Hydrothermal activity plays an important role in hydrological circulation, heat advection and element migration in the crust. Such activity can be related to magmatism, heat budget, crustal strength (Meissner and Wever, 1992; Sibson and Rowland, 2003), and the generation of earthquakes (Zhao et al., 1996; Obara, 2002). In addition, it is also important for industrial and social demands related to the generation of ore deposits (Misla, 2000; Cox, 2005), development of geothermal resources (Bowen, 2011), and evaluation for safety and stability of underground facilities (National Institute of Advanced Industrial Science and Technology, 2016, 2017). Therefore, it is useful to develop techniques to detect evidence of hydrothermal activity and to evaluate the range of their effects.
Thermochronology can be used to constrain the thermal histories of rocks and minerals based on thermal resetting of radiometric ages. Since the 1970s, it has been applied to many geological events, such as, mountain building, basin development, and fault movement (e.g., Reiners et al., 2005; Ault et al., 2019). Elucidating past hydrothermal activity is also an important target of thermochronology. For example, a growing number of studies have applied thermochronometry to detect thermal anomalies derived from fluid activity along fault zones (e.g., Tagami et al., 2001; Murakami et al., 2002; Tagami and Murakami, 2007; Wölfler et al., 2010; Ault et al., 2016; Milesi et al., 2019, 2020; Louis et al., 2019; Sueoka et al., 2019; Jess et al., 2021). Furthermore, a number of studies have reported thermal anomalies and cooling dates related to ore-forming fluids (e.g., Arne et al., 1990; Wilson et al., 2003; Yuan et al., 2009; Márton et al., 2010; Wang et al., 2015). In all previous cases reported, the extent of the thermal anomalies typically ranged from 100 to 102 meters or wider.
In this study, we attempt to detect local thermal anomalies around hydrothermal alteration zones by using a combination of geothermometry and multisystem thermochronology. An alteration zone is formed by paleo-fluid activity along a minor fault or crack. Such a minor fault or crack is expected to act as a temporal or relatively short-term pathway for fluids, in contrast to a major fault zone which is a stable and long-term pathway along which fluid activity over multiple pulses is often recorded (e.g., Tanaka et al., 2007), and a wide range of thermal anomalies are detected (e.g., Tagami et al., 2001; Murakami et al., 2002). However, hot springs are not always distributed along major fault zones (e.g., Kimbara, 2005; Tamburello et al., 2022). Therefore, minor faults and cracks can be important factors that control crustal permeability. Hence, thermal anomalies along alteration zones, namely, fossils of paleo-fluid activity along minor faults and cracks, can provide clues for assessing the stability of individual fluid pathways and understanding the macro-permeability of the crust.
We investigated outcrops of hydrothermal alteration zones in the Hongu area of the Kii Peninsula, southwest Japan, where non-volcanic thermal fluid activity has been reported (e.g., Matsumoto et al., 2003; Umeda et al., 2006; Yamaguchi et al., 2009; Morikawa et al., 2016). The Kii Peninsula contains no Quaternary volcanoes, thus providing a preferable area for studying the thermal effects of hydrothermal events without possible thermal disturbances caused by recent volcanism. The homogenization temperatures of the fluid inclusions in the quartz veins of the alteration zones were measured to estimate the temperatures of the thermal fluids that formed the alteration zones. We also applied thermochronometric methods to the host rock enclosing the alteration zones in an attempt to constrain the timing, duration and spatial range of thermal-fluid activity. These methods include apatite and zircon fission-track (hereinafter called AFT and ZFT), zircon (U–Th)/He (hereinafter called ZHe), and zircon U–Pb analyses. Apatite (U–Th)/He (hereinafter called AHe) method was not adopted because the apatite grains, being generally small and non-euhedral, were not suitable for analysis. Using thermochronometry data, we also discuss the thermal and exhumation histories of accretionary complexes in the study area.
Geological Setting
The Kii Peninsula is located in the Outer Zone of the Southwest Japan Arc, on the forearc side of the Median Tectonic Line (Fig. 1a). The Kii Peninsula features a number of high-temperature hot springs in the Shirahama, Katsu’ura, Ryujin, Tosenji, Totsukawa, and Hongu areas, although they are a few hundred kilometers away from the volcanic front of the Southwest Japan Arc (Fig. 1b). Various geochemical and geophysical studies support the finding that the high-temperature fluids in the Kii Peninsula originate from the Philippine Sea slab beneath this region. For example, 3He/4He ratios as high as ~ 4–5 RA (Matsumoto et al., 2003; Umeda et al., 2006, 2007; Morikawa et al., 2016; 1 RA = atmospheric 3He/4He ratio) and high Li/Cl ratios by weight (> 0.001) (Kazahaya et al., 2014) indicate that the groundwater contains deep-seated fluids. Seismicity, including seismic swarms (Kato et al., 2014) and deep low-frequency tremors (Obara, 2002), supports fluid migration in the crust beneath the Kii Peninsula, and low resistivity (< 10 Ωm) between the Conrad discontinuity and the slab surface (Yamaguchi et al., 2009) suggests upward migration of fluid from the subducting slab.
The Kii Peninsula is composed predominantly of the Cretaceous to Miocene accretionary complexes of the Shimanto Belt and Miocene silicic rocks. The rocks of the Shimanto Belt have been roughly divided into the Cretaceous Hidakagawa, the Eocene Otonashigawa, and the Oligocene to early Miocene Muro Groups from the back-arc to forearc sides (Tokuoka et al., 1981) (Fig. 1b). In the Hongu area, the Haroku Formation of the Otonashigawa Group consists mainly of alternating sandstones and mudstones (Tokuoka et al., 1981). The Haroku Formation yielded Eocene radiolarian fossils (Suzuki, 1993), and detrital zircons from the tuffaceous sandstone yielded U–Pb ages showing the youngest population of 50.8 ± 1.0 Ma (Tokiwa et al., 2016). Illite crystallinity values of 0.54 and 0.64 Δ°2θ were obtained from the northern area of the Otonashigawa Group in the western Kii Peninsula, which were converted into paleo-temperatures of ~ 227°C and ~ 192°C with an error of at least 50°C using the calibration of Underwood et al. (1993) (Awan and Kimura, 1996). For the Hidakagawa Group to the north of the Otonashigawa Group (Fig. 1b), the paleo-temperatures of ~ 310°C and ~ 250°C were similarly obtained in the northern and southern parts, as well as the pressures of ~ 3.6 kbar and ~ 2.2 kbar (equivalent to the burial depths of ~ 14 km and ~ 8 km) based on the illite b0 lattice spacing values (Awan and Kimura, 1996). Considering the southward lowering trend of the paleo-temperatures, the burial depth of the Otonashigawa Group is thought to have been shallower than ~ 8 km.
ZFT and AFT thermochronology have been reported in previous studies that investigated the thermal and exhumation history of the accretionary complexes of the Shimanto Belt on the Kii Peninsula (e.g., Tagami et al., 1995; Hasebe and Tagami, 2001; Hasebe and Watanabe, 2004; Umeda et al., 2007; Hanamuro et al., 2008; Ohira et al., 2016), as well as in other regions of the Southwest Japan Arc (Hasebe et al., 1993b, 1997) (Fig. 2). On eastern Shikoku Island, the maximum temperature during accretion varied regionally. Cretaceous rocks of the Northern Shimanto Belt were heated to the partial annealing zone (PAZ; see Section 3.1 for more details) of ZFT system, yielding grain ages younger than the depositional ages. However, the grains in the rocks of the Southern Shimanto Belt are older than the Eocene–Miocene time of deposition, and as such, have not been heated to temperatures in the ZFT PAZ (Hasebe et al., 1993b; Tagami et al., 1995). In contrast, AFT ages of ~ 10 Ma were obtained in both the Northern and Southern Shimanto Belts. These AFT ages are significantly younger than the deposition ages, reflecting a rapid cooling and exhumation episode straddling the AFT PAZ at ~ 10 Ma (Hasebe et al., 1993b). Fission-track data in western Shikoku Island and Kyushu Island indicate similar thermal histories, except for local reheating episodes, such as the granitic intrusion at ~ 15 Ma (Tagami and Shimada, 1996; Hasebe et al., 1997; Hasebe and Tagami, 2001). Fission-track data for the Kii Peninsula are also largely consistent with other regions, but differ in the following points: (1) AFT ages young southward from ~ 35 Ma to ~ 13 Ma in the Northern Shimanto Belt and ~ 6 Ma in the Southern Shimanto Belt (Hasebe and Tagami, 2001); (2) young AFT ages (~ 5–2.5 Ma) near hot springs have been reported (Umeda et al., 2007; Hanamuro et al., 2008); (3) a patchy age distribution of Miocene ZFT ages is likely related to heat influx (Hasebe and Watanabe, 2004); and (4) accretion-related heating to the upper limit of the ZFT PAZ has been determined for some regions of the Northern Shimanto Belt (Ohira et al., 2016).
Miocene silicic rocks in the Kii Peninsula have been divided into the Kumano Acidic Rocks and Omine Acidic Rocks. The Kumano Acidic Rocks were generally formed by igneous activity of the Kumano Caldera (Miura, 1999; Kawakami et al., 2007), with some exceptions. For example, the Konogi Rhyolite, forming the lower part of the Kumano Acidic Rocks, was produced by subaerial eruptions from fissures preceding the main eruption from the Kumano Caldera (Aramaki, 1965). The lithologic units of the Kumano rocks consist of granite, granite porphyry, and pyroclastic breccia found within the accretionary complex of the Shimanto Belt (Miura, 1999). The Omine rocks are distributed in several separate units and are composed mainly of granodiorite, granite, and granite porphyry (Kawasaki, 1980). The arcuate and semicircular dike swarms of the Omine rocks in the central part of the Kii Mountains are thought to have been formed in association with the formation/collapse of the Omine and Odai Cauldrons, respectively (Sato and Yamato Omine Research Group, 2006), although many of the plutonic stocks of the Omine rocks are apparently not associated with these cauldrons. Radiometric ages for the Kumano Acidic Rocks and Omine Acidic Rocks are generally in the range of ~ 16–14 Ma, as estimated by various methods, including AFT and ZFT (Hasebe et al., 1993a, 2000; Iwano et al., 2007, 2009) (Fig. 2), biotite K–Ar (Sumii et al., 1998; Sumii and Shinjoe, 2003), and zircon U–Pb (Orihashi et al., 2007), suggesting that their formation and rapid post-emplacement cooling occurred during the middle Miocene.