Shear zones develop as a results of progressive deformation during which deformation tends to localise in narrow bands. Deformation is typically continuous and develop without fracturation, deformation is said to be ductile as opposed to brittle. Therefore shear zones develop at a depth where higher temperature (>250ºC) inhibit the development of fractures and faults.
Ductile shear zones can develop as a results of shearing (simple shear strain regime as shown on the sketch on the bottom rigth) or "squeezing" (pure shear strain regime) as illustrated on the sketch below. In both cases the shear zone encompasses a band where strain progressively increases toward the center of the shear zone. As a result a foliation plane (S1), flattening plane analogous to the cleavage plane in folds, develops.
On Google Earth, check the southern part of Madagascar... |
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