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Polyphase Deformation
In orogenic belts where the rocks have been involved in separate collisional
events (e.g. a Grenville event superimposed on an Archean event), or where there has
been a periodic reorientation of the stress system, or where the deformation is manifest
as a set of discrete events, the deformed rocks may exhibit several sets of deformation
structures. It may be possible to observe the presence of two or more lineations or two or
more cleavages. Younger folds may fold older folds producing complex interference
structures, and older lineations and cleavages may be folded by the younger generation
of folds. Pre-existing strong foliation may undergo penetrative microfolding and the
development of both a crenulation cleavage and lineation. Where the crenulations
have very small wavelengths and amplitudes, the axes of the crenulations as seen on the
foliation surface being crenulated may be the more prominent of the deformation
features. The axes then constitute a crenulation lineation.
Rocks exhibiting these structures are said to have suffered polyphase
deformation. Since bedding is a primary planar feature it is usually referred to as S0.
Sequentially younger folds, foliations, and lineations are then respectively referred to as
F1, F2, etc; S1, S2 etc.; L1, L2 etc. The rocks may also undergo several phases of
metamorphic mineral growth (chloritoid, staurolite, kyanite, sillimanite, garnet, etc.)
with the metamorphic events being referred to as M1, M2, etc.
Note that a foliation parallel to the bedding is present in many of the specimens
although they bear no intimation of related folding. This implies that the phase of
folding coeval with the formation of the foliation was likely isoclinal; that is, as a result
of the buckle folding, the beds were rotated into a position normal to the compressive
stresses responsible for the buckling.
Interference patterns produced during polyphase folding may be observed at the
hand specimen, outcrop, or map scale, and are of three basic types:
Type 1 - the egg carton pattern represented by sets of en-echelon domes and
basins
Type 2 - the arrow head pattern
Type 3 - the bent fold pattern
The actual pattern exhibited by parasitic polyphase folds will depend on the
relative location of the folds on the limbs of the earlier and later folds.
The laboratory session to accompany these lecture will permit examination of a
representative suite of polydeformed rock specimens.