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

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.