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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 
	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.


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