Low-grade evolution of clay minerals and organic matter in fault zones of the Hikurangi prism (New Zealand)

Abstract

Clay minerals and organic matter occur frequently in fault zones. Their structural characteristics and their textural evolution are driven by several formation processes: (1) reaction by metasomatism from circulating fluids; (2)in situevolution by diagenesis; and (3) neoformation due to deformation catalysis. Clay-mineral chemistry and precipitated solid organic matter may be used as indicators of fluid circulation in fault zones and to determine the maximum temperatures in these zones. In the present study, clay-mineral and organic-matter analyses of two major fault zones – the Adams-Tinui and Whakataki faults, Wairarapa, North Island, New Zealand – were investigated. The two faults analysed correspond to the soles of large imbricated thrust sheets formed during the onset of subduction beneath the North Island of New Zealand. The mineralogy of both fault zones is composed mainly of quartz, feldspars, calcite, chabazite and clay minerals such as illite-muscovite, kaolinite, chlorite and mixed-layer minerals such as chlorite-smectite and illite-smectite. The diagenesis and very-low-grade metamorphism of the sedimentary rock is determined by gradual changes of clay mineral ‘crystallinity’ (illite, chlorite, kaolinite), the use of a chlorite geothermometer and the reflectance of organic matter. It is concluded here that: (1) the established thermal grade is diagenesis; (2) tectonic strains affect the clay mineral ‘crystallinity’ in the fault zone; (3) there is a strong correlation between temperature determined by chlorite geothermometry and organic-matter reflectance; and (4) the duration and depth of burial as well as the pore-fluid chemistry are important factors affecting clay-mineral formation.

How Clay Mineral Assemblages Affect Instability on the Upper Slope of the Hikurangi Subduction Zone, New Zealand

“…Basement rocks across New Zealand (Figure 1a) consist mostly of illite‐rich metagraywackes and argillites of the Torlesse and Waipapa terranes (MacKinnon, 1983; Sporli, 1978; Warr & Cox, 2016). Younger cover sequences and forearc deposits in the East Coast Basin, and in regions immediately southeast of the Axial Range (Figure 1a), are composed of moderately indurated illite‐rich sedimentary strata (e.g., Browne, 2004; Maison et al., 2018; Neef, 1999). Accordingly, suspensions in the WCC are almost certainly illite‐rich.…”

Composition of fine‐grained sediment in the Hikurangi Trough: Evidence for intermingling among axial gravity flows, transverse gravity flows and margin‐parallel ocean currents

“…It seems logical, therefore, to expect sediment derived from those watersheds to contain abundant detrital illite, along with lesser amounts of detrital chlorite. That expectation is confirmed by analyses of illite‐rich soil samples from steep‐land fields (Officer et al ., 2006) and by XRD analyses of illite‐rich mudrocks in the East Coast Basin (Maison et al ., 2018) (Fig. 12).…”

How clay mineral assemblages affect instability on the upper slope of the Hikurangi subduction zone, New Zealand

“…Younger cover sequences and forearc deposits in the East Coast Basin, and in regions immediately southeast of the Axial Range (Fig. 1a), are composed of moderately indurated illite-rich sedimentary strata with scattered tuff and bentonitic marl (e.g., Neef, 1999;Browne, 2004;Maison et al, 2018;McArthur et al, 2020). Rivers in the area (e.g., the Waipaoa River, Fig.…”