Page 163 - Microtectonics

152 5 · Shear Zones
folding (bf) in veins. Nevertheless, it is useful to plot the can occasionally be used to determine sense of shear. Two
orientation of deformed veins against deformation types of asymmetric boudins can be distinguished in
category (f, b, bf or fb) and to compare the symmetry with layering on mm–m scale, based on detailed analysis of
the theoretically predicted patterns. At least sense of shear field examples and experimental work (Goldstein 1988;
should be deductible; if the pattern is asymmetric, the Goscombe and Passchier 2003): shear band boudins and
sense of asymmetry indicates shear sense (Fig. 5.39, domino boudins (Fig. 5.40). Shearband boudins have a
×Video 5.39). Complexly deformed veins are notably long, curved lenticular shape and large relative displace-
informative. Notice that first folded, then boudinaged ment and synthetic drag on an inter-boudin surface that
veins (fb) can occur in any flow type, but veins with short- is gently inclined to the boudin exterior surface. Domino
ened or folded boudins (bf) are important: such a defor- boudins have an angular shape, an inter-boudin surface
mation sequence can only result from (1) a high rigid steeply inclined to the boudin exterior surface with small
body rotation component of flow such as near rigid ob- relative displacement and unique antithetic flanking folds
jects, or (2) polyphase deformation (Passchier 1990a). In instead of synthetic drag (Fig. 5.40). Development of ei-
most cases, the presence of folded or shortened boudins ther type of asymmetric boudins depends mainly on the
is due to polyphase deformation. initial boudin neck orientation, normal or oblique to the
long axis of the boudinaged object, on the rigid or de-
Asymmetric Boudins formable character of such objects, and on the flow type
(Fig. 5.41). If asymmetric microboudins can be identified
Asymmetric boudins can occur in sedimentary or tec- by their shape as either shearband or domino boudins,
tonic layering. If boudins have an asymmetric (mono- they can be used as shear sense indicators (Fig. 5.42). Shear
clinic) rather than symmetric (orthorhombic) shape, they band boudins are in all reported cases associated with
synthetic slip between boudins, i.e. relative displacement
sense of boudins equals bulk shear sense. Therefore, shear
band boudins are reliable shear sense indicators, like shear
band cleavage. Domino boudins are more difficult to in-
terpret. If they form in a layer parallel to the bulk fabric
attractor, usually close to the mylonitic foliation in mylo-

Fig. 5.40. Two main types of asymmetric boudins. After Goscombe
and Passchier 2003

Fig. 5.42. The use of asymmetric boudins as shear sense indicators.
Shear band boudins are reliable shear sense indicators in all orien-
tations. Domino type boudins are reliable if parallel to the mylonitic
Fig. 5.41. Schematic presentation of the development of asymmet- foliation. If oblique to the mylonitic foliation they may have the op-
ric boudins posite geometry and could be incorrectly interpreted