which was featur4.h detail in Fig. 1. Plane polarized light. Basc is 4.1 mm.

T. H. B E L L & S . E . JOHNSON

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Microprobe points along spiral trace Fig. 18. Probe, data for the spiral probe traverse shown in Fig. 1%. Note that abrupt changes in composition occur at truncation boundaries along the spiral trace. Some of these truncations are shown in detail in Fig. 1.

Figure 24 shows the gross orogen geometry during two cycles of uplift (a and c) and collapse (b and d). In Fig. 24a, crustal compression, presumably caused by convergence of lithospheric plates, causes horizontal shortening and vertical thickening of the crust associated with a steeply dipping foliation. This in turn c a w s gravitational sliding off the crown, and hence extension in the uppermost levels. In Fig. 24b, collapse of the over-thickened crust and

gravitational spreading occur due to gravitational instability caused by the load of the thickened pile. However, space problems prevent expansion below the average level of the earth's crust, resulting in a basal detachment and thrusting onto undeformed rocks. Localized zones of high shear strain form within the collapsing pile (dotted lines). Extension continues in the

crown of the thickened crust during collapse, aided by movement on, and interaction with, the horizontal outer portions of the basal thrust detachment. In Fig. 24c, the pile becomes gravitationally stable

enough for continued horizontal compression to again cause crustal thickening and uplift, which folds the basal detachments and zones of high shear strain lying above them. Gravitationally driven extension of the crown of thickened crust continues. Motion on the outermost portions of the imbrications is assisted by inwards movement. of the crust below the detachment surfaces. In Fig. 24d, near-bulk coaxial gravitational collapse of

the over-thickened crustal pile repeats the process described for part (b). The unroofing of a core of metamorphic rocks has occurred at this stage.

Figure 25 shows the porphyroblast inclusion trail

PORPHYROBLAST INCLUSION TRAILS

geometries that we would expect to find dominating a particular quadrant of an orogenic belt. In Fig. 25a, uplift occurs associated with development of a steeply dipping foliation. NO porphyroblasts grow in this fust foliation- forming event because deformation partitions on too fine a scale (e.g., Bell er al., 1986). h Fig. 25b. a near-horizontal foliation forms, and

porphyroblasts growing early enough during this foliation- forming event (D,) may preserve the orientation of s, and indicate the synthetic sense of shear operating during axial-plane cleavage development on long limbs of folds during D2 (e.g., Bell, 1986). 'Millipede' geometries form in zones of nearcoaxial vertical shortening. The bulk sense of shear affecting rocks depends on

which quadrant (upper left, upper right, lower left and lower right) of the orogen the rocks are in during a particular foliation-farming event. This separation of an orogen into four quadrants with respect to the bulk sense of shear decting an area is a product of the symmetrical geometry of uplift and gravitational spreading. For asymmetric orogens, discussed later, geometric symmetry

pb. 19. (a) Shows a change in the asymmetry of a microfold within a garnet porphyroblast that disrupts the apparent spiral shape. Note the foliation, microfold and quartz pressure-fringe/strain shadow truncations. Section cut parallel to stretching lineation (P-section). Partially crossed polm. Base 4.1 mm. (b) Sketch showing the locations of inclusion trails in (a).

Fig. 19. (b) Sketch showing the locations of inclusion trails in (a).

is disturbed, but the same uplift-collapse processes operate and the orogen can still be divided into four bulk sense-of-shear quadrants.

In Fig. 25c, the combined F:-F: (terminology of Bell & Duncan. 1978) inclusion trail geometries forming in porphyroblasts on the long limbs of F', folds, located on the long limbs of F: folds in the lower half of the orogen, have spiral shapes with an apparent rotation of about 180". The inclusion-trail geometries developed in the equivalent locations on the long limbs of F: folds in the upper half o f the orogen show a change in asymmetry from D, to D3. This is because rocks on long limbs of D3 folds in the right half of the orogen are dected by a left-side-up sense of shear as uplift occurs in the orogen core. Rocks on long limbs of D3 folds in the left half of the orogen are therefore affected by a right-side-up sense of shear.

In Fig. 25d, a spiral-shaped inclusion trail pattern develops on the long limbs of F: folds in the lower half of the orogen, because the sense of shear on long limbs is the same for D, and D,. The asymmetry of crenulations on long limbs reversa in the upper half of the orogen from D,


W

pb. #). Model showing the p r o m v e development of spiral-shaped inclusion trails in a garnet porphyroblast. See text for explanation. Note how the pressure-fringe/strain-shadow quark gets crenulatcd by the subsequent stage of orthogonal foliation formation.


Implications for structural relationships in thrust terrains

IQg. 21. Garnet porphyroblast, the rim of which has overgrown a crcnulation with an anastornosing axial plane trace. Note that the quartz-rich region in this photo is a strain shadow and not a pressure fringe. Crossed polars. Base 5.2 mm.

to D,, resulting in inclusion-trail geometries that superficially resemble a spiral shape, and, without careful examination, may not be recognized as a change in asymmetry. ‘Millipede’ geometries form in zones of nearcoaxial vertical shortening, and may form in areas where ‘millipedes’ have already formed leading to porphyroblasts that preserve overprinting ‘millipedes’.

Implications for the interaction between continental collision and thrusting

Elliott (1976), in a now classic paper, argued that the motion of thrust sheets is due to gravitational spreading resulting from uplift and the formation of a topographic high. However, many researchers appear to have disregarded the full implications of his work, with a few notable exceptions (e.g., Platt et al., 1983; Law et af., 1984, 1986; Platt, 1986). This may be due to the obvious relationship of orogeny in the Himalayas to horizontal compressive forces arising from continent-continent collision, and Chapple’s (1978) model indicating that an external driving force could emplace thrusts, plus Elliott’s inability to satisfactorily resolve the problem of what causes uplift. However, Chapple’s (1978) model requires that stress be transmitted through a relatively strong medium that does not deform much, which does not apply to rocks from the cores of orogenic zones, such as the Appalachians and the Himalayas.

Our results provide a solution to the problem of uplift that Elliott did not resolve. The progressive development of successive near-orthogonal foliations and the deformation history evidence recorded in these garnet porphyroblasts (e.g., the ‘millipedes’) indicate that orogenesis initially involves crustal shortening and the development of steep foliations with vertical stretching lineations and consequent uplift. This leads to gravitational instability generating vertical, near-coaxial bulk shortening of the u p lifted pile, which transforms into noncoaxial shortening plus shearing and associated thrusting on the margins of the orogenic zone (Figs 24 and 25).

Complex histories of foliation development have been mgnized in thrust terrains for a number of years, wherever detailed meso-and microstructural work has been done (e.g., Carreras et al., 1977; Bell, 1978; Mitra & Elliott, 1980; Hutton, 1983; Platt, 1983, 1984; Platt et al., 1983; Hanmer, 1984; Mitra, Yonkee & Gentry, 1984; Verhoef ct al. 1984; Mitra & Yonkee, 1985; O’Brien, 1985; Brunel, 1986; Meneilly & Storey, 1986; Platt & Behrman, 1986; Casas & Munoz, 1987; Lacassin, 1987; Ratschbacher & Oertel, 1987). We suggest that many, if not most of these structures have a similar origin to the small-scale overprinting structures preserved in porphyroblasts described in this paper.

Figures 24 to 26 show how the shallowly dipping foliations (including mylonitic foliations formed along thrust horizons) associated with gravitational spreading and the basal detachment to this zone (Figs 24b & 25b) are folded during subsequent shortening when horizontal compression dominates over gravitational instability because of the reduction in height of the collapsed pile (Figs 24c, 25c, 26a). When the height of the orogenic pile has increased to the stage that it becomes gravitationally unstable, spreading occurs and associated shallowly dipping foliations form (including mylonitic ones along zones of more intense deformation). These overprint the previously formed foliations (Figs 2 4 , Z d , 26b). In mylonite zones, they result in the overprinting and truncating mylonitic foliations and refolded geometries that are so typical of these rocks (documented by many of the above authors). Some of these geometries are products of remylonitization of anastomosing mylonitic foliation (e.g. Bell & Hammond, 1984), but many could result from remylonitization of a mylonite zone folded about steeply dipping axial planes. The steep axial planes of folded mylonitic foliations rotate towards the mylonitic foliation forming during the subsequent gravitational spreading stage of orogenesis. All stages of this transition can be preserved (e.g., Platt, 1983).

Implications for P-T-1 paths during metamorphism

The model for orogenesis shown in Figs 24, 25 & 26 has considerable implications for P-T-t paths during metamorphism (cf. Thompson & England, 1984). Spear & Selverstone (1983a and b), Selverstone, Spear, Franz & Morteani (1984), Spear, Selverstone, Hickmott, Crowley & Hodges (1984), and Spear & Rumble (1986) have shown that P-T-t paths can be obtained from zoned minerals such as garnet. However, some garnet zoning is controlled by and can be correlated directly with the various stages of near-orthogonal foliation development described in this paper (Figs 16-18).

The inclusion-trail geometries in these garnet porphyroblasts, in combination with meso- and macroscale fold and foliation geometries, can be used to track the location of a


Fig. 22. (a) Garnet porphyroblast from Vermont that has well-developed spiral-shaped indusion trails. Partially crossed polars. Base 13.2 mm. (b) Detailed examination of the porphyroblast shown in (a) rev& multiple nenulations, which were formed by crenulation of the foliation surrounding the garnet during a foliation-overprinting event, and subsequently overgrown and preserved by the garnet. Note the foliation truncations. Section cut perpendicular to stretching lineation (N-section). Partially crossed polais. Base 5.28 mm. (c) Sketch of the location of inclusion trails in (b).

Fig. 23. Series of sketches showing how single, and overprinting ‘millipedes’ can form. See text for explanation.


\

(d)

pb. 24. Series of schematic diagrams showing the tectonic modcl necessary to explain the orthogonal overprinting character of foliations preserved as inclusion trails in porphyroblasts, and shown schematically in Fig. 25. See text for dicrcussioo.

Fig. 25. A series of schematic diagrams showing how the inclusion trail geometrics that we obse' e can develop and would vary in a fold belt. The folds and inclusion trail geometries are drawn for long limbs of overprinting folds because these should dominate any particular volu6e of rock. Near-vertical S, (---) forms in (a), near-horizontal S, (.f-) forms in (b), near-vertical S, (.--.) forms in (c) and near-horizontal S , (dots) forms in (d). See text for discussion.

T


Fig. 26. Series of sketches showing schematically the complications possible in porphyroblast inclusion trail geometries when the opposite sense of shear exists in the zones of shortenjng shown in Figs 25c and 25d. This results in the development of short Limbs of folds in the previously formed deavage. on which crenulations develop with the opposite asymmetry. The location of the inclusion trail geometries within these folds is shown by dots in (a). In (b) the inclusion trail geometries labelled A, B, etc. occur in the sites with those labels. The two possible inclusion trail geometries for each site result from D4 overprinting of the F: asymmetries in (a).

rock during orogenesis in an orogenic belt (Figs 25 & 26). This information can potentially be combined with P-T-t path determinations from the compositional zoning (Spear trails '

& Selverstone, 1983a,b; Spear et uf., 1984), allowing a more detailed understanding of the relationships between The sense of shear determined from spiral-shaped structure and metamorphism during orogenesis. inclusion trails formed in the manner described here is

Implications for sense of shear determined from porphyioblasts containing spiral-shaped indusion

306 T. H. BELL & S. E . JOHNSON

exactly opposite to that inferred when the porphyroblast is interpreted to have rotated during growth (e.g., Bell & Hammond, 1984; Bell, 1985). It is noteworthy that very few papers record, in any detail, attempts to use spiral-shaped inclusion trails to detezmine the sense of shear, with the exception of Rosenfeld (1968).

We would expect considerable problems to arise if one tried to determine the sense of shear with spiral-shaped inclusion trails using all the garnet porphyroblasts in several thin sections cut in three mutually orthogonal orientations from an oriented specimen. For instance, the sense of shear of ‘spirals’ in porphyroblasts in Figs 1 & 2, which superficially have circular inclusion trails around their rims and more obvious ‘spirals’ in their cores, is not consistent, with three one way and five the other. Meneilly & Storey (1986) mentioned that out of ten thin sections containing garnet porphyroblasts, eight with spiral-shaped inclusion trails showed one sense of shear and two showed the other. This may explain why, even though these garnet porphyroblasts are common in some orogenic belts, such as those in the Himalayas (Powell & Vernon, 1979; Brunel, 1986), Vermont (Rosenfeld, 1968), New Caledo- nia (Bell & Brothers, 1985), New Zealand (Johnson, 1989), European Caledonides (Schoneveld, 1977), Alps (Schoneveld, 1977) and the South Orkney Islands (Meneilly & Storey, 1986), they are seldom used to determine senses of shear.

Figures 25 and 26 show some of the potsntial complications in sense of shear obtained from inclusion trails in porphyroblasts that nucleate and grow on different fold limbs in an orogenic belt. Fully spiral-shaped inclusion trails dominantly develop only in the deeper parts of orogenic belts, as shown in the lower half of Fig. 25c and 25d. They can develop in the upper half of the metamorphic belts, provided long limbs and short limbs of folds alternately overprint one another, as shown in Fig. 26b (the central garnet porphyroblasts A and B). Changes in sense of shear produce quite subtle effects on the inclusion trail geometry (Fig. 26). They generally go unnoticed because careful, detailed microstructural analysis is needed to differentiate between them (e.g. Figs 8d and 9d).

Hence, inclusion trail geometries that superficially appear to have spiral shapes, can involve a change in sense of shear. Once the detailed analyses are done, a statistical approach must be employed before these porphyroblasts can be used as indicators for bulk-scale shear, owing to the complications of repeated fold and foliation overprinting illustrated in Fig. 26. Long limbs should dominate the volume of rock affected by these folding events (Fig. 25), and the sense of shear determined from porphyroblasts collected on these long limbs should give the bulk shear for that part of the orogenic belt.

Implications for lack of porphyroblast rotation

Bell (1985) argued that during a general ductile deformation history involving partitioning of the deformation, porphyroblasts do not rotate relative to geographic

coordinates. Studies in which the orientations of inclusion trails in porphyroblasts with relatively simple inclusion- trail geometries were compared over large areas, have strongly supported this argument (e.g., Fyson, 1975, 1980; Johnson, 1989; Steinhardt, 1989).

It is also strongly supported by the results of this study. The porphyroblasts come from areas regarded as containing classic examples of spiral-shaped inclusion trails produced by rotation during growth of the garnet. Yet, we have not found one example of a porphyroblast that can be demonstrated to have rotated during growth. On the contrary, we have demonstrated that they preserve up to eight foliations (depending on the area) with consistent orientations from porphyroblast to porphyroblast in rocks that have undergone intense non-coaxial deformation at various stages during their development. This has significant implications for fold mechanisms (T. H. Bell, 1985, unpublished data; Johnson, 1989; Steinhardt, 1989). Similar overprinting and truncational microstructures occur in other porphyroblasts such as plagioclase and cordierite (T. H. Bell, unpublished data; S. E. Johnson, unpublished data) and non-equidimensional porphyroblasts such as staurolite and andalusite (F. van der Hor, unpublished data; C. K. Steinhardt, unpublished data; C. Wilkins, unpublished data).

Implications for structural symmetry across orogenic belts

The orogenic model is drawn symmetrically in Figs 24-26. Although these diagrams are schematic, we expect a gross degree of structural symmetry across an orogenic belt, especially if it has been only affected by one orogeny because uplift of a linear belt of rock will generally result in a similar surface slope in both directions out from the belt’s margins. A number of well known orogens are structurally symmetric in cross-section, such as the Pyrenees (Zwart, 1979), the Grampian orogeny, Central Highlands, Scotland (Thomas, 1979, figs 4 & 5; Thomas, 1980, fig 8) and the Otago Schist, New Zealand (S. E. Johnson, unpublished data). However many superficially asymmetric orogens have a strong degree of symmetry at the surface which is minimized at depth by the tectonic prejudices of the person drawing the cross-section. For example crustal-scale cross-sections through the Hima- layas commonly minimize the change in asymmetry of folds and thrusts that has been mapped by Le Fort (1975, fig. 5) and Searle, Windley, Coward, Copper, Rex, Rex, Tingdong, Xuchang, Jan, Thakur & Kumar (1987, fig. 5). Recent work by Wang & Chu (1988, fig. 3) shows that further to the east, this orogen is almost perfectly symmetrical. Of course the high Tibetan Plateau could affect this symmetry further to the west. Similarly, recent cross-sections through the Alps suggest that the Insubric Line marks a deep rooted structure that overthrusts to the south (Heizman, 1987), giving this orogen a degree of symmetry at deep as well as at shallow levels (e.g., de Jong, 1979).

Where the orogen overprints previously deformeb-and


metamorphosed rocks close to a continental margin, gravitational collapse and resultant thrusting on the edges of the orogen would be structurally more asymmetrical. However, the subduction zone running along the western edge of the Andes is mirrored by a major thrust belt that runs along their eastern edge (e.g., Farrar, Clark, Kontak & Archibald, 1988).

Another cause for asymmetry may be that crust on one side of the orogen behaves more rigidly than the crust on the other, as shown by Weber (1984, fig. 5 ) for the Variscan Orogen. However, this asymmetry could simply be the effect of overprinting by a subsequent orogeny and uplift to the SE causing overthrust of the Mid German Crystalline Rise over the rocks to the NE.

Many orogens are only half preserved, in that the core of highest grade rocks occurs on one side, being truncated by a subsequently formed major extensional fault that drops the crystalline rock and forms a basin for sedimentation above; e.g., the eastern margin of the Adelaide Geosyncline (C. K. Steinhardt, unpublished data) and the NE margin of new Caledonia (Brothers, 1985). Perhaps the hot core of the orogen acts as a site for nucleation of a basal detachment surface during subsequent extension.

Implications for orientation of thin sections used in andysing orogenic belts

Microstructural examination of rocks based on N, P and S sections relative to foliations and lineations in the matrix provides limited direct information about the orogenic development of mountain belts. This is because the matrix foliation and lineation are subject to all the rotation effects of reactivation of earlier foliations, in particular reactivation of bedding (Bell, 1%), and progressive orogenesis as described here. This includes the heterogeneous rotation of the fold belt towards the bulk compressive movement direction that causes orogenesis (T. H. Bell & S. E. Johnson, unpublished data). A more useful approach is to orient thin sections relative to the bulk movement direction of the compressive event driving the orogeny. This direction can be bound by determining the orientation of the stretching lineation in shallowly-dipping foliations preserved in the cores of porphyroblasts. Sections in three dimensions allow determination of the orientation of this foliation within the porphyoblasts. A section parallel to this foliation enables the stretching lineation within the porphyroblast to be determined.

The first thin sections cut for all rocks across the orogen should be vertically dipping and contain this direction, regardless of the orientation of the matrix foliation and lineation. They should be cut looking in the same direction relative to the orogenic belt, with the long axis of the section accurately and constantly oriented perpendicular or parallel to the horizontal. This enables direct comparison of the asymmetry of foliation overprinting relationships preserved as inclusion trails in the porphyroblasts as well as in the matrix. It also enables these relationships to be

plotted directly onto a cross-section, where they are most useful for determining movement paths during orogenesis.

CONCLUSIONS

1. Spiral-shaped and other complex inclusion trails in garnet porphyroblasts indicate that orogenesis involves a repeating cycle of uplift and collapse. Horizontal compression, presumably caused by convergence of lithospheric plates, generates a near-vertical foliation with a down-dip stretching lineation, which accommodates uplift. When the pile reaches a critical height and thickness, it becomes gravitationally unstable. The unstable pile collapses, resulting in nearaaxial vertical shortening on the bulk scale, gravity spreading and formation of a near-horizontal foliation. The pile becomes gravitationally stable, completing one uplift-collapse cycle, at which point uplift again dominates. ‘Ihe cycle is repeated many times, as indicated by porphyroblast and matrix microstructures in Nepal, Pakistan and Vermont, which record up to four full cycles.

2. P-T-t paths in synmetamorphic thrust belts may be complex and highly variable (e.g., Chamberlain & Karabinos, 1987). Changes in garnet porphyroblast and inclusion composition from core to rim can be used to track the P-T-t path in relation to the various stages of foliation development preserved within the porphyroblasts as inclusion trails, and hence the structural/metamorphic pathway the rocks have followed during orogenesis.

3. None of the many hundreds of garnet porphyroblasts we have studied, including those with spiral-shaped inclusion trails, require a history of rotation with respect to geographical coordinates during growth. Therefore, any discordance between the orientation of inclusion trails and the surrounding matrix foliation is due entirely to rotation of the matrix foliation. This suggests that many structures that look like S and C planes and ‘extensional crenulation cleavages’ are rotated relics of a complex history and can give the wrong bulk sense of shear if interpreted

4. Porphyroblasts with spiral-shaped inclusion trails that formed without rotation during growth preserve microstructures that indicate the opposite sense of shear to that inferred when the porphyroblast is interpreted to have rotated during growth. Hence, considerable care must be taken when using garnet porphyroblasts with spiral-shaped inclusion trails as sense of shear indicators.

5. The deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis may be relatively uncommon, and spatially quite restricted in continental crust. In conditions of ductile deformation, this environment is confined to vertical, ductile tear/transcurrent faults, across which there is no component of bulk shortening or transpression. In conditions of brittle deformation, this environment is confined to zones of brecciation and faults other than thrusts, with a rotational component of movement.

superficially.

308 T. H. BELL & S . E . JOHNSON

ACKNOWLEDGEMENTS

We thank Mike Rubenach for the specimens from Nepal and Ron Vernon and Chris Powell for the specimens from Pakistan. Bell acknowledges the support of NSF Grant EAR 8406572 that enabled him to map and collect specimens from Vermont. Johnson acknowledges support from an NSF Graduate Fellowship and James Cook University. Mark Hinman, Pat James, Kingsley Mills and Ron Vernon constructively criticized various versions of this paper and we thank them for their comments.

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fold axes in a ductile mylonite zone, Central Australia. Tectonophysia, 44,285-320.

Bell, T. H., 1981. Foliation development: the contribution, geometry and significance of progressive bulk inhomogeneous shortening. Tectonophysics, 75, 273-2%.

Bell, T. H., 1985. Deformation partitioning and porphyroblast rotation in metamorphic rocks: a radical re-interpretation. Journal of Metamorphic Geology, 3, 109-118.

Bell, T. H., 1986. Foliation development and refraction in metamorphic rocks: reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning. Journal of Metamorphic Geology, 4,421-444.

Bell, T. H. & Brothers, R. N., 1985. Development of P-T prograde and P-retrograde/T-prograde isogradic surfaces during blueschist to eclogite regional metamorphism in New Calcdonia as indicated by progressively developed porphyroblast microstructures. J o u d of Mekzmorphic Geology, 3,

Bell, T. H. & Duncan, A. C., 1978. A rationalized and unified shorthand terminology for fold axes and lineations in tectonites. Tectonophysics, 46, Tl-T5.

Bell, T. H., Duncan, A. C. & Simmons, J. V., 1989. Deformation partitioning, shear zone development and the role of undeformable objects. Tectonophysics, 15s.

Bell, T. H., Fleming, P. D. & Rubenach, M. J., 1986. Porphyroblast nucleation, growth and dissolution in regional metamorphic rocks as a ‘function of deformation partitioning during foliation development. Journal of Metamorphic Geology,

Bell, T. H. & Hammond, R. L., 1984. On the internal geometry of mylonite zones. Journal of Geology, 92, 667-686.

Bell, T. H., Reinhardt, J. & Hammond, R. L., 1988. Multiple foliation development in thrust terrains: A new interpretation of the regional structure of the Mt Isa Block. Australia. Journal of Structural Geology, submitted.

Bell, T . H. & Rubenach, M. J., 1980. Crenulation cleavage development - evidence for progressive bulk inhomogeneous shortening from ‘millipede’ microstructures. in the Robertson River Metamorphics. Tectonophysics, 68, T9-Tl5.

Bell, T. H. & Rubenach, M. J., 1983. Sequential porphyroblast growth and crenulation cleavage development during progressive deformation. Tectonophysics, 92, 171-194.

Brothers, R. N., 1985. Regional mid-Teriary blueschist-eclogite metamorphism in northern New Caledonia. Geologie de la France, 1, 37-44.

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Carreras, J., Estrada, A. & White, S., 1977. The effects of folding on the c-axis fabric of a quartz mylonite. Tectonophysics, 39,

Caras, J. M. & Munoz, J. A., 1987. Sequences of mesostructures related to the development of Alpine thrusts in the Eastern Pyrenees. Tectonophysia, Us, 67-75.

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Received 16 November 1987; revbion accepted 10 May 1988

APPENDIX

Indusion-trail analysis

Several N, P and S thin sections were examined for each rock from Nepal, Pakistan and Vermont. Every garnet porphyroblast in these thin sections was photomicrographed, keeping the section in a constant orientation so that the orientations of inclusion trails in the porphyroblasts could be compared from photomicrograph to photomicrograph. Each porphyroblast was then examined at high magnification under the microscope, and the microstructural relationships marked on the photomicrographs.

After examining hundreds of garnet porphyroblasts, photomicrographs of eight N-section porphyroblasts, one P-section porphyroblast and one S-section porphyroblast from each of the three areas were chosen for presentation. Thcsc porphyroblasts are shown in mosaic figures (Figs 2-9) so that comparison of inclusion-trail orientations can be made from porphyroblast to porphyroblast. These porphyroblasts where chosen as repre- sentative samples based on three criteria: clarity of inclusion trails, quantity of inclusion trails and variation in the orientation of inclusion trails. We chose to present eight N-section porphyroblasts, rather than eight P or S section porphyroblasts, because this orientation best reveals crenulation relationships in the matrix and in the porphyroblast inclusion trails. The P and S sections were included to show that similar microstructural relationships occur in all three sections. The preservation of most of the foliations in all three orthogonal sections is partly a function

310 T. H. BELL 81 S . E . JOHNSON

of the somewhat spherical shapes of the garnet porphyroblasts. There is a strong tendency for foliations to anastomose around them in three dimensions. These sections are also generally oblique to the shallowly dipping foliation and stretching lineation preserved in the porphyroblasts, due to the rotation effects of progressive orogenesis on the matrix, but not on the porphyroblast (see implications for section orientations in text).

Correlahn of indudon M s

Figures 2-7 show the oriented N section garnet porphyroblasts from Nepal, Pakistan and Vermont respectively. Figures 8 and 9 show one P section and one S section for each of the three areas. Different criteria were used to correlate inclusion trails for each of the three areas, because of variations in the composition and concentration of inclusion trails. For example, the Nepal porphyroblasts (Figs 2 & 3) have a core with a high concentration of inclusion trails bound by a consistently oriented truncation surface (&). It commonly indudes a smaller core with an even higher concentration of inclusion trails. Outside this core, the inclusion trails in all the garnet porphyroblasts are lcss concentrated and have generally changed composition, with an increase in the number of opaque grains and a decrease in the number of quartz grains. Working inwards from the S, truncation surface, a maximum of two earlier foliations arc seen in most of the porphyroblasts (e.g., Figs 2a-f). Thus, the truncating foliation dividing this core from the outer portions of the porphyroblasts

with lower concentration of inclusion trails was labelled S3. Working outward from the S, foliation, a maximum of three later foliations are seen in the porphyroblasts. Thus, up to six separable foliations are preserved in the garnet porphyroblasts, most of which are not preserved in the matrix.

In the Pakistan garnet porphyroblasts (Figs 4 & 5). the inclusion trail concentration is consistent from the core to the outer margins, except for a decrease in quartz content at stage 6 and a tendency for a decease in carbon content at stage 5 . There is a tendency for quark pressure hinge/strain shadows and ‘millipedes’ to occur at a particular stage of the garnet development. Working inwards and outwards from this ‘millipede’ geometry led to the labelling of the foliation associated with this ‘millipede’ forming event as h.

In the Vermont garnet porphyroblasts (Figs 6 & 7), one correlation criterion used is the sudden graphite enrichment of the inclusion trails .a t a particular stage of garnet development. Working inward and outward from this graphite-rich truncating foliation leads to labelling it S,. Another criterion used in the Vermont garnets is the consistent drop in the amount of blocky quark pressure and strain shadows at the S, truncation, and an overall decrease in quartz content at stage 6.

In summary, garnet porphyroblasts from each of these areas show some major change in composition, concentration or shape of inclusion trails at some point in their history. These changes are consistent for garnets from any OM area and were used as ‘structural markers’ for inclusion trail correlations.

Bell, T.H. and Johnson, S.E., 1989a. Porphyroblast inclusion trails: the key to orogenesis

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