Page 19 - Microtectonics

1.2 · Establishing and Interpreting Deformation Phases 5
any longer. However, we feel that the concept continues to For all overprinting relations it is necessary to determine
be useful to classify structures in a sequential order, if whether they could have formed during a single phase of
used with care. Deformation phases refer only to the rela- deformation under similar metamorphic conditions. The
tive age of structures in a limited volume of rock, (com- following criteria may help to determine whether overprint-
3
monly in the order of a few hundred km ) and are gener- ing relations correspond to separate deformation phases:
ally not equivalent to tectono-metamorphic events of re-
gional significance. It is therefore necessary to determine a Two overprinting structures composed of different min-
the tectonic significance of local deformation phases. To eral assemblages that represent a gap in metamorphic
establish deformation phases it is important to define sets grade must belong to different deformation phases.
of structures based on reliable overprinting criteria, such b Foliations that overprint each other commonly represent
as a foliation (S ) that has been folded (D n+1 folds), and deformation phases on thin section scale (Sect. 4.2.10.2),
n
not just on style, orientation, tightness of folds etc., which but exceptions such as oblique fabrics (Sect. 5.6.2) and
are criteria that may change from one outcrop to the next shear band cleavages (Sect. 5.6.3) exist.
in structures of the same age. It is also important to take c Overprinting folds with oblique axial surfaces represent
metamorphic conditions during deformation into con- different deformation phases. Care should be taken with
sideration, since these are not subject to rapid change refolded folds with parallel axes (Type III of Ramsay
(Fig. 1.1). A final warning must be given for the extrapo- 1967), especially in the case of isoclinal folds since these
lation of phases from one area to another, or even from may form during a single deformation phase (Fig. 1.2).
one outcrop to the next. The criteria for subdivision re- d Shortened boudins are commonly formed by over-
main subjective in the sense that different workers may printing of two deformation phases (Passchier 1990a;
define a sequence of deformation phases in a different Sect. 5.6.13).
way, resulting in a variable number of phases for the same e Some structures preserved in porphyroblasts repre-
area. This, however, does not necessarily mean that one sent separate deformation phases (Sects. 7.3–7.5).
of these workers is right, and the others in error; it may f Intrusive veins or dykes can be important to separate
just be a matter of different criteria for definition. phases of deformation and their associated foliations.
Fig. 1.4.
Only relative ages of deforma-
tion can be established by over-
printing criteria; overprinting
structures shown at centre right
could form over any time inter-
val, e.g. over 2000 Ma (upper
bar) or over 30 Ma (lower bar)

Fig. 1.5.
Schematic representation of an
active accretionary prism. At A
no deformation is occurring
whereas at B a first deformation
phase D is responsible for ocean-
1
ward thrusting, probably accom-
panied by the development of foli-
ations and folds in deep levels.
While such D 1 structures develop
at B, a second phase of deforma-
tion D related to back thrusting
2
is already overprinting D 1 struc-
tures at C (see also ×Video B.7.1)