Page 190 - Microtectonics

6.3 · Fringe Structures 179
depends on the relative rotation of fringe an core object, bres converge on the core object (Köhn et al. 2000). Curva-
which normally deviates from the orientation of the ex- ture of the fibres is strongly dependent on the rotation rate
tensional ISA, even up to 90°. Fringes can therefore con- of the core object with respect to the fringes (Fig. 6.25). This
tain tracking, non-tracking and miscellaneous fibres, rotation rate, in turn, depends on coupling between the
while tracking fibres are not generally opening in the core object and the matrix (compare Sects. 5.6.7, 5.6.8).
direction of ISA (Fig. 6.23, ×Video 6.23).
The rotational behaviour of core-object and fringes 6.3.3
is quite different from that of a single, isolated rigid ob- Fringes on Angular Core Objects
ject in a ductile flow, since all three objects influence each
other (Köhn et al. 2003). Rotations of each element, and Most fringes in nature develop on angular core objects
the fringe object as a whole, are slower than would be with planar faces such as pyrite or magnetite crystals in
expected from their aspect ratio, and the rotation rate fine-grained metapelite or carbonaceous slate at low
generally decreases with fringe growth (Box 5.4; Ghosh metamorphic grade (Figs. 6.3, 6.21, 6.27). Because of the
and Ramberg 1976; Passchier 1988a; Köhn et al. 2003). smooth crystal faces of such core objects, most elongate
In fact, the rotation and growth of fringes is too complex crystals or fibres tend to be non-tracking in adjacent
to be approached analytically and can only be predicted fringes. However, one, two or more crystal faces can be
by numerical or analogue modelling (e.g. Kanagawa 1996; in contact with a fringe at any time. As a result, a divid-
Köhn et al. 2000, 2001a,b, 2003). ing line or suture may be present in the fringe separating
If a fringe is not rigid, it may deform internally. Since populations of fibres or elongate crystals with different
distal parts of antitaxial fringes are oldest, they are most orientation. Such sutures are attached to the corner be-
strongly deformed. A non-deformed fringe can be rec- tween two faces of the core object (Figs. 6.21, 6.23, 6.27,
ognised by an undeformed ‘cast’ of the core object at the ×Photo 6.21, ×Video 6.24b, 6.27). In fact, sutures are
distal part of the fringe (Fig. 6.20) and by fibres which trapped by corners of core objects just as individual fi-
lack undulose extinction and grain boundary migration bres are on asperities in the contact (Köhn et al. 2000).
structures. In some cases rigid strain fringes may start Figures 6.22 and 6.24 show the development of strain
to act as nuclei for second-generation fringes at the distal fringes around an angular core object in coaxial and non-
ends of the fringe (Fig. 3 in Choukroune 1971). coaxial progressive deformation. In the case of displace-
ment-controlled growth and non-coaxial progressive
6.3.2 deformation, the fibres are strongly curved in the outer,
Fringes on Spherical Core Objects oldest parts of the fringe and are more straight on the
inside. This is a result of decreasing angular velocity of
The simplest type of fringe develops on spherical core the fringe when its aspect ratio increases (Box 5.4). In
objects. Common examples are globular aggregates of the case of face-controlled growth, curvature of the fi-
pyrite with a raspberry-like external form known as bres is complex and directed towards the suture lines,
framboidal pyrites (Fig. 6.20). Matrix material is pulled which are therefore more prominent than in displace-
away from the rigid sphere by the flowing matrix and ment-controlled fibres. The shape of the sutures is usu-
new fringe material is depositing in the gap. In coaxial ally less complex than that of the fibres and is more use-
progressive deformation displacement-controlled fibres ful as a shear sense indicator; if the combined sutures of
form on rough objects and are simply parallel to the long both fringes define an S shape, shear sense is dextral; if
dimension of the fringe. Face-controlled fibres form on they define a Z shape, shear sense is sinistral (Fig. 6.24,
smooth objects and can be predicted to show inward ×Video 6.24b). The external shape of the fringes is usu-
curvature of fibres that gradually decrease in width ally curved or even “hooked” with a similar sense as the
(Fig. 6.22; Köhn et al. 2000). suture lines and as the external geometry of fringes
Modelling of the interaction of fringe growth, rota- around spherical core objects. However, the external ge-
tion and deformation predicts patterns as shown in ometry of fringes on angular core objects is less reliable
Fig. 6.24 (Choukroune 1971; Malavieille et al. 1982; Et- as a shear sense indicator than those on spherical core
checopar and Malavieille 1987; Köhn et al. 2000). In all objects; the orientation of the angular core object at the
cases, the fringes are curved into an S-shaped spiral for onset of fringe growth determines the final shape of the
dextral shear sense (Fig. 6.24, ×Video 6.24a). If most fi- fringe. Fringes on elongate core-objects can therefore be
bres are tracking, they will show a similar curvature as subdivided into four geometry classes; nw, n, w and wn-
the outline of the fringe, but be inclined at a steeper an- type, depending on the orientation of the core object at
gle to the flow plane (Figs. 6.20, 6.24, ×Video 6.24a). If the onset of fringe growth and its subsequent rotation
few fibres are tracking most will be face controlled and (Fig. 6.26). An additional complication is that the aspect
radiate outward. The curvature of the fibres will be mostly ratio of elongate core objects will also influence fringe
in the same sense as the curvature of the fringe, but fi- shape (Passchier 1987b; Köhn et al. 2003; Sect. 5.6.7).