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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

海成粘土質/珪質堆積物中の層理面に平行な粘土質せん断帯の起源特定; テフラ起源のせん断帯と母岩起源の断層ガウジの識別

石井 英一

Ishii, Eiichi

スメクタイトを含むせん断帯はスメクタイトの独特な物理化学特性により地球科学, 地球工学の様々な分野で重要となっている。同せん断帯は海成粘土質/珪質堆積物中に広く挟在するが、その起源としてテフラ起源と母岩起源のものが考えられ、両者は潜在的にスメクタイト含有量, 拡がり, 形成時期の点で異なる。しかし、その起源を観察により特定することは困難である。本論はその特定手法として、粘土鉱物中のイライトに対するスメクタイト含有量と全岩化学組成のAl$$_{2}$$O$$_{3}$$/TiO$$_{2}$$(wt%/wt%)を用いた手法を提案する。高いスメクタイト含有量はテフラ起源の証拠となり得るが、この値はせん断変形に伴う鉱物学的擾乱/変質により低下することがあり、識別が困難になる。先行研究に基づくと、母岩と異なるAl$$_{2}$$O$$_{3}$$/TiO$$_{2}$$もテフラ起源の証拠となり、その測定誤差もスメクタイトの含有量の誤差と比べれば非常に小さいが、例えそのせん断帯がテフラ起源であっても母岩と同じAl$$_{2}$$O$$_{3}$$/TiO$$_{2}$$を示すことがある。これらの指標を褶曲した北海道の珪質泥岩に適用した結果、両指標を組み合わせることにより、せん断帯の起源をより簡便に、より高精度で特定できることが確認できた。

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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