On the formation and rheology of polymineralic ultramylonites

Gilgannon, James (2020). On the formation and rheology of polymineralic ultramylonites (Unpublished). (Dissertation, Bern University, Institute for Geological Sciences)

Text Gilgannon (2020) - PhD - On the formation and rheology of polymineralic ultramylonites.pdf - Published Version Restricted to registered users only Available under License BORIS Standard License. Download (91MB)

The work contained within this thesis forwards the understanding of which microphysical processes facilitate the formation of polymineralic shear zones and discusses how the activity of these processes will affect a shear zone's mechanical state. To do this, experimental data and samples from several low and high strain deformation experiments on monomineralic carbonates are revisited and re-evaluated. Here, the intention is to test if physical processes can activate in a monomineralic aggregate that would allow an evolution to a polymineralic ultramylonite. For this purpose, mechanical data sets from experiments conducted on carbonates that span the calcite-dolomite compositional series are reviewed and related. Additionally, novel image analysis techniques are used to map representative microstructural changes in experimentally deformed Carrara marble samples that have undergone mylonitisation. In this way the mechanical and microstructural evolution of a carbonate during mylonite formation is extensively reappraised and the specific roles of i) chemical impurities, ii) grain-size, iii) syn-kinematic porosity and iv) second-phases are considered. In this regard, the thesis contains several new critical findings: 1) a magnesium-sensitive diffusion creep flow law has been defined for carbonates in the calcite-dolomite compositional series ; 2) dynamic recrystallisation has been shown to produce syn-kinematic pores, called creep cavities, during shear-zone formation; and 3) these creep cavities have been found to emerge and evolve systematically in oriented, periodic, porous domains. All of these findings have consequences for the formation and rheology of polymineralic shear zones. Firstly, if mass is available to nucleate new phases, the emergence of creep cavities provides a spontaneous path for a monomineralic mylonite to transition to a second-phase controlled polymineralic ultramylonite. Secondly, the porous domains identified in this thesis represent a hydromechanical anisotropy that may allow ultramylonites to transition from flow to fracture if individual pore sheets become mechanically unstable. Alternatively, if this porous anisotropy is stable and acts instead as a network for new material to precipitate into, it may explain how phyllonitic rocks could develop in the absence of classical brittle fracturing. Lastly, experimental data acquired for natural second-phase controlled ultramylonites fits with the new diffusion creep flow law, suggesting that the rheological controls in both pure and impure fine-grained carbonate shear zones are similar. Together these results point to the fact that the ongoing viscous deformation of a shear zone is far more complex than currently assumed. The difference between how deformation occurs in monomineralic and polymineralic rocks is not obviously defined and much work still needs to be undertaken to understand this. Furthermore, it is apparent that the current model of viscous rocks in the crust is incomplete and should in future account for the unambiguous emergent behaviours of mylonitic rocks found in this thesis.

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