The Geometry, Spatial Distribution and Texture of Slate‐Hosted Calcite Veins in the Helvetic Flysch Units—Insights in Structural and Fluid Processes Within a Paleo‐Accretionary Complex

Abstract

Abstract The exhumed Infrahelvetic Flysch Units in the eastern central Alps in Switzerland are a field analog to modern accretionary wedges at active plate boundaries. In these seismically active convergent settings, water‐saturated sediments undergo consolidation, and diagenetic to low‐grade metamorphic processes cause complex fluid‐rock interactions. To contribute to the understanding of structural and fluid processes and their interaction with seismic activity, we present quantitative information on the geometrical and spatial distribution of slate‐hosted calcite veins from the Infrahelvetic Flysch Units that show mutual overprinting relationships with the ductile phyllosilicate‐rich matrix. Two vein systems that form in the deeper part of the inner wedge are characterized: (a) layer‐parallel veins (meter‐scale) forming spatially repetitive vein‐arrays and (b) pervasively distributed, steep micron‐veinlets, that cross‐cut the thicker layer‐parallel veins and the ductile matrix. Synchrotron X‐ray Fluorescence Microscopy (XFM) is instrumental in detecting previously unseen densely spaced micron‐veinlets. The spatial distribution of micron‐veinlets indicates pervasive layer‐perpendicular fluid transport in response to dissolution‐precipitation creep through the wedge. Layer‐parallel veins form vein‐arrays with thicknesses on the meter‐scale suggesting that fluids are progressively localized in channels up‐scale. Both vein sets form in an alternating fashion with two different enhanced flux directions, which could be indicative for a critically stressed wedge with near‐lithostatic fluid pressures. The layer‐parallel veins and vein‐arrays could represent seismic events with low magnitude earthquakes (Mw up to 4.0) or slow‐slip events currently found at active plate boundaries, while micron‐veinlets and dissolution‐precipitation processes accommodate slow interseismic deformation.