Protolith identification of bedding-parallel, smectite-bearing shear zones in argillaceous and siliceous marine sediments: Discriminating between tephra-derived shear zones and host-rock-derived fault gouges

Abstract

Abstract Smectite-bearing shear zones are important in a variety of geoscientific and geoengineering fields owing to the unique physicochemical properties of smectite. Such shear zones occur globally in argillaceous and siliceous marine sediments, and are typically a product of either host-rock deformation, or exploitation of weak, altered tephra horizons within the sedimentary sequence. However, it is difficult to discriminate between a tephra-derived and host-rock-derived origin based on simple fault-rock observations. Here, a method is proposed that integrates two discriminators: the ratio of smectite to illite, sm/(ill\u202f+\u202fsm) (wt%/wt%), in the clay fractions of shear zone samples; and the ratio of conservative elements, Al2O3/TiO2 (wt%/wt%), in the bulk chemistry of samples. Although high sm/(ill\u202f+\u202fsm) ratios may indicate a tephra-derived origin, deformation processes can lower the ratio, making protolith identification difficult. Previous studies have suggested that different Al2O3/TiO2 ratios between shear zones and host rocks may also imply a tephra-derived origin. However, the Al2O3/TiO2 ratios within shear zones may be intrinsically comparable with those of the host rock, even if the shear zones were derived from tephra layers. Despite these ambiguities, application of these two indicators to the protolith identification of bedding-parallel, smectite-bearing shear zones in a folded siliceous mudstone (Hokkaido, Japan) demonstrates that combining sm/(ill\u202f+\u202fsm) and Al2O3/TiO2 ratios is a useful starting point when attempting to identify the protolith of smectite-bearing shear zones in argillaceous and siliceous marine mudstones.