Abstract
The Paleoproterozoic is characterised by a global-scale period of crust formation, extensive mafic to granitic magmatism, crustal reworking and intracrustal partial melting. Since a dramatic strength drop is associated with the presence of melt in crystallising or melting rocks, Paleoproterozoic continental deformation is thought to have been largely accommodated by shearing of high-grade gneisses and syntectonic granitoids. The Paleoproterozoic Nyong complex, which is made up of volumetrically dominant granitoids, metapelites and granitic gneisses, quartzites and banded iron formations; has been affected by the Eburnean/Transamazonian polyphase ductile and brittle deformation responsible of S1/2 foliation and S2 schistosity, L2 stretching and mineral lineations, F3 meso- and large-scale folds, S2/C3 structures and faults. Its shear zone strain anisotropies represent a natural laboratory to study the effect of crystal plastic deformation of felsic minerals and amphibole as predicted by numerical experiments and tectonic models. This paper describes the geometry of the deformation and focus on microstructural evidence of dynamic recrystallization features that were active under granulite conditions (>800 °C, >8.5 Kbar). These thermal conditions can be extended in similar Proterozoic worldwide shear zones.
Highlights
Geological and geophysical boundaries show that deformation in the Earth’s crust is heterogeneous, with large displacements confined into faults and shear zones
Since a dramatic strength drop is associated with the presence of melt in crystallising or melting rocks, Paleoproterozoic continental deformation is thought to have been largely accommodated by shearing of high-grade gneisses and syntectonic granitoids
This paper describes the geometry of the deformation and focus on microstructural evidence of dynamic recrystallization features that were active under granulite conditions (>800 °C, >8.5 Kbar)
Summary
Geological and geophysical boundaries show that deformation in the Earth’s crust is heterogeneous, with large displacements confined into faults and shear zones. Shear zones display microstructurally very different strain anisotropy to protoliths They are associated to mineral dynamic recrystallizations where magmatic and metamorphic textures are overprinted by later deformations or annealing processes and the physical conditions (pressure, temperature, strain rate, differential stress, and water content). They offer to geologists, best examples of mineral recrystallization from microscopic to mesoscopic and megascopic scales. Interpretation of microstructure and orientation and intensity of fabrics is critical in order to constrain space/time/temperature/deformation relationships during rock crystallization (Stipp et al, 2002)
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