Abstract

Granitic rocks are here described as mixtures of weak and more resistant fractions whose proportions vary with strain and temperature and control some macroscopic characteristics of shear structures produced during deformation. Three main structural fields are distinguished on a strain/temperature diagram: (1) discrete shear zones anastomosed around low-strain domains, (2) microscale associations of pervasive foliation and ductile shear bands (e.g., C-S structures), and (3) "steady state" homogeneous foliations. We outline the following: a sharp drop in bulk strength is expected to occur around 500 °C within wet granites; the widespread occurrence of pervasive ductile shear bands is diagnostic of retrograde deformation histories, especially as encountered within syntectonic plutons; and most prograde deformation histories, as expected during reworking of pretectonic basement, should produce heterogeneous strain patterns associated with discrete shear zones.

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