Page 17 - Microtectonics

1.2 · Establishing and Interpreting Deformation Phases 3
possibilities of recognising overprinting relations in thin Time intervals of no-deformation activity are postulated
section and to determine the conditions at which they between the deformation phases during which metamor-
formed. The aim is then to translate overprinting relations phic conditions changed significantly while the volume
in terms of deformation phases and metamorphic events. of rock under consideration was ‘passively’ transported
Deformation phases are thought to be distinct peri- to another position in the crust (e.g. by erosion and up-
ods of active deformation of rocks on a scale exceeding lift). The deformation phases are accompanied by meta-
that of a single outcrop, possibly separated by time inter- morphic events, which may lie on the retrograde leg of a
vals with little or no deformation during which metamor- single metamorphic cycle (Sect. 1.3). The size of the area
phic conditions and orientation of the stress field may over which these deformation phases can be recognised
have changed (Sects. 1.2, 2.11). The concept was originally should now be investigated and gradients in style and
created in relation to groups of structures that can be sepa- orientation monitored. Finally, the synchronous or dia-
rated in the field by overprinting criteria (Sect. 1.2). Meta- chronous nature of a deformation phase can in some cases
morphic events are episodes of metamorphism charac- be established by absolute dating of minerals associated
terised by changes in mineral assemblage in a volume of with structures visible in thin section, or by dating cross-
rock. Such changes are thought to reflect changes in meta- cutting intrusions. Comparison with similar data on a
morphic conditions. larger scale, either from the literature or by carrying out
Once deformation phases and metamorphic events are further field and thin section research, can establish the
defined, it is necessary to determine to what extent they regional significance of deformation phases with relation
correspond to tectonic events or metamorphic cycles, i.e. to tectonic events. Because such large-scale analysis is not
events on a larger scale such as those associated with plate part of the subjects covered in this book, we restrict our-
motion or collision. Finally, orogenies (e.g. the Alpine selves to the establishment of overprinting relations, de-
orogeny) may encompass several tectonic events with as- formation phases and metamorphic events from data
sociated metamorphic cycles. The following example il- obtained in thin section. The following section gives an
lustrates this concept. In thin sections from several out- outline of some of the problems involved in establishing
crops, a horizontal biotite foliation is overprinted by a overprinting relations and deformation phases.
steeply dipping chlorite foliation, and both are cut by brit-
tle faults (Fig. 1.1). Based on these overprinting relations 1.2 1.2
we could argue that a first ‘deformation phase’ with a com- Establishing and Interpreting Deformation Phases
ponent of vertical shortening formed a foliation under
conditions suitable for growth of biotite; later, a second The concept of deformation phases has been used exten-
‘deformation phase’ of oblique shortening was accompa- sively in the geological literature in reconstruction of the
nied by chlorite growth under lower-grade metamorphic structural evolution of rock units with complex deforma-
conditions. A third deformation phase affected both ear- tion patterns (e.g. Ramsay 1967; Hobbs et al. 1976; Ramsay
lier structures at very low-grade or non-metamorphic and Huber 1987; Marshak and Mitra 1988). The underly-
conditions or at high strain rate, to cause brittle faulting. ing idea is that permanent deformation in a volume of
rock occurs when differential stresses (Sect. 2.11) are rela-
tively high and that the orientation of the stress field may
change between such periods of permanent deformation
without visible effects on the rock fabric. The older fabric
is not always smoothly erased or modified to a new fab-
ric, since deformation in rocks is commonly partitioned
(that is: concentrated in certain domains and less con-
centrated or absent in others); relicts of older fabric ele-
ments may be locally preserved. A foliation that is short-
ened parallel to the foliation plane may develop folds,
commonly with a new crenulation cleavage developing
along the axial surface. The older foliation will be com-
pletely erased only at high strain or by recrystallisation
and grain growth under favourable metamorphic circum-
stances (Box 4.9). Boudins and tight or isoclinal folds may
Fig. 1.1. Schematic diagram of a biotite foliation (horizontal), a be refolded but remain recognisable up to very high strain.
chlorite foliation (inclined) and a brittle fault. The sequence of over- Lattice-preferred orientation may be preserved in less
printing relations is: biotite foliation-chlorite foliation-fault. The
three structures may represent different deformation phases since deformed lenses up to high strain and porphyroblasts may
they overprint each other, have different orientation and represent preserve relicts of older structures as long as the porphyro-
probably different metamorphic conditions blast phase remains intact (Sects. 7.3–7.7).