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Sources and effects of fluids in continental retrograde shear zones

Stenvall, Christian 2020. Sources and effects of fluids in continental retrograde shear zones. PhD Thesis, Cardiff University.
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Abstract

Exhumed mid-crustal retrograde shear zones provide a rare natural laboratory for investigating the effect of uids on rheology. In active plateboundary retrograde shear zones, geophysical observations have highlighted localised uid-rich structures, at and below the frictional-viscous transition. Because rocks in retrograde metamorphic settings have typically been dehydrated, and mylonitic shear zones are viewed as tabular zones, the wet and localised active shear zones raise questions regarding uid origin, as well as the spatiotemporal distribution of mineral-scale deformation mechanisms within mylonites. This thesis presents field observations, microstructural analyses, oxygen stable isotopes and bulk rock compositions, to investigate two exhumed, mid-crustal, strike-slip shear zones: the Kuckaus Mylonite Zone (KMZ), Namibia, and the Outer Hebrides Fault Zone (OHFZ), Scotland. In the OHFZ, the uid likely originated from magmatic and metamorphic sources, and induced weakening predominantly by facilitating pervasive growth of phyllosilicates within which deformation localised. In the KMZ, at least some of the uids were sourced from the surface, and whereas phyllosilicate growth facilitated incipient weakening, the ultramylonitic cores comprise a fine-grained polyphase mixture deforming by uid enhanced grain size sensitive creep. The greatest effects of uids in retrograde shear zones may be the metasomatism and enhancement of grain size sensitive creep they facilitate. The weakening effect includes replacement of strong minerals by weaker and finer grained phases. Therefore, uid presence may facilitate drastic weakness in mid-crustal, strike-slip shear zones, without elevated uid pressures, by allowing grain size sensitive creep at very low driving stresses in thin (< 1 m) ultramylonites. This weakening may create conditions favourable for low stress drop phenomena, such as slow slip and tremor and provides alternative explanations for such events observed at active plate boundary shear zones.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Funders: ERC
Date of First Compliant Deposit: 6 March 2020
Last Modified: 09 Mar 2020 10:04
URI: http://orca.cf.ac.uk/id/eprint/130195

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