Clay minerals in Alpine Fault gouge: First results from the DFDP-1B pilot hole

Clay mineralization is increasingly recognized as a key process along fault systems in the upper crust. The Alpine Fault in New Zealand is a major active fault zone with locally large earthquakes. Samples from this fault zone offer excellent opportunity to investigate recent and ancient rupture zones, and the mechanical role of clay mineral transformations and fluid-rock interactions in particular. The Alpine Fault drilling project (DFDP-project) on the South Island of New Zealand sampled two shallow pilot holes; DFDP-1A was drilled down to 100.6 m and DFDP-1B drilled down to 151.4 m. Five samples from borehole DFDP-1B have been investigated by X-ray diffraction, X-ray texture goniometry and electron microscopy. These samples were taken at ~143.3 m (sections 69_2 to 69_2) and ~128.1 m depth (sections 59_1 to 59_1); the latter is the area of principal slip. The bulk rock mineralogy shows similar compositions in all samples with quartz, phyllosilicates (muscovite, chlorite), calcite, zeolite and clay minerals; the dominant clay phases in all samples are illite and chlorite. Importantly, abundant discrete smectite is uniquely present in gouge zones at sections 69_2 (~143.4 m) and 59_1 (~128.1 m). Smectite was likely formed by dissolution-precipitation reactions during displacement and movement of aqueous fluids along permeable fractures, at the expense of host rock minerals. Electron microscopy of fault gouge at section 69_2 shows small illite and smectite particles with pseudo-hexagonal shapes and variable amounts of K, Ca, Mg and Fe, growing adjacent to each other. Some distinct illite and smectite mineral veins form epitaxially on quartz-feldspar mineral surfaces. Clay fabric intensity, measured by X-ray goniometry, is higher outside the gouge zones (true cataclasite, section 69_1) with average fabric intensities of m.r.d. 3.5. Both gouge zones at sections 59_2 and 69_1 exhibit uniformly weak fabrics for illite and chlorite (m.r.d. ~2.5 on average). The weak fabrics in fault gouge support the interpretation that clay fabric development in shallow fault rocks is a function of authigenic mineral growth and not stress-induced particle orientation. The unique localization of smectite in fault gouge indicates that swelling clay minerals may dominate fault behavior at shallow depth, and that temporal and fluid histories may be obtained from study of these fault rocks and samples from planned, deeper drilling.