Résumé
The development of millimetre-scale continuous shear zones in the Sidi Ali Bou Nab (Algeria) granite and their progressive transformation into discrete shear zones are presented. Increases in strain, stress and strain-rate magnitudes, as well as changes of deformation mechanisms are suggested by an evolution of the microstructure across these narrow shear zones. Temperatures are assumed to be constant.
From boundaries to core, the most striking aspects of this evolution are:
1.
(1) a decrease in the angle between the foliation and the shear zone
2.
(2) a decrease in the recrystallized grain-size of quartz together with the development of a crystallographic orientation characterized by a strong concentration of c-axes around the
Y-axis of the finite-strain ellipsoid
3.
(3) the development of very-fine-grained feldspathic aggregates or layers, probably through dynamic recrystallization of original large crystals, and
4.
(4) the replacement of original biotite by muscovite and a new generation of biotite.
In the continuous shear zones the deformation is controlled by the behaviour of quartz. This mineral undergoes extensive dynamic recrystallization which probably favours a higher strain rate and a change in the active slip system from a dominant basal one in the outermost part of the structures, to a dominant prismatic one in its core. Paleopiezometry and strain computations show a shear strain up to 10 and an inferred steady-state differential stress to 100–130 MPa, respectively. The high stress level is suspect because the estimates are derived from recrystallized grain size which may be sensitive to strain and variations of chemical environment.
Discrete shear zones develop in relation with dynamic recrystallization of feldspars in the core of continuous shear zones. Dynamic recrystallization of feldspars produces very-fine-grained aggregates (grain size of 6–7 μm) which coalesce and form a continuous layer at the central part of the shear zones. This fabric may result from
1.
(1) a change in the driving flow mechanism from dislocation creep of quartz to dislocation creep or/and grain-boundary sliding of feldspar neoblasts, and
2.
(2) a large strain softening that would favour strain concentration in the fine-grained matrix and the development of sharp shear-zone boundaries.