**2. Geological setting of Okitsu melange and the northern boundary fault zone with pseudotachylytes**

The Shimanto Belt is an ancient accretionary complex exposed on land from the Kanto region to the Okinawa islands, Japan, almost parallel to the Nankai Trough (Fig. 1). The Shimanto Belt is divided into two units on the basis of age, the northern Cretaceous Unit and the southern Tertiary Unit (Taira et al., 1988) (Fig. 1A). On Shikoku Island, the 4th largest island in Japan, the Shimanto Belt is bounded by the Butsuzo Tectonic Line from the Chichibu Belt, a Jurassic accretionary complex (Fig. 1B), and is further classified by its lithology as a melange unit and a coherent unit (Taira et al., 1988) (Fig. 1B). A melange unit is composed of chaotic rocks representing blocks in matrix textures. Most of the melange in the Shimanto Belt is tectonic in origin (e.g., Kimura and Mukai, 1991; Onishi and Kimura, 1995; Hashimoto and Kimura, 1999). A coherent unit is composed mainly of an alternation of sandstone and mudstone with weaker deformations (Taira et al.1988).

The study area is in the Okitsu mélange, Shikoku Island, SW Japan. The Okitsu melange is located at the southern end of the Cretaceous Unit (Fig. 1). The main lithology of the Okitsu melange is sandstone and black shale showing tectonic melange textures, minor basalts, cherts, red shale, and tuff. Foliations are well developed within the shale matrices. The mélange foliations strike ENE–WSW and dip steeply to the north. The radiolarian age of the Okitsu melange is Cenomanian to Turonian from cherts, and Santonian to Campanian from black shale (Taira et al., 1988). The paleo-maximum temperature of the mélange is about 270 (± 30)°C, based on vitrinite reflectance (Sakaguchi, 1999).

The northern boundary fault bounds the Okitsu mélange from Nonokawa formation, a coherent unit in the north (Fig. 1B). The fault zone is about 5–10 m wide (Fig. 2), and mainly develops within the Okitsu melange. Basalt blocks, and black shale matrices mixed with tuffaceous shale in some parts, are included within the fault zone, which strikes ENE–WSW, almost parallel to the melange foliations (Fig. 2).

to help understand rock-fluid interactions at the time of seismogenesis or related

The studied fault is a fossil seismogenic fault along a subduction interface, in the Okitsu melange, the Cretaceous Shimanto Belt, SW Japan. The Shimanto Belt is the most studied on-land accretionary complex in the world, with lithology, age, thermal structure, and deformation structures available. These studies have revealed that the Shimanto Belt includes a deformation along its subduction interface from underthrusting to underplating, and that the Shimanto Belt is experienced at the seismogenic depth on the basis of the thermal model for seismogenic zones (Hyndman and Wang, 1993; Oleskevich et al., 1999). At the northernmost boundary fault of the Okitsu melange, the first pseudotachylyte within the sedimentary rocks was reported (Ikesawa et al., 2003), indicating that the fault was

We conducted an X-ray diffraction (XRD) analysis on the host and fault rocks along the fossil seismogenic fault, and examined mineralogy, iron and magnesium substitution in chlorite, illite crystallinity, and semi-quantification of illite and chlorite to determine the clay characteristics for seismogenic fault rocks, in comparison with those of the host rocks. Finally, characteristic rock-fluid interactions in seismogenic faults, due to melt lubrication

**2. Geological setting of Okitsu melange and the northern boundary fault zone** 

The Shimanto Belt is an ancient accretionary complex exposed on land from the Kanto region to the Okinawa islands, Japan, almost parallel to the Nankai Trough (Fig. 1). The Shimanto Belt is divided into two units on the basis of age, the northern Cretaceous Unit and the southern Tertiary Unit (Taira et al., 1988) (Fig. 1A). On Shikoku Island, the 4th largest island in Japan, the Shimanto Belt is bounded by the Butsuzo Tectonic Line from the Chichibu Belt, a Jurassic accretionary complex (Fig. 1B), and is further classified by its lithology as a melange unit and a coherent unit (Taira et al., 1988) (Fig. 1B). A melange unit is composed of chaotic rocks representing blocks in matrix textures. Most of the melange in the Shimanto Belt is tectonic in origin (e.g., Kimura and Mukai, 1991; Onishi and Kimura, 1995; Hashimoto and Kimura, 1999). A coherent unit is composed mainly of an alternation of sandstone and

The study area is in the Okitsu mélange, Shikoku Island, SW Japan. The Okitsu melange is located at the southern end of the Cretaceous Unit (Fig. 1). The main lithology of the Okitsu melange is sandstone and black shale showing tectonic melange textures, minor basalts, cherts, red shale, and tuff. Foliations are well developed within the shale matrices. The mélange foliations strike ENE–WSW and dip steeply to the north. The radiolarian age of the Okitsu melange is Cenomanian to Turonian from cherts, and Santonian to Campanian from black shale (Taira et al., 1988). The paleo-maximum temperature of the mélange is about 270

The northern boundary fault bounds the Okitsu mélange from Nonokawa formation, a coherent unit in the north (Fig. 1B). The fault zone is about 5–10 m wide (Fig. 2), and mainly develops within the Okitsu melange. Basalt blocks, and black shale matrices mixed with tuffaceous shale in some parts, are included within the fault zone, which strikes ENE–WSW,

formed by melt lubrication along the subduction interface.

along the subduction interface, are discussed.

mudstone with weaker deformations (Taira et al.1988).

(± 30)°C, based on vitrinite reflectance (Sakaguchi, 1999).

almost parallel to the melange foliations (Fig. 2).

**with pseudotachylytes** 

phenomena.

Fig. 1. A) Distribution of the Shimanto Belt along SW Japan. B) Distribution of mélange and coherent units in SW Shikoku Island. Study area is also shown.

Fig. 2. A) A photo of the study area from ENE to WSW. B) Route map of the northern boundary fault zone between Okitsu melange and Nonokawa formation.

Cataclasites of 2–3 m in thickness are well observed along the fault zone (Fig. 2). The cataclasites include pseudotachylyte, as reported by Ikewasa et al. (2003). They estimated a temperature rise for the fault of at least 450°C, based on the mineral assemblage of the pseudotachylyte.

Fig. 2. A) A photo of the study area from ENE to WSW. B) Route map of the northern

Cataclasites of 2–3 m in thickness are well observed along the fault zone (Fig. 2). The cataclasites include pseudotachylyte, as reported by Ikewasa et al. (2003). They estimated a temperature rise for the fault of at least 450°C, based on the mineral assemblage of the

boundary fault zone between Okitsu melange and Nonokawa formation.

pseudotachylyte.

We focused on two kinds of fault rocks. One is the tectonic melange, as the host rock, and the other is the cataclasites containing pseudotachylyte, as fossil seismogenic fault rocks. The latter type are developped within the host rocks of the tectonic melanges, and thus a comparison of clay minerals in the host rocks and the cataclasites provides the characteristic modification of clay minerals along seismogenic faults.
