Brittle–viscous deformation of vein quartz under fluid-rich lower greenschist facies conditions
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Abstract. We studied by Electron BackScatter Diffraction (EBSD) and optical microscopy a coarse-grained (ca. 0.5–6 mm) quartz vein embedded in a phyllonitic matrix to gain insights into the recrystallization mechanisms and the processes of strain localization in quartz deformed under lower greenschist facies conditions, broadly coincident with the brittle–viscous transition. The vein deformed during faulting along a phyllonitic thrust of Caledonian age within the Porsa Imbricate Stack in the Paleoproterozoic Repparfjord Tectonic Window in northern Norway. The phyllonite hosting the vein formed at the expense of a metabasaltic protolith through feldspar breakdown to form interconnected layers of fine, synkinematic phyllosilicates. In the mechanically weak framework of the phyllonite, the quartz vein acted as a relatively rigid body. Viscous deformation in the vein was initially accommodated by quartz basal slip. Under the prevailing deformation conditions, however, dislocation glide- and possibly creep-accommodated deformation of quartz was inefficient, and this resulted in localized strain hardening. In response to the (1) hardening, (2) progressive and cyclic increase of the fluid pressure, and (3) increasing competence contrast between the vein and the weakly foliated host phyllonite, vein quartz crystals began to deform by brittle processes along specific, suitably oriented lattice planes, creating microgouges along microfractures. Nucleated new grains rapidly sealed these fractures as fluids penetrated the actively deforming system. The grains grew initially by solution precipitation and later by grain boundary migration. We suggest that the different initial orientation of the vein crystals led to strain accommodation by different mechanisms in the individual crystals, generating remarkably different microstructures. Crystals suitably oriented for basal slip, for example, accommodated strain mainly viscously and experienced only minor fracturing. Instead, crystals misoriented for basal slip hardened and deformed predominantly by domainal fracturing. This study indicates the importance of considering shear zones as dynamic systems wherein the activated deformation mechanisms may vary through time in response to the complex temporal and spatial evolution of the shear zone, often in a cyclic fashion.Keywords:
Greenschist
Deformation mechanism
Brittleness
Mylonite
Grain Boundary Sliding
Misorientation
Deformation mechanism
Dynamic Recrystallization
Diffusion creep
Recrystallization (geology)
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Diffusion creep
Deformation mechanism
Pressure solution
Superplasticity
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Diffusion creep
Deformation mechanism
Climb
Differential stress
Flow stress
Grain Boundary Sliding
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Mylonite
Deformation mechanism
Grain Boundary Sliding
Diffusion creep
Dynamic Recrystallization
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Polycrystalline aggregates lacking four independent systems for the glide of dislocations can deform in a purely viscoplastic regime only if additional deformation mechanisms are activated. Since this is the case of most minerals of the upper mantle (olivine, pyroxene, …), we anticipate that, besides dislocation creep that is responsible for the development of strongly pronounced lattice preferred orientations, other deformation mechanisms such as diffusion, grain boundary sliding, disclinations, … must be active in-situ. It has been established that such an accommodation mechanism completely controls the effective flow stress of the mantle rock. In this presentation, using a scale transition model that allows relating polycrystal and grain behaviors, we will show that they also have a huge influence of the effective stress sensitivity (stress exponent). For example if one consider that n=3.5 as for the slip of dislocation in individual grains, one cannot reach the commonly adopted value n=3.5 at the polycrystal scale, if the accommodation mechanism is linear (n=1) as for e.g. diffusion creep. This raises the question about possible deformation mechanisms active in-situ.
Grain Boundary Sliding
Deformation mechanism
Diffusion creep
Viscoplasticity
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