Bell, T.H. and Hammond, R.L., 1984. On the Internal Geometry of Mylonite Zones

On the Internal Geometry of Mylonite ZonesAuthor(s): T. H. Bell and R. L. HammondReviewed work(s):Source: The Journal of Geology, Vol. 92, No. 6 (Nov., 1984), pp. 667-686Published by: The University of Chicago PressStable URL: http://www.jstor.org/stable/30070500 .Accessed: 09/03/2012 23:24

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ON THE INTERNAL GEOMETRY OF MYLONITE ZONES1

T. H. BELL AND R. L. HAMMOND

Department of Geology, James Cook University, Townsville, Q. 4811, Australia

ABSTRACT

Mylonite zones are generally characterized by abrupt and very large strain transitions, which commonly result in excessively anastomosing schistosities on a wide range of scales when compared with non- mylonitic foliations. This geometry is very susceptible to remodification during progressive mylonitization, resulting in unusual and complex fold, lineation, and foliation geometries and interrelationships. Open folds of the mylonitic foliation with axes parallel to the stretching lineation in the surrounding mylonite cannot have formed by the rotation of fold axes through a large angle within their axial planes, as has been usually proposed for isoclinal and sheath folds in mylonitic zones. Open folds initiate with axes parallel or close to the stretching lineation due to the geometric effects of folding a mylonitic foliation, which anastomoses around an ellipsoidal pod of less deformed material. This initial geometry also allows the generation of fold axes curved within their axial plane through 1800 about the stretching lineation at the time of nucleation. Successive mylonitic foliations develop during this folding and refolding process with boundaries that truncate and isolate earlier fold hinges and portions of fold limbs. As a consequence, stretching and intersection lineations can vary from plane to plane through the mylonite zone, although careful examination often reveals a weak overprinting stretching lineation parallel to the bulk movement direction for the whole zone. Fold asymmetry in mylonite zones is a potential indicator of shear sense across a zone, if the fold axes lie at an angle to the bulk stretching lineation direction. In such circumstances, however, a single asymmetry projected onto a plane perpendicular to the mylonitic foliation and containing the bulk stretching lineation can indicate either sense-of-shear depending on a variety of factors. These include whether the foliation folded is primary or mylonitic, and in the latter case whether the mylonite zone formed with a steep-dip and horizontal stretching lineation or in some other orientation. The most satisfactory sense-of-shear indicator is the asymmetry of S and C planes.

INTRODUCTION

Over the last decade considerable ad- vances have been made in our understanding of the microstructural development of mylonitic rocks. However, their development on a macro- and mesoscale has received far less

critical examination. These rocks commonly contain complex internal geometries formed in a single overall mylonitization period and consequently differ from slate and schist belts where geometrical relationships appear to be more straightforward (e.g., Hobbs et al. 1976).

Thus mylonites commonly contain structures which fold and refold the mylonitic foliation and whose axial surfaces parallel the mylonite zone. How are such structures developed? These folds often fold elongation and intersection lineations that lie on the mylonitic foliation surface. They also have axes which bend through 1800, indicating appar-

1 Manuscript received January 9, 1984; revised June 13, 1984.

[JOURNAL OF GEOLOGY, 1984, vol. 92, p. 667-686] © 1984 by The University of Chicago. All rights reserved. 0022-1376/84/9206-005$1.00

ently very complex geometries. How can the stretching lineation vary sufficiently from the bulk movement direction to be redeformed?

Other folds of the mylonitic schistosity appear to have formed parallel to the stretching lineation in the foliation that is folded; in particular, this includes relatively open structures which could not conceivably have rotated into such an orientation by inhomogeneous strain without becoming tight to isoclinal (Sanderson 1973). Does the old problem of fold axes forming parallel to the movement direction (tectonic "a" direction, e.g., Cloos 1946) still exist and is their origin similar to that suggested by Christie (1963) or perhaps Nicolas and Boudier (1975)?

Fold asymmetry is a criterion sometimes used to interpret the sense-of-shear in mylonite zones. However, many zones exhibit both senses of asymmetry (Christie 1963; Bryant and Reed 1969; Bell 1978; Mawer 1983) whereas others only exhibit one sense (Ponce de Leon and Choukroune 1980). What is the significance of this and can asymmetry be used in this manner?

This paper attempts to explain the origin of these and other unusual geometrical relationships.

667


THE ANASTOMOSING CHARACTER

OF MYLONITIC FOLIATION

The intensely heterogeneous strain associated with mylonitic rocks appears to us to be the predominant cause of the complex array of geometries they exhibit. The anastomosing geometry of mylonitic foliation (Bell 1978; Mitra 1978; Choukroune and Gapais 1983; Mawer 1983; Simpson 1983) on various scales is an obvious characteristic of major mylonite zones especially near their margins (fig. 1). This aspect, which can vary from porphyro- clast to mountainside, produces a geometry that is susceptible to later modification during progressive deformation.

Strain transitions are commonly very abrupt in mylonitic rocks (fig. 2). Geometri- cally this necessitates a highly anastomosing foliation for a coaxial or non-coaxial bulk inhomogeneous shortening deformation history (Bell 1981). Similar characteristics are probably also shared by rocks deformed by progressive inhomogeneous simple shear. Such a strain history possibly could produce extreme strain heterogeneity without the development of any anastomosing character, but this requires that the flow rate along any one plane parallel to the shear zone remains constant. Ductility contrasts in the primary rock, at least on the grain scale (e.g., quartz and mica versus feldspar), render this unlikely. Consequently, mylonitic foliation developed by such a deformation history would also have to anastomose around these in- homogeneities if the zone were to remain planar (notably this will generate local thickening and thinning of layers).

The excessively anastomosing character of most mylonites provides a relatively unique micro-, meso-, and macroscopic source for many of the unusual internal geometries developed in mylonite zones; this theme is extensively developed in this article. The development of planar slaty mylonites (phyllonites) is also considered.

FOLDS IN MYLONITIC ROCKS

Two types of folds can be observed in mylonite zones, though the second type pre- dominates. They are:

1. Folds involving earlier non-mylonitic foliation or layering (Ss) with mylonitic schistosity (Sm) forming parallel to their axial planes (Fsm; terminology after Bell and Dun- can 1978). These folds are geometrically similar to those found in slate, schist, or gneiss belts, and consequently are not considered in detail.

2. Folds of the mylonitic foliation itself, yet with axial planes parallel or approximately parallel to the mylonite zone boundaries which occasionally have newly developed mylonitic foliation on their limbs. These appear to be the most common folds in many mylonite zones (Rhodes and Gayer 1977; Bell 1978; Williams 1978; Cobbold and Quinquis 1980) and are apparently mainly confined to such zones (fig. 3). We use the notation FM as an abbreviation of F +, where n can range from 1 to at least 5. Multiple generations of mylonitic foliation can apparently form during the one protracted period of mylonitization (e.g., fig. 4), and consequently designat- ing Smi, Sm2, Sm3, etc. is meaningless except on a small scale where all generations of foliation can sometimes be identified (e.g., fig. 11A in Bell 1978).

FMl folds can occur on almost any scale, for example up to 10 m across in the Bruers Knob area on the Mann Fault Mylonite Zone. Two or three refolds of the mylonitic schistosity frequently can be recognized within their hinges (e.g., fig. 5). FM folds are often truncated on one or both sides by mylonitic foliation identical in appearance to that folded. This results in isolated pods of fold hinge (fig. 4b) or mylonitic foliation, abruptly truncated by that immediately adjacent (fig. 4a). Such structures are some of the most distinctive and characteristic features of many mylonite zones and could be unique to

FIG. 1.-A structural map of the Bruer's Knob area in the Mann Ranges, Central Australia. F5 folds up to 10 m across, cut off by mylonitic foliation, occur in this area. They are sometimes located around the ends of ellipsoidal pods of granulite facies gneissic country rock. The mylonitic foliation anastomoses and has been folded (by progressive mylonitization) on a large variety of scales and as a result has a complex final distribution even though no post-mylonite deformation has occurred in these rocks. Note the anastomosing character of Sm in the north to south transition zone from country rock to mylonite.

668

STRUCTURAL GEOLOGY OF BRUERS KNOB, MANN RANGES

100 metres

Country rock toisotion S~

6 mineral elongation CS

~ Mylonitic schistosity Sm

+ intersection lineotion otten porollel to L~

+ mineral m elongation Cm

~ Fold axis FM orF~ M

-e Fold axis FM M

669


FIG. 2.-A. Photograph and B. sketch of an abrupt transition from a pod of country rock (right of photo) to mylonite (left of photo) in the Woodroffe Thrust Mylonite Zone. The country rock foliation Ss is variably sheared against the mylonitic schistosity Sm and the sense of rotation changes from the top to bottom of the outcrop. The stretching lineation Lm plunges at a shallow angle to the outcrop surface. The geometrical relationships shown are indicative of a deformation history of bulk inhomogeneous shortening on the scale of the photograph.

them. We regard these structures as criteria for distinguishing mylonitic rocks, which are otherwise difficult to recognize (e.g., Ham- mond in prep).

Origin of Fu4 Folds and Multiple Sm Gener- ations.-In the early stages of mylonitization (c.f. the margins of mylonite zones) the foliation commonly anastomoses on a wide range

FIG. 3.-Photograph of an FS fold; i.e., a fold of the mylonitic schistosity yet with mylonitic foliation developed on its limbs parallel to the axial surface and commonly enclosing it so that it is in- trafolial in character.

of scales around ellipsoid-shaped pods of less deformed country rock (fig. 1; c.f., Bell 1978; Mitra 1978; Choukroune and Gapais 1983; Mawer 1983; Simpson 1983; Garcia Celma 1983). This variably oriented foliation, which does not everywhere lie in the XY plane of the bulk strain ellipsoid, is susceptible to modification by folding (figs. 3, 5, and 6) and/ or remylonitization as mylonitization of the enclosed ellipsoidal pod continues (fig. 7c, 7e, and 7g). This process does not cease once the country rock pod has been completely mylonitized because portions of older mylonitic foliation lie oblique to the bulk foliation plane, generally surrounded by anastomosing slightly younger mylonitic foliation. Consequently, the process can continue, forming multiple generations of Fj folds and mylonitic foliation (c.f. fig. 4) during a single deformation event.

Open F J Folds Parallel to Stretching Line- ation Lm.-FM folds vary considerably in tightness and in their effect on lineations generated during the earlier stages of my-

670

FIG. 4.-Photographs of mylonite from the Woodroffe Thrust showing small scale FM folds truncated by Sm on one (A) or both limbs (B). Note the abruptness of the strain transition from one generation of mylonitic foliation to the next and the similar character of the foliation in both cases. Note also the development of oblique foliations on the right hand side of figure 4A. Of particular significance is the portion of younger mylonitic foliation which stops abruptly in the centre of the photograph shown in A (c.f. fig. 16). Length of photograph A is 13 cms and B is 6.5 cms.

671


FIG. 5.-Photograph of two FMt folds of different generations in the Mann Fault Mylonite Zone, Central Australia. Mylonitic schistosity Sm has been folded around the smaller scale fold on the right side and then refolded around the larger fold on the left side during progressive mylonitization. The axial plane of this second fold is parallel to the mylonite zone boundaries. The geology hammer in the figure centre is used as a scale.

lonitization. However, one particular fold shape and lineation geometry is quite common though seemingly difficult to explain. From the sketch in figure 8 it is apparent that, although the fold is quite open, the stretching lineation on the folded surface (Lm) is approximately parallel to the fold axis as well as the lineation in the surrounding unfolded mylonitic schistosity. This structure cannot be formed by rotation of the fold axis into the stretching lineation as the strain involved in a 900 rotation would make it nearly isoclinal (Sanderson 1973; Cobbold and Quinquis 1980).

The anastomosing character of mylonitic foliations provides a simple geometric solution. Figure 9 shows an ellipsoidal pod of mylonitic foliation anastomosing around a core of less mylonitized rock. When a new zone of shear strain initiates and cuts across the earlier anastomosing foliation, folds form as is shown in figure 6b. Along the sides of these ellipsoidal pods (figs. 6b and 6c) developing

folds have axes subparallel to the stretching lineation on the folded foliation.

Curved Fold Axes within the Mylonitic Schistosity.-The apparent rotation of Fs and F1 fold axes toward the stretching lineation in mylonites has often been observed. However, it is not generally recognized that apparent rotation can be a result of two quite different phenomena: (1) actual rotation of the fold axes towards the stretching lineation due to inhomogeneous strain, as shown in figure 10. This can occur with Fsm or F} folds (e.g., Rhodes and Gayer 1977; Bell 1978; Wil- liams 1978; Cobbold and Quinquis 1980); (2) apparent rotation which can only occur with FM folds by the mechanism outlined above (fig. 6). Because these ellipsoidal pods are generally elongate parallel to the stretching direction (fig. 6a) fold axes will, to a large extent, form close to parallel to the stretching lineation (fig. 6b), and proportionally only a small volume of rock will contain FM fold axes at a high angle to Lm. Consequently, fur-

672

INTERNAL GEOMETRY OF MYLONITE ZONES

FIG. 6.-a. Sketch of an ellipsoidal pod surrounded by anastomosing, mylonitic foliation similar to that shown in figure 9 showing the divergence of Lm as well as Sm around the pod with respect to the bulk strain axes X, Y, and Z. b. A sketch showing a sheath fold produced by imposing a zone of inhomogeneous shear on the upper part of an ellipsoidal pod such as that shown in figure 6a. Note the open style of the fold where it is oriented sub-parallel to the stretching lineation and that it has formed a sheath geometry without involving rotation of the fold axis. The X-axis of the bulk strain ellipse is approximately parallel to the long axis of the pod, and consequently much of the anastomosing mylonitic schistosity lies at a marked angle to the bulk XY plane. Sm in such localities is susceptible to folding during progressive mylonitization. c. Attenuation and tightening of the sheath fold produced in 6b due to further inhomogeneous deformation.

ther homogeneous or inhomogeneous strain will enhance the sheath-like character (fig. 6c).

Discussion. -Progressive mylonitization appears to be a function of strain softening effects which have been discussed in some detail recently (White et al. 1980) and consequently are dealt with no further here. How- ever, it appears that it is this sequential development combined with the commonly anastomosing character of mylonitic foliations discussed earlier that produces multiple stages or generations of mylonitic foliation and associated lineations ascribable to one overall event.

Sheath-like Fs folds presumably formed by

rotation of the fold axes towards the stretching lineation Lm, and similarly F fold hinges formed initially at a high angle to L, may have rotated towards Lm (c.f. Bell 1978; Cob- bold and Quinquis 1980). However, FM folds would tend to nucleate with axes for the most part almost parallel to L as a consequence of the triaxial ellipsoid shape of the pods surrounded by anastomosing mylonitic foliation. Proportionally only a small volume of rock will contain F fold axes at a high angle to Lm (fig. 6b and 6c). If sheath-like F folds were generally produced by rotation of fold axes nucleated at a high angle to L then one would expect to see a lot more folds axes with this geometry on the margins of, and

673


FIG. 7.-Diagrammatic sketches of the effects of deformation histories involving progressive bulk inhomogeneous shortening and progressive inhomogeneous simple shear on an ellipsoidal pod of anastomosing mylonitic schistosity (modified from Bell 1978). (A) Shows an ellipsoidal pod and the orientation of the bulk X, and Z axes of the strain ellipsoid for the mylonite deformation. (B) Shows the effect coaxial progressive bulk inhomogeneous shortening on the upper half of this pod. Y is perpendicular to the page. (D) Shows the effect of non- coaxial progressive bulk inhomogeneous shortening on the upper half of this pod. (F) Shows the effect of non-coaxial progressive inhomogeneous simple shear on the upper half of this pod. (C), (E), and (G) share the deformation histories of (B), (D), and (F) respectively, but in these cases the deformed pod has been cut by a new Sm. Note: these deformation histories generate folds of both asymmetries. The width parallel to the z-axis of B, C, and D would generally be decreased relative to A, although this is not shown.

FIG. 8.-A sketch of open F1 folds showing the parallelism of fold axes and the stretching lineation (Lm) on the folded mylonitic foliation.

perhaps within, mylonite zones. Such folds presumably form at any time during mylonitization and consequently all stages of their development should commonly be preserved. If rotation was the dominant process, examples of geometries somewhat similar to those shown in figure 11, where Lm is also rotated, could be expected to be more common even though the rotation was non-passive. Re- orientation of stretching lineations is observed in some mylonite zones (e.g., fig. 12), but the geometry illustrated in figure 1lb is rare, suggesting that passive rotation of fold axes is relatively uncommon and that the geometry shown in figure 12 is a result of the refolding of an anastomosing portion of the mylonitic foliation that does not lie in the bulk XY plane, as is described above and illustrated in progressive stages in figure 13.

A possible alternative FM fold mechanism involves inhomogeneous flow rates analo- gous to those of a flowing viscous fluid. This requires a localized variation in flow rate (strain rate) such that the uppermost limb of the fold overrides the lowermost, leading to an asymmetric, local thickening and thinning of the foliation. There appear to be two end members representing the conceivable range of three-dimensional geometries produced by this process. The first results from a zone of slow flow that fades out both forward and laterally. The resultant folds form behind the above zone and swing forward towards the

674


FIG. 9.-Photographs of mylonitic foliation (Sm) anastomosing around an ellipsoidal pod of less mylonitized material or country rock in an outcrop which is almost totally exposed in 3-D. Photograph A (top) is taken almost at 90° to B (bottom) as shown by the orientation of the hammer.

675

T. H. BELL AND R. L. HAMMOND 676

FIG. 10.-A series of sketches showing the progressive development of sheath folds (C) due to the rotation of axes initiated at a high angle to the bulk stretching direction (A).

flow direction as they decrease in amplitude and intensity (fig. 14a). The second occurs when the flow rate behind the developing folds is increased. This produces folds with the same asymmetry but arcuate in the re- verse sense (fig. 14b). However, if formation of folds by this mechanism involved only progressive inhomogeneous simple shear, specific lineation geometries relative to fold axes would result. The flow direction and stretching lineation orientation prior to fold development would be relatively constant around the developing fold. Consequently, the lineation would bend in a plane which is perpendicular to the flow plane, contains the stretching direction, and lies approximately orthogonal to the axial plane. In our experience, such lineation geometries around fold hinges are rare.

Other proposals for the formation of F M folds are the "inhomogeneity parallel to the mylonitic foliation" suggestions offered by

FIG. 11.-Two sketches showing the effect on an earlier formed stretching lineation of the formation and subsequent rotation of an FM fold initiated at a high angle to the bulk stretching direction. Figure 1 la shows the fold geometry soon after its formation. Figure 11 b shows the effect on the earlier Lm after subsequent rotation of the fold toward the bulk stretching direction. The older lineation could be expected to locally maintain a high angle to the fold axis. This geometry is rarely observed.

Rhodes and Gayer (1977), Williams (1978), Berthe and Brun (1980), Cobbold and Quin- quis (1980), Ponce de Leon and Choukroune (1980), and Lister and Williams (1983).

The striking characteristic of total obliteration of earlier developed mylonitic foliations on fold limbs over very short distances, and the development of new ones such that old and new are almost identical in appearance (figs. 4 and 15), is a significant consequence of multiple generations of F folds and Sm.


FIG. 12.-A. Photograph of the long limb (sketched in 12B) of a specimen of folded mylonitic schistosity from the Woodroffe Thrust, Central Australia. It shows that the older stretching/intersection lineation is curved around a weaker, newly formed Lm which is parallel to the length of the photograph. Width of the photo is 9 cm. B.-A 3-D sketch of the specimen in figure 12A. Short, heavier dashes on the surface shown photographed in figure 12A indicate the younger, weakly developed, mineral elongation lineation which was oriented parallel to the bulk stretching direction of the mylonite zone. C. A 3-D sketch of the underneath surface of the specimen in 12A showing the geometry of the earlier formed stretching/intersection lineation at a much lower angle to the F~ axis than that which occurs on the surface shown in A and B.

Abrupt transitions occur at a high angle to Sm

(figs. 4B and 15) and along the Sm plane itself (fig. 4A). This is a strong geometric indication of the deformation history involved because progressive inhomogeneous simple shear, without volume change, can accommodate abrupt changes in strain in one, but not in two, orthogonal directions without the cre- ation of voids (fig. 16d) or faults (fig. 16f). On the other hand, progressive bulk inhomoge- nous shortening can readily accommodate abrupt strain changes in two orthogonal directions, and the 3-D effects of this are dem-

onstrated in figure 16a, b, and c. Shear strain transitions in two directions at a high angle result in a zone of high strain that must thin relative to its original thickness (e.g., fig. 16e) if the deformation occurred at constant volume. Therefore, the deformation involved progressive bulk inhomogeneous shortening, and such a deformation history is suggested for that portion, at least, of the mylonite (c.f. fig. 2). The specimen shown in figure 4a comes from the Woodroffe Thrust which formed in the upper amphibolite facies by dominantly plastic deformation and recrystal-

677


FIG. 13.-A. Sketch of an ellipsoidal pod of mylonitic schistosity projected onto a plane parallel to the XZ plane and perpendicular to the Y axis of the bulk strain ellipsoid. The anastomosing mylonitic schistosity has been affected by a zone of shear strain parallel to and above S-S' and this has produced an asymmetric fold on the upper half of the ellipsoidal pod. The stretching lineation (Lm) is shown as faint lines. B-D. Sketches of the projec- tion of the lower surface (short limb) of the fold in la onto the plane S-S'. Figures 13C and 13D show

the effects of higher strain on the fold in 13A. The lineation geometry shown in figure 13D represents the highest strain and is similar to the lineation geometry which occurs on the fold limb in figures 12A and 12B. The lineation geometry on the upper surface of the fold (long limb) is unaffected by the folding and is similar to that shown in figure 12C.

lization mechanisms (Bell and Etheridge 1976). Consequently, we can interpret that there would have been little or no volume loss due to solution transfer. Mylonitic microstructures in general indicate dominance of plastic over solution transfer deformation mechanisms, which suggests that the strain rates involved are too great to be ac- commodated by solution transfer especially at moderate to high metamorphic grades.

LINEATION IN MYLONITIC ROCKS

Apparent Variations in Lm on Adjacent Sm Surfaces.-Another very common feature of mylonite zones is a variation of up to 400 in the stretching lineation (often an intersection lineation may lie parallel to it) in closely

678

FIG. 14.-The possible range of three dimensional geometries for folds produced by a model involving localized variation in the flow rate. Fig- ure 14A shows the result of a local decrease in flow rate in front of the developing folds. The zone of slow flow thus produced fades back into the pre- vailing flow rate both forward and laterally. Figure 14B shows the geometry produced by a localized increase in flow rate behind the developing folds and is a geometry often observed in mylonites although the lineation geometries produced by this model are very specific (see text) and not common in our experience.

spaced Sm planes (fig. 17). More extreme variations sometimes also occur (fig. 18).

An explanation can again be found in the exaggerated anastomosing character of mylonitic schistosity (especially in the earlier stages of mylonitization). Figure 6a shows that the stretching lineation on an anastomosing surface of mylonitic schistosity must vary around the ellipsoidal pod which it envel- opes, such that portions of it lie oblique to the bulk Lm even in plane projections onto the bulk Sm. Consequently, progressive mylonitization through this ellipsoidal pod (fig. 7) can generate stretching lineation on a later formed foliation plane which is oblique to earlier formed lineation on one immediately adjacent (fig. 15).

The origin of these relationships in mylonites can readily be determined because Lm on one of the S-surfaces, is bound to be close to Lm on the bulk scale. The others commonly show weakly developed younger Lm at an angle to earlier developed Lm (e.g., fig. 12a) which may or may not be parallel to Lm (the


B

FIG. 15.-A sketch showing in 3-D a new, localized zone of mylonitic schistosity cutting an ellipsoidal pod (A). The top segment of the pod would generally be displaced by this process although this is not shown (cf. fig. 7). Such a situa- tion would result in almost parallel, adjacent schistosity surfaces showing considerable divergence in their respective stretching lineations as is illustrated in b.

FIG. 16.-Schematic diagrams showing how an abrupt strain transition (at constant volume) in two directions at a high angle to each other (as photographed in fig. 4A), due to the effects of deformation histories involving bulk inhomogeneous shortening or inhomogeneous simple shear, affect a block of rock. (a) Shows the effect of coaxial progressive bulk inhomogeneous shortening. (b) Shows the effect of non-coaxial progressive bulk inhomogeneous shortening. (c) Cross-section through the diagram shown in b at the location indicated. (d) Shows the effect on non-coaxial progressive inhomogeneous simple shear. A void must form as shown if the shear zone is required to maintain a constant width. (e) Shows the result of an abrupt change in shear strain without void formation (as occurred in d). Note how the shear zone thins and consequently the deformation history involved was non-coaxial progressive bulk inhomogeneous shortening and not progressive inhomogeneous simple shear. (f) Shows the effect of non-coaxial progressive inhomogeneous simple shear with the formation of a fault instead of a void.

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FIG. 17.-Block diagrams showing Lm variation on closely spaced schistosity surfaces. In this case the cause is porphyroclasts around which Sm anastomoses and the stretching lineation diverges (c.f. fig. 7).

intersection of Ss on Sm) or the intersection lineation of mylonitic against mylonitic schistosity. This requires close examination as the eye tends to be distracted by the strong, earlier intersection/stretching lineation (e.g., figs. 12a and 18). It should be emphasized that intersection lineations generated from older on younger mylonitic foliations, or vice versa, will have the same geometry with respect to stretching lineations, as the equiva- lent Frt fold axes described earlier.

A further means of producing such variations in Lm lies with the folding process outlined earlier. At higher strains, F folds become tight to isoclinal, resulting in limb regions which may not have been re-mylonitized and retain the older Lm (figs. 4 and 12) or hinges so tight they remain unrecognized. This may produce considerable variation in lineation orientation (e.g., fig. 13). Detailed examination of slabs of mylonite showing lineation variation from surface to surface (figs. 15b and 18) reveals superb examples of the multiple generation of mylonitic foliation (e.g., fig. 4).

Obliteration of the Stretching Lineation in Slaty Mylonites.-Slaty mylonites (or phyllonites) are often developed in the central portions of major mylonite zones and commonly contain very weak to apparently nonexistent (to the naked eye) stretching lineations. These portions of the mylonite zone have generally undergone the highest strain and the most extensive recrystallization. They are phyllonitic due to the nucleation and growth of the various minerals that comprise

FIG. 18.-Photograph of extreme variations in Lm on closely spaced schistosity surfaces. Specimen from Woodroffe Thrust, Central Australia. Width of photo is 20 cms.

680


the mylonite, such that these minerals are relatively uniformly distributed with few porphyroclasts remaining (c.f. Bell and Etheridge 1976).

The paucity of stretching lineation rem- nants is due partly to the extent of the above homogenization of microstructures. How- ever, it is also considerably assisted by the mesoscopic effects of repeated mylonitization described above. Slaty mylonites occur in the most intensely deformed portions of mylonite zones and have probably undergone several cycles of mylonitization during progressive development of the zone. Conse- quently, the first-developed stretching lineations, which were probably the strongest, were obliterated as shown in figure 19 by the development of second generation stretching lineations (fig. 7). An angle greater than perhaps 100 between first and second generation lineations would mean that formation of the latter should geometrically disrupt and dis- aggregate the earlier mineral aggregates too long to be homogeneously rotated into the new stretching direction. After several such episodes the breakdown in primary grain width would be so great that no strong stretching lineation defined by elongate grain aggregates could develop.

Commonly accompanying this lineation weakening is a marked decrease in the anastomosing character of mylonitic foliations on both micro- and meso-scopic scales until they become quite planar. This is paralleled by a decrease in the preferred orientation of quartz c-axes in the mylonites on the Wood- roffe Thrust (Bell and Etheridge 1976), suggesting enhancement of the role of grain boundary sliding as a deformation mechanism. The microstructure developed in these rocks would facilitate this latter mechanism as they consist of generally equidimensional quartz and feldspar grains which abutt uniformly oriented micas (fig. 13 in Bell and Etheridge 1976).

FOLD ASYMMETRY AND SENSE-OF-SHEAR

IN MYLONITE ZONES

Fold asymmetry in mylonite zones has sometimes tentatively been used to determine the sense of movement (e.g., Christie 1963). There are problems associated with this because of the anastomosing character of the foliation in mylonitic rocks and the inher-

FIG. 19.-Sketch illustrating the disruption of a mineral aggregate that forms a stretching lineation on an Sm surface during progressive remylonitization of anastomosing portions of Sm. This process contributes to the development of phyllonites and is described in the text.

ent, very high strains, and they are discussed below.

Commonly fold axes in mylonites lie parallel to the stretching lineation whether the folds are of the Fm or Fi varieties. Such folds cannot be used to give a sense-of-shear because they could have rotated from either direction into the stretching lineation as

681


FIG. 20.-Sketches demonstrating how the fold asymmetry shown in (a) can be formed by rotation of the fold axis from either direction (b) and (c) into this orientation. The asymmetries in (b) and (c), projected onto a plane containing the stretching lineation Lm and drawn perpendicular to Sm, are opposite one another. Consequently if one attempts to use fold asymmetry to interpret shear sense, only fold axes oblique to the bulk stretching lineation can be used, and this asymmetry should be projected as shown. The interpretation of the sense of shear from that asymmetry is problemat- ical however (see text for discussion).

shown in figures 10 and 20. This changes their original asymmetry with respect to the stretching lineation (fig. 20), and therefore either sense-of-shear was originally possible. As a consequence of this the sense-of-shear can only ever be assessed using the asymmetry of folds with axes oblique to the bulk stretching lineation. It is essential to note their asymmetry in combination with their axial relationships to the bulk stretching lineation direction. Thus, the asymmetry should be projected onto a plane perpendicular to Sm which contains the bulk stretching lineation direction Lm, as shown in figure 20b and c.

Mylonite zones often show both asymmetries even when the relationships described above are taken into account, although one sense may dominate (e.g., Christie 1963; Bry-

FIG. 21.-Schematic diagram illustrating the effect of differing amounts of shear strain on pre- existing layering. Depending on the initial orientation of this earlier foliation, either sense of asymmetry is possible (a and b). At high strains however, only the asymmetry shown in b will be present. Consequently folds which fold a pre- mylonitic foliation and have a well developed mylonite schistosity as axial plane, should have the asymmetry shown in b (relative to the sense of shear) because the shear strain involved in such a deformation is very high.

ant and Reed 1969; Bell 1978). How can the movement direction be interpreted in such cases and what is the basis of this interpretation if both asymmetries can be produced for the one bulk sense-of-shear? Is fold asymmetry useless as a movement indicator, or can a number of criteria be cautiously applied which enable one to make an educated judge- ment about what the asymmetry means?

Pre-existing layering forming Fs folds will tend to be rotated by the large shear strains inherent in mylonites to give the sense of movement shown in figure 21b. Naturally, at lower strains on the margins of the mylonite zones (for example) the opposite asymmetry is possible, depending on the initial orientation of the folded early layering (foliation) with respect to the newly formed mylonite zone (e.g., fig. 21a). In well developed mylonite, however, the shear strain component should be so high that the asymmetry will have always converted to that shown in figure 21b. This is independent of the deformation history, although either progressive inhomogeneous simple shear or progressive bulk inhomogeneous shortening could cause local development of the opposite asymmetry on fold limbs (e.g., Carreras et al. 1980; Mawer 1983).

If the mylonitic foliation is refolded forming FM folds, a variety of asymmetries can develop depending on the strain history and degree of non coaxiality.

1. For example, if the mylonite zone has formed by progressive simple shear, uniform development of this deformation history across an earlier developed pod of anas-

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A

FIG. 22.-Sketches showing the fold geometry produced by progressive simple shear on an ellipsoidal pod of anastomosing mylonitic foliation. (A) shows the unfolded anastomosing mylonitic foliation, with the boundaries of the local zone of progressive simple shear sketched in figure 22B and the orientation of the bulk X and Z axes of the mylonite deformation strain ellipsoid. Y is perpendicular to the page. (B) shows the effects of a local zone of progressive simple shear on the upper half of the pod and uniformly distributed progressive simple shear on the lower half. Note that the asymmetry produced in both cases is the same for the same sense of shear irrespective of where the folds are located on the ellipsoidal pod.

tomosing mylonitic foliation or its localiza- tion in narrow zones (fig. 22) will only generate one fold asymmetry relative to the movement direction regardless of the degree of shear strain. This asymmetry is generally exactly the opposite for the same movement direction to that of folds developed in a pre- existing layering in well developed mylonites (c.f. Berth6 and Brun 1980; figs. 5 and 21b). The relationships observed by Berth6 and Brun are particularly appropriate for the South Armorican Shear Zone, as this is a ver- tical mylonite zone with a horizontal stretching lineation which may have formed without any component of bulk shortening (c.f. Bell 1981), although some extension in Y is possible. It also has the independent confirmation of the sense of shear from S and C planes (Berth6 et al. 1979, see below).

2. If the deformation history involves progressive bulk inhomogeneous shortening or progressive inhomogeneous simple shear, both asymmetries can occur as shown in figure 7. However, there may tend to be a dominance of the asymmetry generated by progressive simple shear especially in mylonites developed with a highly non-coaxial

FIG. 23.-Rhodes and Gayer (1977) used inclusion trails in garnets to interpret the sense of shear in mylonitic rocks in the Kalak Nappe complex. This sketch has been drawn from the figure they used. Bold arrows are proportional to the shear strain. They interpreted pre-S2 to early syn-S2 porphyroblast growth. However, an equally plausible alternative interpretation gives the opposite shear sense to that shown. This interpretation is simply that of syn-D2 porphyroblast growth which has preserved crenulated S1. The shear planes for this deformation history would lie closer in orientation to S2 than Si (c.f. Bell 1981).

strain history (e.g., Moine Thrust, Christie 1963; the Blue Ridge and Table Rock Thrusts, Bryant and Reed 1969). This is not necessarily the case, however, as exemplified by the Woodroffe Thrust (Bell 1978).

OTHER SHEAR SENSE INDICATORS

1. Syntectonic porphyroblast growth and the asymmetry of curved inclusion trails can also be used to determine the sense-of-shear. Caution must be exercised with this tech- nique, however, as it is totally dependent on the very local strain history and on the timing of porphyroblast growth, and can indicate either sense-of-shear if these are not known. For example, Rhodes and Gayer (1977) feature sketches of garnets containing S1 inclusion trails which they suggest grew partly prior to D2, or early syn-D2 (fig. 23). They interpret that shear occurred parallel to bed- ding and that the outer rim of the garnet porphyroblast grew early syn-D2, thus preserving a rotated S1 inclusion pattern. This gives a dextral sense-of-shear for the diagram shown in figure 23. However, if the deformation history was progressive bulk inhomogeneous shortening around the porphyroblast, the outer rim (and even the core) could have grown syn D2, with the garnet overgrowing crenulated Si. This would give the opposite

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sense-of-shear to that shown in figure 23. No- tably in this case the shear component of the strain would tend to be approximately parallel to S2 and not So. The various angular relationships between So, SI, and S2 fit either case equally well.

2. The best internal geometric indicators of movement direction in mylonite zones are the S and C planes first described by Berth6 et al. (1979). Similar structures have subsequently been described by White et al. (1980) which they call shear bands, and by Platt (1979) and Platt and Vissers (1980) which they call extensional crenulation cleavage. However, the original terminology of Berthe et al. (1979) appears perfectly adequate and descriptive. They argue that these structures developed approximately synchronously during mylonitization whereas White et al. (1980), and to a degree Platt (1979) and Platt and Vissers (1980), argue that they formed at a later stage in the mylonitization. Geometrically the C planes are simply zones of locally extremely high shear strain (fig. 24) and they can form throughout the mylonitization as argued by Berth6 et al. (1980). They can form in progressive bulk inhomogeneous shortening or progressive inhomogeneous simple shear (fig. 24a and b) and have been synchronously developed in experimentally deformed mylonites by one of us. However, if conjugate sets develop (Platt and Vissers 1980) the sense of shear may be indeterminate.

The reader is referred to Simpson and Schmid (1983) for a further treatment of shear sense criteria.

CONCLUSIONS

The complex internal geometric relationships of folds, lineations, and foliations in mylonite zones can be explained in terms of the extreme heterogeneity of strain and the anastomosing character of the foliation within them.

Fold axes parallel to the stretching lineation in the surrounding mylonite or which curve through 1800 about it, need not have rotated into this orientation and in many cases initially nucleated with this geometry due to the effects of folding ellipsoidal pods of mylonitic schistosity.

Stretching and intersection lineation variation from one mylonitic schistosity surface to

FIG. 24.-Sketches showing the geometry of S and C planes produced by non-coaxial progressive, inhomogeneous simple shear (a) and progressive bulk inhomogeneous shortening, (b). In the latter case, the tails on quartz and feldspar grains reveal the asymmetry as shown in heavy dots. The C- planes are defined by closely spaced lines.

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another is a product of the anastomosing character of earlier-formed mylonitic foliation combined with abrupt transitions into later formed foliation and its associated folding effects.

A specific fold asymmetry in mylonites can indicate either sense-of-shear and, therefore, should be interpreted with considerable caution. S and C planes provide the best means of determining the overall sense-of-shear in a mylonite zone.

F1 folds cut off by apparently identical

foliations may well be unique to rocks which have undergone mylonitic deformation histories and can be used to identify mylonite zones previously unrecognized because of a lack of grain size reduction within them.

ACKNOWLEDGMENTS.-Cees Swager read and criticized the manuscript for us and the reviewers also provided valuable criticism. Bell acknowledges the support of research grants from ARGS, and Hammond from a Commonwealth Post-graduate Award.

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Bell, T.H. and Hammond, R.L., 1984. On the Internal Geometry of Mylonite Zones

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