Changes in illite crystallinity within an ancient tectonic boundary thrust caused by thermal, mechanical, and hydrothermal effects: an example from the Nobeoka Thrust, southwest Japan

Geological setting and borehole core lithology

Geological setting

The Nobeoka Thrust is a large thrust bounding the northern and southern Shimanto Belt, the Cretaceous-Paleogene accretionary complex in Kyushu, Southwest Japan (Kimura 1998; Murata 1998; Kondo et al. 2005; Figure 1a,b). The Nobeoka Thrust presents the largest lithological and paleothermal gaps in the Shimanto Belt in Kyushu and is traced for approximately 100 km. The thrust is well exposed along the rocky coastline near Nobeoka City (Figure 1c). The hanging wall of the thrust is the Eocene Kitagawa group, composed of phyllitic shales and sandstones (Okumura et al. 1985; Kondo et al. 2005), whereas the footwall is the Hyuga group, consisting of a cataclastic mélange of shale matrix with sandstone and basaltic blocks (Nishi 1988; Kimura et al. 1991; Kondo et al. 2005). Microfossil assemblages indicate the depositional age to be from middle Eocene to early Oligocene (Saito et al. 1996). The paleotemperatures of the hanging wall and footwall based on vitrinite reflectance analysis are estimated to be approximately 320°C and 250°C, respectively (Kondo et al. 2005).

The 70°C temperature difference between the hanging wall and footwall is explained by the displacement of approximately 8.6 to 14.4 km along the thrust under a depth of up to 11 km beneath the seafloor (Kondo et al. 2005; Hara and Kimura 2008; Raimbourg et al. 2009). Such a displacement is comparable to that of the deep portion of the modern megasplay fault in the Nankai Trough (Kondo et al. 2005; Hamahashi et al. 2013). Several detailed structural investigations focused on good surface exposure have been performed (Kondo et al. 2005; Okamoto et al. 2006; Yamaguchi et al. 2011; Kimura et al. 2013).

Borehole core lithology

The Nobeoka Thrust Drilling Project (NOBELL) was conducted (see Figure 1c for the drill sites) to investigate the mechanics and evolution of the fossilized tectonic boundary thrust. Coring and geophysical logging operations were performed from July to September 2011. Continuous core samples down to 255 m below the ground surface (hereafter referred as 'mbgs') were retrieved at the excellent recovery rate of 99.82% (Figure 2), and valuable datasets of geophysical logging were also collected.

The fault core (at the principal slip zone) of the Nobeoka Thrust was obtained at a depth of 41.3 to 41.8 mbgs. The fault core is composed of random, non-foliated cataclasite including angular to subangular clasts of sandstones, shales, and quartz veins (Hamahashi et al. 2013) (Figure 2d). Phyllites of alternating beds of sandstone and shale make up the hanging wall Kitagawa group (0 to 41.3 mbgs) (Figure 2a). Deformation structures such as phyllitic cleavages, kink folds of alternating beds of sandstones and shales, and quartz-carbonate veins characterize the hanging wall. A brittle damage zone of abundant boudinaged blocks of sandstone can be observed (Figure 2b,c), and broken phyllite containing pseudotachylyte is also present in the lowest part of the hanging wall (38.8 to 41.3 mbgs). The footwall Hyuga group (41.3 to 255 mbgs) is composed of scaly shale, tuffaceous shale, sandstone, and acidic tuff. All of these are strongly deformed to display cataclasites derived from mélange with abundant faults, fractures, and mineral veins on the core surfaces. The footwall is divided into four lithologic units (Figures 3 and 4) based on the abundance of sandstone and tuff. Random fabric cataclasites are dominant in unit 1, whereas units 2 to 4 are dominated by foliated cataclasites. A fault zone with fine-grained breccias comprises the boundary between units 3 and 4.

Experimental procedure

We conducted grinding experiments using a planetary ball mill housed at the Tokyo Gakugei University (FRITSCH P-6). A borehole core sample selected from 35.49 mbgs was used as the starting material of the grinding experiment. The sample had a relatively small IC value (0.27 (Δ°2θ), 35.49 mbgs) and high peak intensity (638 cps, 35.49 mbgs) and was therefore expected to be appropriate as the starting material to trace IC changes during the experiment. We also conducted grinding experiments with a sericite standard sample (JCSS-5102; The Clay Science Society of Japan [Miyawaki et al. 2010]) as the starting material, as it has an IC value (0.21 ∆°2θ), which is similar to that of the borehole core sample selected from 35.49 mbgs.

Dry grinding of the samples was conducted at a rotation speed of 400 rpm, with an orbital radius of 7 cm. The grinding vessel was made of tungsten carbide with a volume of 45 mL. We used 180 tungsten carbide balls (1.00 g each) and 6.0 g of sample together in the vessel. The grinding time ranged from 1 to 90 min. After the experiments, the IC values of the experimental products were analyzed as described in the following section.

Paleotemperature

The IC values show a clear difference between the hanging wall and footwall (Figure 4a, Table 1). Relatively homogenous IC values ranging from 0.26 to 0.32 ∆°2θ above 38.8 mbgs represent the maximum diagenetic burial temperature in the hanging wall. As for the footwall, the IC values range from 0.51 to 0.75 ∆°2θ. The samples collected from the two major fault zones (at approximately 41 and 114 mbgs) have large IC values and high illite peak intensities, suggesting crystal formation of illite due to hydrothermal activity (see the next section). We therefore excluded the IC values of the samples within these fault zones from the paleotemperature estimation.

The estimated paleotemperatures are 288°C to 299°C (294°C average with standard deviation of 4.3°C) in the hanging wall and 198°C to 249°C (228°C average with standard deviation of 12.5°C) in the footwall, respectively. These temperatures are approximately 20°C to 30°C lower than the previously estimated values based on a vitrinite reflectance analysis (approximately 320°C and approximately 255°C for the hanging wall and footwall, respectively; Kondo et al. 2005); however, a large temperature difference between hanging wall and footwall is nonetheless exhibited.Negative anomalies of IC values (i.e., illite crystal growth under higher temperature conditions) were not detected along the fault core (Figure 4). Their absence suggests that neither the temperature rise caused by frictional heat and/or hot hydrothermal fluid flow along the fault core nor its duration is enough to affect IC value.

Factors controlling illite crystallinity within fault zone

The IC values of the hanging wall damage zone below 38.8 mbgs (Figure 4b) increase systematically toward the fault core of the Nobeoka Thrust; in other words, the crystallinity of illite decreases toward the fault core. Similar IC behavior near a fault core was reported, for instance, from the Matapedia fault in Canada (Duba and Williams-Jones, 1983), the Sashiu fault in Southwest Japan (Yamakita et al. 1995), and the Akaishi Tectonic Line in central Japan (Tanaka et al. 1995). Tanaka et al. (1995) argued that IC values increase toward the fault because muscovite crystals are buckled, fractured, and comminuted into finer grain sizes or replaced by fine crystals via hydrothermal alteration and the recrystallization of micaceous clay minerals. Although Hara and Kimura (2000) pointed out that lithological differences, deformation, hydrothermal alteration, and weathering are the controlling factors for IC value, we first focused on the effect of deformation and fragmentation due to the faulting before moving on to examine the other effects such as hydrothermal alteration on the basis of bulk-rock mineralogy.

In our grinding experiments, IC value increased with grinding time, whereas illite peak intensity decreased with grinding time (Figure 5b, Table 2) regardless of whether natural (35.49 mbgs) or standard samples were used. The experimental results suggest that the poorly crystallized illite progressively increases as a result of comminution (Figure 6a,b). The IC values and peak intensities of the samples from 20 to 41 mbgs in the hanging wall exhibited a similar tendency to those from the grinding experiments. In the natural borehole samples of the hanging wall damage zone, IC value increases as it approaches the Nobeoka Thrust to approximately 0.30 Δ°2θ, whereas illite peak intensity decreases toward the Nobeoka Thrust to approximately 170 cps, at depths 40.95 mbgs (the blue, green, yellow, and orange solid squares in Figure 5a; the original data are shown in Table 2), except for three points characterized by high illite peak intensities (approximately 680 to 1300 cps) instead of relatively large IC values (0.38 to 0.40 Δ°2θ). The similarity between the experimental (Figure 6a,b) and natural (Figure 6,d) samples suggests that the degree of comminution increases progressively toward the fault core of the Nobeoka Thrust. Deformational features of the core samples (Figure 2a,b) are also consistent with this tendency.However, an abrupt increase in both IC value and illite peak intensity was observed just above the fault core (41.10 to 41.26 mbgs; two red points in Figure 5a): the IC value is 0.62 Δ°2θ and the peak intensities are approximately 2,239 to 2,269 cps. This trend is completely opposite to the tendency we observed in the grinding experiments and core samples above 40.95 mbgs, and it cannot be explained by comminution. The trend observed just above the fault core might be explained by an authigenic formation of small illite crystals (Figure 6f,g) caused by the circulation of hydrothermal fluid along the fault core, which is geologically recorded as mineral veins accumulated just above the fault core (Figure 2c).

Wintsch et al. (1995) demonstrated that the transformation of feldspar into phyllosilicate such as chlorite and mica results in the weakening of fault zones. Jefferies et al. 2006a,[b] also reported that plagioclase altered to sericite within the fault core of the Median Tectonic Line (MTL) in Japan. Hydrothermal alteration, which causes the breakdown of feldspar and the new formation of white mica, is a reasonable mechanism to explain the increase in the amount of illite in the fault core at around 41.34 mbgs. Interestingly, feldspar breakdown in fault core is opposite to some other thrusts in subduction setting. For example, plagioclase was newly formed by >350°C water-rock interaction in the fault core of the Pasagshak Point Thrust in the Kodiak accretionary complex, Alaska (Yamaguchi et al. 2014), although both the Nobeoka Thrust and the Pasagshak Point Thrust were active at similar background temperatures of approximately 250°C.There are a couple of possibilities to explain the three exceptional points in Figure 5a, that is, the high illite peak intensities (approximately 680 to 1,300 cps) with relatively large IC values (0.38 to 0.40 Δ°2θ). One possibility is that high peak intensities resulted from extremely enriched clay minerals in the host rock. Another possibility is a transitional or beginning stage of hydrothermal alteration that produced large amounts of illite. Further mineralogical investigations are necessary to reconcile the origin of illite with the high peak intensities and IC values.Based on the results of the study, we propose that the increase in IC values in the hanging wall damage zone of the Nobeoka Thrust is due to mechanical comminution, in light of our comparisons of borehole core samples (Figures 5a and 6c,d) and products of grinding experiments (Figures 5b and 6a,b). Moreover, the abrupt increase in IC value accompanied by high illite peak intensity (Figure 5a) is explained by hydrothermal alteration that causes plagioclase breakdown and formation of white micas (Figure 6e,f,g). The IC values within fault zones have the highest potential utility for detecting the effects of comminution and alteration rather than for the estimation of paleotemperatures.

The following conclusions were drawn from this study