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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

Ishii, Eiichi 

Smectite-bearing shear zones are crucial in a variety of geoscientific and geoengineering fields due to the unique physicochemical properties of smectite. Although such shear zones worldwide occur in marine argillaceous/siliceous sediments, tephra-derived and host rock-derived are possible as their origin, and these shear zones potentially differ in amount of smectite, extent of the spatial distribution, and time of the formation. Nevertheless, it is hard to discriminate these origins by observation. This paper proposes a method utilizing the following two indicators for the discrimination: one is a proportion ratio of smectite to illite, sm/(ill+sm), in the clay fractions of the samples; the other is a concentration ratio of conservative elements, Al$$_{2}$$O$$_{3}$$/TiO$$_{2}$$ (wt%/wt%), in bulk chemistry of the samples. Although high sm/(ill+sm) values can be an evidence indicating tephra-derived origin, the values may decrease by mineralogical disturbance/alteration during shear deformation, which make the discrimination difficult. The Al$$_{2}$$O$$_{3}$$/TiO$$_{2}$$ values different from those of the host rock can also be an evidence of tephra-derived origin, based on previous studies, and the measurement error is also very small, compared to the analytical error of sm/(ill+sm). But the Al$$_{2}$$O$$_{3}$$/TiO$$_{2}$$ values of shear zones may intrinsically be the same as those of the host rock even if those shear zones were derived from tephra layers. Application of these indicators to the protolith identification of bedding-parallel clay-rich shear zones in a folded siliceous mudstone (Hokkaido, Japan) demonstrates that combining sm/(ill+sm) and Al$$_{2}$$O$$_{3}$$/TiO$$_{2}$$ is useful as a first diagnosis to identify the protolith more easily and more confidently.

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