Page 246 - Microtectonics
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8
Primary Structures
8.1 I n this chapter a number of microstructures are presented that originated in ig-
Introduction neous or sedimentary rocks and that could be mistaken for microstructures from
metamorphic rocks. This problem may appear in low grade metasediments and in
8.2 igneous rocks that were subjected to solid state deformation after crystallisation.
Primary Structures In igneous rocks and in migmatites a distinction can be made between mag-
in Rocks of Igneous Origin matic, submagmatic and solid state flow. Criteria to recognise these flow types in
or in Migmatites thin section are listed and discussed.
Many primary structures in sedimentary rocks, such as slump folds and sedi-
8.3 mentary breccias may be mistaken for tectonic structures induced during the
Primary Structures post-sedimentary evolution. Thin section study of these structures will rarely pro-
in Sedimentary Rocks vide conclusive evidence, but in specific cases such as compaction structures
around concretions, or pseudo boudinage, microscopic observation may help.
,
Some special structures, such as “loop bedding” resembling isoclinal folds, are
also discussed in this chapter.
