Page 18 - Microtectonics

4 1 · A Framework of Microtectonic Studies
Although the concept of deformation phases seems over the ramp can induce overprinting relations. In this
fairly simple and straightforward, there are some prob- case, the final fabric will show separable deformation
lems with its general application, as outlined below. phases on thin section and outcrop scale, but these will
be part of a single deformation phase on a regional scale.
1. Overprinting relations may be produced by a single
deformation phase 5. Deformation phases may be diachronous

Non-coaxial progressive deformation (Sect. 2.5.2) may Deformation may affect volumes of rock in a progressive
produce overprinting relations between structures with- way, starting from one side and reaching the other end
out a major change in the large-scale orientation and much later (Hobbs et al. 1976). As a consequence, an over-
magnitude of the stress field. Especially in mylonitic rocks printing structure labelled D may be older in a certain
2
developed in shear zones, it is common to find folds area than a D fabric in another. A common setting for
1
(often sheath folds) that deform the mylonitic foliation such an evolution may be in accretionary wedges, where
and which are clearly the result of the same deformation undeformed rocks arrive at a subduction zone, and sub-
phase that produced the mylonitic foliation in the first sequently become incorporated in the wedge (Figs. 1.5,
place (Sect. 5.3.2); such folds can be formed at any time B.7.1, ×Video B.7.1).
during progressive deformation (Fig. 1.2, ×Video 1.2). Because of the problems mentioned above and also
because of the subjective nature of subdivision in sets of
2. Subsequent deformation phases do not necessarily structures (e.g. Hobbs et al. 1976), some geologists have
produce overprinting relations become reluctant to use the deformation phase concept

Two subsequent deformation phases with a similar ori-
entation of the stress field and a similar metamorphic
grade may be indistinguishable in the final fabric (Fig. 1.3).
For example, a 2400 Ma-old hornblende foliation, formed
under amphibolite facies conditions, could be overprinted
by a 1600 Ma phase of amphibolite facies deformation in
a stress field with approximately the same orientation. The
result would be strengthening of the earlier foliation. In
such cases, only detailed microprobe work or mineral
dating may reveal the correct sequence of events.

3. Only the relative age of deformation phases can be
established

Identical overprinting relations may develop where a Paleo-
proterozoic foliation is overprinted by a crenulation cleav- Fig. 1.2. Sequence of events in a shear zone to show how overprinting
age of Phanerozoic age, or in a thrust nappe within an in- relations may form during a single phase of progressive deforma-
terval of several hundred thousand years only (Fig. 1.4). If tion if some heterogeneity is present to cause folding
metamorphic conditions are significantly different for two
deformation phases, a minimum time separation can be
established but otherwise absolute age dating is required.
4. The significance of deformation phases depends on
the scale of observation

During development of a fold the axial planar foliation
may be rotated to such an extent that a crenulation cleav-
age is locally formed, overprinting the earlier formed fo-
liation (Williams 1972a). Such overprinting relations form
during a single phase of deformation. The same effect may
Fig. 1.3. Subsequent deformation phases, represented as grey blocks
occur on a larger scale; consider a volume of rock in a thrust
on the time bar, do not necessarily produce overprinting relations.
sheet that is transported over a ramp; the sudden changes If metamorphic conditions and stress orientation are similar, struc-
in orientation of the rock volume when it moves up and tures like folds may just be further tightened