The evolution of slate microstructures during the accretion of foreland basin sediments and implications for mechanical strength, fluid flow and seismicity in accretionary wedges

Akker, Ismay Vénice (2020). The evolution of slate microstructures during the accretion of foreland basin sediments and implications for mechanical strength, fluid flow and seismicity in accretionary wedges (Unpublished). (Dissertation, Institute of Geological Sciences, Faculty of Science)

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Exposed sediment successions from exhumed accretionary complexes, as well as studies on active accretionary wedges, have shown a strong partitioning in deformational style between the aseismic outer and seismic inner wedge. These changes are due to variations in mechanical strength as the initially unconsolidated and in water oversaturated sediments dehydrate due to compaction in the outer wedge and transform into well-consolidated and foliated rocks towards the inner wedge. The central question of this thesis is how progressive accretion adapts the microstructure of phyllosilicate-rich rocks as well as associated physico-chemical processes and what the response is of the wedge in terms of mechanical strength, fluid flow and potential seismic processes. For this study, we focus on slates from the Alpine paleo-orogenic wedge in the central European Alps. This wedge is exhumed and exposes a metamorphic gradient correlating to a background strain gradient covering the paleo outer and inner wedge in the Glarus Alps (eastern
Switzerland). The microstructural development is documented with a combination of microscopic techniques, ranging from petrographic microscopy to high-resolution scanning electron microscopy (SEM) on broad ion beam (BIB) prepared cross sections (BIB-SEM) and finally to Synchrotron X-ray Fluorescence Microscopy. The latter allows for linkage of the microstructure with geochemical compositions and was complemented by data from electron microscopy and the isotopic K-Ar system. From wedge- down to thin section-scale the slates show a large internal heterogeneity of alternating layers of calcite, quartz and mica. As a function of the original sedimentary input, each layer displays a unique microfabric. Therefore, the slates consist of a composite rheology of microlayers with different strengths. Also the type of porosity is affected by the original sedimentary layering and generally a reduction in porosity is seen with increasing metamorphic grade. The decrease in porosity goes in hand with dehydration of the sediments by release of pore water. With progressive accretion a narrow spaced foliation forms by dissolution-precipitation and recrystallization processes. Recrystallization of phyllosilicates
with a strong preferred orientation does not only form foliation planes, but also increasingly affects the chemical resetting of the isotopic K-Ar system and the individual phyllosilicate chemistry. Changes in the phyllosilicate chemistry by metamorphic reactions releases water causing further dehydration. As the result of the reduction in matrix porosity, fluid flow through these slates must be controlled by fracturing parallel to the foliation network, which increases in density towards the inner wedge resulting in a highly
anisotropic fluid flow parallel to the main foliation. In terms of deformation mechanisms, slow deformation in the slate matrix by ductile dissolution precipitation creep alternates with brittle fracturing. In this way, there exists feedbacks between grain-scale slow
fluid liberation, strain localization, fracturing and fluid flux. Fracturing temporarily enhances fracture porosity and is documented by i) pervasively distributed micron veinlets, with very high spatial densities and ii) by localised vein arrays, where strain is high and the foliation increasingly dense. Whereas the micron veinlets document a pervasive and disperse fluid flow though former micro fractures, fluid flow in the vein arrays through a series of foliation parallel fractures, is rather localised and episodic. Vein arrays are identified over a range of scales from the mm-m, the 100 m up to the km-scale and are therefore important structures facilitating fluid flow over larger scales. Given the size and reoccurrence intervals of the observed vein arrays, we infer that these are potentially displaying the seismic nature of episodic deformation in the inner wedge as observed in active accretionary prisms.

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