Page 15 - Microtectonics

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A Framework of Microtectonic Studies

1.1 asic concepts of microtectonics are introduced and discussed in this chapter.
Introduction BThe study of thin sections of rocks, which was originally mainly petrographic,
has evolved during the last century to include a number of structural characteris-
1.2 tics, that constitute the microstructure or fabric of a rock. The study of this fabric
Establishing can be used to reconstruct the structural and metamorphic history, but also to
and Interpreting improve the understanding of deformation and metamorphic processes.
Deformation Phases Successive stages in the deformational and metamorphic evolution of a rock
are commonly preserved as part of a fabric and the recognition and correct in-
1.3 terpretation of these fabrics is essential for the understanding of this evolution.
Deformation Phases Two concepts are important: deformation phases and metamorphic events.
and Metamorphic Events The first relates to specific periods during which a rock is deformed under the
influence of a deviatoric stress field, leaving visible records such as folds, cleav-
ages or lineations. Successive deformation phases may be superposed on each
other leaving overprinting structures such as folded foliations, refolded folds or
folded lineations.
Metamorphic events correspond to the formation of a specific mineral assem-
blage, thought to reflect particular P-T conditions or the crossing of a reaction
equilibrium curve in P-T space. These may also be superposed on each other, leav-
ing only a recognizable record if the equilibrium during the last event(s) was
incomplete.
The main objective of a microstructural analysis is therefore to unravel the
sequence of deformation phases in a specific area and to link this sequence to
metamorphic events, in order to establish a P-T-t path.