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- Title
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.
- Authors
Akker, Ismay Vénice; Schrank, Christoph; Herwegh, Marco; Berger, Alfons; Jones, Michael; Kewish, Cameron M.
- 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. Plain Language Summary: Convergent plate boundaries are locations where lithospheric plates collide. Plate collision produces a wide variety of seismic activity, which is a major natural hazard posing socio‐economic risks. When oceanic plates are involved in collision, an accretionary wedge made of sediments forms near the plate boundary. Here, we study an ancient accretionary wedge in the Swiss Alps to examine the potential traces of paleo‐seismic activity left in the rock record. We focus on ubiquitous veins, mineralized fractures that acted as fluid pathways in such active seismic domains. We documented veins from the field (m‐scale) to the grain scale (μm‐scale). Synchrotron X‐ray Fluorescence Microscopy (XFM) enabled mapping of trace‐element concentrations in the vein minerals, capturing tracers of paleo‐fluid transport at 2 μm resolution. The innovative XFM technique unveiled the presence of micron‐veinlets with widths less than that of a human hair. These micron‐veinlets form widespread, densely spaced clusters throughout these rock sequences. Our microanalytical observations demonstrate that these micron‐veinlets facilitate fluid transport via dissolution‐precipitation creep during interseismic deformation of the wedge while the m‐scale veins could potentially represent small earthquakes or slow slip events. Key Points: Cyclic fracturing, veining, and pressure solution are characteristic in the sediment‐rich inner part (>250°C) of the accretionary wedgeSynchrotron XFM documents pervasive micron‐veinlets enabling layer‐perpendicular fluid migration during slow wedge deformationLayer‐parallel veins arrange in vein‐arrays indicating localized fluid transport and potentially small seismic or slow‐slip events
- Subjects
ALPS; SWITZERLAND; VEINS (Geology); FLYSCH; PLATE tectonics; X-ray fluorescence; CALCITE; HAIR; VEINS
- Publication
Geochemistry, Geophysics, Geosystems: G3, 2023, Vol 24, Issue 10, p1
- ISSN
1525-2027
- Publication type
Article
- DOI
10.1029/2023GC010873