Page 106 - Microtectonics

94 4 · Foliations, Lineations and Lattice Preferred Orientation
4.2.9.2 Box 4.7 Strain nomenclature
Foliations and the XY-Plane of Tectonic Strain
Strain may be subdivided into parts related to periods of the
progressive strain history. The following terms are currently
The symmetric relationship between secondary foliations
in use:
and the axial plane of folds has led early workers, from
th
the middle of the 19 century onwards (e.g. Sorby 1853) Diagenetic strain – strain resulting from diagenetic proc-
to realise that such foliations may be parallel to the esses such as compaction and dewatering.
XY-plane of a finite strain ellipsoid. To be more specific, Tectonic strain – strain induced by tectonic deformation,
usually after diagenesis.
it is now generally believed that many secondary foliations
Incremental strain – (infinitesimally small) increment of
approximately trace the XY-plane of tectonic strain re- strain.
lated to the deformation phase in which they developed Finite strain – part of the tectonic strain, i.e. strain accumu-
(Box 4.7). However, exact parallelism is expected to be rare lated over a specific period of time. It may, for example, refer
for reasons outlined below. to the strain of the D 1 deformation episode in comparison
to the combined strain acquired during D and D , or even
Many sediments and igneous rocks have been de- 2 3
to the tectonic strain. The term ‘finite’ (accumulated over a
formed before they start to develop secondary foliations; measurable period of time) is also used as a contrast to ‘in-
common examples are diagenetic foliations in sediments cremental’.
and flow banding in batholiths. Consequently, an over- Total strain – normally this term refers to the total accumu-
printing secondary foliation, even if it is ‘passive’ and lated strain of a rock, including diagenetic- and tectonic
strain.
traces the XY-plane of tectonic strain during progressive
deformation, does not represent the total strain in the rock, A more detailed description of this terminology is given in
which includes diagenetic compaction and other early Means (1979).
deformation (Treagus 1985). Another problem is inho-
mogeneous deformation where foliation planes are ‘ac-
tive’ as faults or shear zones. However, if an older anisotropy existed, several paths can
‘Passive foliations’ act as material planes in a homoge- be followed. If an old foliation lies at a high angle to the
neous flow. If they develop from a random fabric, con- shortening direction, it may rotate towards a new orien-
tinuous foliations can form in fine-grained rocks, or tation without development of folds or new foliation
spaced foliations in coarse-grained material (Gray 1978). planes (Fig. 4.17b). If the older anisotropy plane is ob-
Another type is shape preferred orientation formed by lique to the shortening direction, a new foliation may de-
flattening of grains or rock fragments (Box 4.2). Ideally, velop oblique to the previous one, gradually replacing it
passive foliations will be parallel or subparallel to the (Fig. 4.31a); this is the case for many disjunctive foliations.
XY-plane of tectonic strain (Fig. 4.17a,c; Williams 1972a). Alternatively, the earlier anisotropy may cause microfold-
Fig. 4.31.
Three situations where a foliation
is not parallel to the XY-plane of
tectonic strain. a A diagenetic folia-
tion (grey crystals) is overprinted
by oriented growth of new micas
(white). The resulting foliation has
a mixed orientation and is oblique
to tectonic strain axes. b A pre-
existing foliation in non-coaxial
flow may develop microfolds that
become overgrown in the hinges by
oriented new micas. The resulting
mean fabric is oblique to tectonic
strain axes. c An aggregate of dy-
namically recrystallising grains
obtains an oblique foliation repre-
senting only the last increments of
strain (Box 4.7). This fabric is ob-
lique to the ellipsoid of tectonic
strain (Sect. 5.6.2). Grey domains
represent the material contained
originally in two grains at left: these
domains are stretched, while recrys-
tallised grains retain the same ori-
entation and slightly oblong shape