Fissure fills along faults: Variscan examples from Gower, South Wales

• Published in: Geological magazine Vol. 146; no. 6; pp. 890 - 902
• Main Authors: WRIGHT, V., WOODCOCK, N. H., DICKSON, J. A. D.
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 01.11.2009
• Subjects: active faults; breccia; Calcite; carbonate rocks; Carboniferous; chemical composition; crust; deformation; Europe; fault breccia; faults; fissures; fluid phase; folds; geochemistry; geometry; Glamorgan Wales; Gower Peninsula; Great Britain; hydrocarbons; limestone; lithostratigraphy; organic compounds; orogeny; Paleozoic; permeability; rock, sediment, soil; sedimentary rocks; South Wales; stratigraphic units; structural analysis; Structural geology; tectonics; United Kingdom; upper crust; Variscan Orogeny; vein; veins; void; Wales; water-rock interaction; Western Europe; Carboniferous; fault breccia; void; vein
• ISSN: 0016-7568; 1469-5081
• DOI: 10.1017/S001675680999001X

The extent to which persistent, rather than transient, fissures (wide planar voids) can exist along upper crustal faults is important in assessing fault permeability to mineral and hydrocarbon-bearing fluids. Variscan (late Carboniferous) faults cutting Dinantian (Lower Carboniferous) limestones on the Gower peninsula, South Wales, host clear evidence for fissures up to several metres wide. Evidence includes dendritic hematite growth and elongate calcite growth into open voids, spar ball and cockade breccia formation, laminated sediment infill and void-collapse breccias. Detailed mapping reveals cross-cutting geometries and brecciation of earlier fissure fills, showing that fissures were formed during, rather than after, active faulting. Fissures therefore probably formed by geometric mismatch between displaced fault walls, rather than by solution widening along inactive faults.