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Mylonitic shear zones are important fluid conduits in the Earth's crust. They host transient and permeable porosity that facilitates fluid transfer and controls fluid-rock interaction. Here we present microstructural observations from a mid-crustal ultramylonite with very large pores that occupy the strain shadows of albite porphyroclasts. Our non-invasive three-dimensional X-ray microtomographic data show that the largest of these strain shadow megapores have substantial volumes of as much as ~1.7 × 105 µm3. Given that the sample shows no signs of retrogressive overprint or weathering, these pores must be synkinematic. Importantly, the close proximity of the pores to creep cavities in dynamically recrystallized quartz ribbon grains suggests a potential hydraulic link between fluid in the strain shadow megapores and fluid in the creeping rock matrix. The evolving megapores constitute very large syndeformational local fluid reservoirs in mylonites that likely fed into the granular fluid pump established by the dynamically evolving creep cavities. Our findings add to an emerging picture of the dynamic transport properties of ultramylonitic shear zones, where the formation and destruction of porosity are intrinsically linked to microscale deformation processes. They also suggest that despite many studies on porphyroclast systems, open questions remain, especially concerning the interaction of clasts with their matrix.

In experiments designed to understand deep shear zones, we show that periodic porous sheets emerge spontaneously during viscous creep and that they facilitate mass transfer. These findings challenge conventional expectations of how viscosity in solid rocks operates and provide quantitative data in favour of an alternative paradigm, that of the dynamic granular fluid pump model. On this basis, we argue that our results warrant a reappraisal of the community's perception of how viscous deformation in rocks proceeds with time and suggest that the general model for deep shear zones should be updated to include creep cavitation. Through our discussion we highlight how the integration of creep cavitation, and its Generalised Thermodynamic paradigm, would be consequential for a range of important solid Earth topics that involve viscosity in Earth materials like, for example, slow earthquakes.

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.

Supporting information datasets for gilgannon_2019GL086172

We present 230Th-normalized dust and export production fluxes for two contrasted marine sediment cores spanning the Antarctic Polar Front, close to the Kerguelen Plateau in the Southern Indian Ocean, covering the last glacial cycle. We report glacial lithogenic fluxes comparable to the South Atlantic and higher than in the South Pacific sectors of the Southern Ocean. Structural and temporal discrepancies with dust reconstructions from Antarctic ice cores and the Atlantic and Pacific sectors of the Subantarctic Zone (SAZ) point towards Southern Africa and/or the Kerguelen Plateau as an additional source of lithogenic material to the Southern Indian Ocean during the last ice age. In the SAZ, export production proxies respond to iron (Fe) fertilization with total organic carbon (TOC) fluxes as high as those previously reported from the Atlantic sector of the Southern Ocean. However, the correlation between export production and dust proxies is weaker than in the other sectors, and shows a muted response of export production during peak glacials. We hypothesize that this muted response may be related to macronutrient (co-)limitation imposed on phytoplankton growth possibly induced by a northward displacement of wind-driven upwelling and/or the polar frontal system during peak glacials. The Antarctic Zone (AZ) record depicts the typical pattern of enhanced export production during interglacials and comparatively low productivity during glacials suggesting a decrease in the supply of macronutrients to the AZ surface during ice ages compared to warm periods, as previously proposed. However, a muted response of opal fluxes during marine isotope stage (MIS) 5e argues for a southward migration of the frontal system during warmer MIS 5e, possibly causing silicic acid (co-)limitation at this site.