Rheology of fault rocks - an experimental study on the brittle-viscous transition in mafic rocks

Marti, Sina. Rheology of fault rocks - an experimental study on the brittle-viscous transition in mafic rocks. 2017, Doctoral Thesis, University of Basel, Faculty of Science.


Official URL: http://edoc.unibas.ch/diss/DissB_12979

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In this thesis, the rheology, deformation mechanisms and microstructural evolution of a mafic fault rock are investigated, over a range of conditions where a transition from dominant brittle to dominant viscous deformation is observed.
 A Griggs-type deformation apparatus was used to perform experiments at elevated confining pressures (Pc) of 0.5 to 1.5 GPa, and over a temperature (T) range from 300 - 900 °C. The sample material is a ’simulated’ fault rock of mafic composition, fabricated from pre-crushed plagioclase-pyroxene mixtures. 0.18 or 0.11 wt.-% H2O is added to the samples to allow for solution-mass transport and mineral reactions during the experiments. Most experiments were performed in a general shear set-up, at constant displacement rates of ∼ 1e-4 down to 1e-6 mm/s. Mechanical data and microstructural observations are used to determine the rheology and identify the deformation mechanisms. Microstructural descriptions mainly base on observations made from scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Different methods of image analysis were used for microstructural quantifications.
In the experiments, the brittle-viscous transition (BVT) occurs within the temperature range of 600 °C ≤ T ≤ 800 °C, at confining pressures of 0.5 GPa ≤ Pc ≤ 1.5 GPa. The transition is effected by a switch from brittle fracturing and cataclastic flow, to dissolution-precipitation creep and grain boundary sliding (GBS). Viscous flow is enabled by intense grain size refinement resulting from heterogeneous nucleation during syn-kinematic mineral reactions. The reaction products are fine-grained (< 1 µm) neo-crystallized plagioclase and the new mineral phases amphibole, quartz and zoisite. In the BVT regime, the mechanical response of the sample is a mixed-mode between brittle and viscous rheology and microstructures associated with both brittle and viscous de- formation are observed.
Microstructural evolution is seen to be a crucial factor determining the bulk sample rheology, highlighting the importance of considering strain dependent rheological changes for natural fault zones. Especially in the BVT regime, the initially more brittle dominated rheology is suggested to be only transient, evolving with increasing strain to a more viscous rheology.
In the viscous dominated 800 °C experiments, the following findings are furthermore made:
1) A method was developed to study amphibole reaction corona thicknesses as a function of direction around their pyroxene host clasts. During the initial ∼ hydrostatic stage of the experiments (during lead run-in) reaction coronas grow isotropically in all directions. Upon applying a differential stress however, corona thicknesses decrease in directions correlating with high stress sites and increase in low stress sites. The results are interpreted to show the ability of amphibole to deform by dissolution-precipitation creep - a mechanism that is frequently described for naturally deformed amphibole but so far has seldom been reproduced in deformation experiments.
2) Electron backscatter diffraction (EBSD) orientation mapping revealed a weak but consistent crystallographic preferred orientation (CPO) of fine-grained albite within shear bands. No indicators for the activity of dislocation glide or creep are observed and the CPO is interpreted to form during deformation by dissolution-precipitation creep and GBS.
In the low-T experiments at T of 300 - 600 °C, samples deform in a dominant semi-brittle manner by cataclastic flow. Within these experiments, the syn-kinematic formation of amorphous material in high strain zones was observed. The amorphous material is interpreted to evolve from an ultra-cataclastic pre-stage by mechanical wear, where plagioclase is seen to be particularly susceptible to this form of amorphisation. Flow structures within the amorphous material indicate its potential viscous flow behaviour. The mechanical data does show significantly lower strengths for samples deformed at higher temperatures and based on microstructural observations and Brillouin spectroscopy measurements it is seen as likely that the amorphous material is causing this temperature sensitivity. The occurrence of amorphous material in natural brittle fault zones might thus bear the potential to introduce a viscous contribution to fault rock rheology.
Advisors:Heilbronner, Renée and Bystricky, Misha
Faculties and Departments:05 Faculty of Science > Departement Umweltwissenschaften > Ehemalige Einheiten Umweltwissenschaften > Rock deformation (Heilbronner)
UniBasel Contributors:Heilbronner, Renée
Item Type:Thesis
Thesis Subtype:Doctoral Thesis
Thesis no:12979
Thesis status:Complete
Bibsysno:Link to catalogue
Number of Pages:1 Online-Ressource (176, 3 Seiten)
Identification Number:
Last Modified:01 Jul 2020 12:49
Deposited On:25 Mar 2019 15:20

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