Introduction
In naturally deformed quartz-rich rocks, the
microstructures show systematic differences with increasing
temperatures of deformation, similar to the different
dislocation creep regimes in experimentally deformed
examples. The strain rate and stress conditions of the
naturally deformed rocks usually are unknown or poorly
constrained, so that a direct correlation with dislocation
creep regimes could be difficult. However, the dominant
recrystallization mechanisms can be determined from the
microstructures and can be compared with the experimentally
established dislocation creep regimes. Three microstructural
regimes corresponding to three main mechanisms of dynamic
recrystallization can be distinguished:
1. Bulging recrystallization which is dominated by local
grain boundary migration (slow migration) and occurs at the
lowest temperatures of deformation. The grain boundary lobes
are very small. Favorite sites for bulging are triple
junctions, and - if present - fractures.
2. Progressive subgrain rotation which is dominated by
polygonization of old grains and formation of newly
recrystallized grains. This recrystallization occurs at
intermediate temperatures.
3. Grain boundary migration recrystallization which is
dominated by fast grain boundary migration and occurs at
high temperatures. During this recrystallization, whole
grains may be swept. Progressive subgrain rotation is only
important for the initial formation of new grains.
Generally, the recrystallization mechanisms listed above
as 1,2,3 correspond approximately to the dominant
recrystallization processes identified in the respective
experimental dislocation creep regimes 1,2,3.
The Heavitree quartzite is from the Ruby Gap duplex (RGD)
which forms a part of the internal ductile zones of the
Alice Springs orogen in central Australia. The RGD consists
of 5 thrust sheets of Heavitree quartzite deformed under
greenschist-facies conditions (less than 400°C). Sheets
1, 2 and 3 form an imbricate system. Finite strain and
temperature of deformation generally increase upward through
the duplex. The microstructures are typical of the full
spectrum of dislocation creep regimes.
The chert samples are from the Warrawoona syncline which
is part of an Archean greenstone belt accumulated between
3450 Ma and 3320 Ma. The syncline is a tight keel structure
developed between two granitic domes. In the axis of the
syncline, a chert series was deformed and recrystallized
under greenschist-facies conditions. The tectonites are very
strongly lineated in the central part of the syncline, and
shape and crystallographic fabric analyses indicate a
deformation in constriction. During
deformation-recrystallization, the chert underwent
substantial grain growth.
The quartz veins are from the Tonale Line, a major strike
slip fault in the Alps. At the eastern end, the synkinematic
Adamello intrusion has imposed a thermal gradient across the
fault. The quartz samples shown here come from the Edolo
shists which were deformed under greenschist-facies
conditions.
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