Page 88 - Microtectonics

76 4 · Foliations, Lineations and Lattice Preferred Orientation

Box 4.2 Shape fabrics
In deformed rocks it is common to find a fabric composed of elon- of a deformed rock, deformation by dislocation creep will form a
gate or disc-like grains or grain aggregates that define planes, or GSPO that can be oblique to finite strain axes for the last deforma-
lineations if they share a linear direction. In general, this type of tion phase. If the fabric consists of an alternation of grains of dif-
fabric is known as a shape preferred orientation (SPO). ferent minerals, as in a granite, some minerals will deform more
If the fabric is composed of elongate or disc-like single grains of strongly than others. Moreover, grains do not always deform up to
minerals, which normally form equidimensional grains in an very high strain, but may be affected by recrystallisation to form new
undeformed rock, such as quartz or calcite, the fabric is known as a grains with a low aspect ratio, so that aggregates consist of grains of
grain shape fabric or grain shape preferred orientation (GSPO). A different shapes, depending on when they formed during the defor-
GSPO can be planar, linear of both. A linear GSPO is known as a mation process. The mean aspect ratio of all these grains will only
grain lineation (Fig. B.4.2; Sect. 4.3). The term shape preferred ori- reflect part of the finite strain. GBAR and static recrystallisation
entation or GSPO is not used for a preferred orientation of platy can also change the shape of grains during and after deformation.
minerals such as micas or amphiboles that also have an elongate If flow is coaxial a GSPO will at least lie approximately parallel to
shape in undeformed rocks. finite strain axes, even if the aspect ratio of the grains is not the
GSPO can be developed in primary grains such as sand grains in same as the finite strain ratio. However, in non-coaxial flow the fi-
a quartzite or oolites. Monocrystalline ribbons (Boullier and nite strain ellipse rotates away from the orientation of ISA with pro-
Bouchez 1978) can be regarded as an extreme case of such GSPO. gressive deformation (Sect. 2.7). If grains recrystallise to form dur-
These ribbons form mostly in minerals where only a single slip ing this deformation process, their mean orientation will only re-
system operates such as orthopyroxene but can also form in quartz flect part of the finite strain, and the GSPO will lie somewhere be-
or feldspar under certain metamorphic conditions (Sect. 3.12). tween the orientation of the flow ISA and finite strain axes.
Monocrystalline ribbons do not only form by deformation; they Besides a preferred orientation of single grains or subgrains,
can also develop from polycrystalline ribbons that are bounded by aggregates of grains can also have a preferred orientation, which is
other minerals due to grain boundary migration, e.g. in GBM or visible, if individual aggregates are bounded by grains or aggregates
static recrystallisation (Sect. 3.10, 3.11). of other minerals. This type of fabric could be named an aggregate
More commonly, GSPO develops in aggregates of secondary, re- shape preferred orientation (ASPO; Fig. B.4.2). This kind of fabric
crystallised grains (Means 1981; Lister and Snoke 1984; Figs. B.4.2, is also generally referred to as a shape fabric, either a planar shape
4.31c, 5.10f). Examples are shown in Figs. 4.25, 5.24, 5.30, 5.31. GSPO fabric or a linear shape fabric. The term polycrystalline ribbon is
can develop by crystalplastic processes such as dislocation creep or also occasionally used in thin section descriptions. Another name
solid-state diffusion (Sect. 3.4, 3.8) but solution transfer may also for a linear shape fabric is an aggregate lineation (Sect. 4.3).
play a role. In the case of dislocation creep, the deformation inten- ASPO most commonly forms by deformation of older aggregates
sity of each individual grain depends on its lattice orientation, since of polycrystals such as conglomerates, or by deformation and re-
the activity of slip systems is a function of their orientation with crystallisation of large grains (Fig. B.4.1; Sect. 4.2.7.5). Piazolo and
respect to the kinematic frame (see below) (Fig. 4.24). This can ex- Passchier (2002a) demonstrated that, even if an original fabric is
plain why some quartz grains in a deformed quartzite may be much undeformed, the strength of an ASPO depends not only on strain
less deformed than others (Fig. 4.24); however, other reasons may intensity, but also on the initial mineral distribution and grain size
be a considerable difference in original grain shape or late prefer- of the rock (Fig. B.4.1). Since the size of dynamically recrystallised
ential grain growth of some crystals. At high homologous tempera- grains depends on differential stress (Sect. 9.6.2), a fine-grained
tures (Sect. 3.14), diffusion of ions through a crystal lattice becomes poly- or monomineralic rock will flatten but may recrystallise to
increasingly important (Nabarro-Herring creep). Grains can be flat- grains of the same size. In such cases, no ASPO can form. Only if a
tened in this case without activity of slip systems or the presence of rock is polymineralic, and if the original grain size or aggregate
an intergranular fluid. This process may aid development of a grain size exceeds that of the new recrystallised grains a new ASPO will
shape-preferred orientation in high-grade rocks, but its importance form. An example of this influence is shown in Fig. B.4.1 where the
is uncertain since the number of active slip systems also increases effect of original grain size and fabric on development of an ASPO
with temperature. during deformation and dynamic recrystallisation is shown. Other
If subgrains obtain an elongate shape, they may define a weak possible mechanisms to form an ASPO are breakdown of large
foliation on thin section scale, which is named a subgrain shape grains to other phases such as the common reaction of garnet to
preferred orientation (SSPO). SSPO and GSPO grade into each other plagioclase upon decompression, followed by deformation (e.g.
where SGR recrystallisation transforms subgrains into new grains garnet to plagioclase), and by boudinage of a layer into rods or discs.
(Sect. 3.7.2). As for GSPO, there is a relation between ASPO and finite strain.
The strength and orientation of a GSPO depends on finite strain, ASPO is not easily reset by recrystallisation and therefore has a ten-
but there is no simple relationship. In an ideal case, a GSPO would dency to lie close to the XY-plane of the strain ellipsoid, provided it
form by deformation of a set of spheres with isotropic rheological formed from equidimensional older elements, and is not overprint-
properties. In this case, the GSPO would exactly mimic the geom- ing an older ASPO. However, there is usually no good correlation
etry of the strain ellipsoid; a planar shape fabric would be parallel between the 3D aspect ratio of aggregates and 3D finite strain ge-
to the XY-plane of finite strain, and a linear shape fabric with the ometry. Freeman and Lisle (1987) have shown that viscous spheres
X-axis. However, in nature an older SPO may be overprinted and of a certain rheology embedded in a material of an other rheology
the resulting shape will not reflect finite strain of the latest defor- do not mimic the shape of the strain ellipsoid, but tend to be more
mation phase. Also, grains are not passive spheres, especially if they linear if the viscosity is higher and more planar if the viscosity is
deform by dislocation creep. In this case, they only deform along lower than that of the matrix. In conclusion, ASPO can in some cases
certain slip planes and, depending on the overall flow field, some be used to find the orientation of finite strain axes, but has to be
will deform more strongly than others (Chap 4.2.7.4; Fig. 4.24; interpreted with great care.
Wilson 1984). If a rock consists of equidimensional grains that have A special type of shape fabric can be defined by domains com-
an older lattice preferred orientation, e.g. by static recrystallisation posed of grains that share a certain crystallographic preferred ori-