Page 105 - Microtectonics

4.2 · Foliations 93

Box 4.6 Geometric development of foliations
Irrespective of the processes involved in foliation development,
the geometry of a developing fabric in rocks may change dra-
matically during its evolution. The simplest possible situation is
deformation of a random fabric into a foliation or lineation,
where fabric development simply reflects increasing strain. If
an older fabric exists, its deformation may lead to inhomoge-
neities such as folding or boudinage, but may also lead to ho-
mogeneous deformation at some scales; such homogeneous de-
formation can show drastic changes in the geometry of the strain
ellipsoid and the associated fabric. A well-documented example
is the development of slaty cleavage at a high angle to diagenetic
foliation and bedding (Reks and Gray 1982; Ramsay and Huber
1983). After deposition, a pelitic sediment will undergo dia-
genetic compaction that may lead to a significant volume loss,
associated with expulsion of part of the pore fluid. This causes
development of a diagenetic foliation parallel to bedding
(Sect. 4.2.2; Fig. B.4.5b, ×Video B.4.5). Subsequent superposition
of a tectonic strain usually causes development of a new folia-
tion oblique to the diagenetic foliation. At small tectonic strain,
the tectonic and diagenetic strains may produce the same de-
gree of anisotropy, and result in an effectively linear fabric. If
such rocks are uplifted and eroded, the result has been called a
pencil cleavage (Graham 1978; Reks and Gray 1982, 1983; Ramsay
and Huber 1983; Fig. B.4.5c, ×Video B.4.5). If tectonic strain
increases beyond development of a linear fabric, a new foliation
is formed, usually with relicts of the diagenetic foliation in mi-
crolithons (Figs. B.4.5d, 10.4, ×Video B.4.5). This sequence of
fabrics seems to be common in development of a first slaty cleav-
age in pelitic sediments (Reks and Gray 1982; Ramsay and Huber
1983). It also illustrates the relevance of the distinction between
diagenetic, tectonic and total strain (Box 4.7). It should be noted
that linear fabrics similar to pencil cleavage can also form by
constrictional strains under some circumstances, rather than by
foliation overprint (Ramsay 1981).

Fig. B.4.5. Development of slaty cleavage. a, b During diagenesis,
the rock is vertically compacted and water is expelled, leading to
considerable volume loss and a bedding parallel diagenetic folia-
tion (Fig. 4.6). c With onset of tectonic shortening, a foliation starts
to develop oblique to the diagenetic foliation. The interference of
both gives rise to a linear fabric, pencil cleavage. d With increas-
ing tectonic strain, the new foliation increases in strength and a
slaty cleavage develops. In microlithons of the slaty cleavage, rel-
icts of the diagenetic foliation may be preserved (e.g. Figs. 4.14,
10.4; see also ×Video B.4.5)