Deformation characteristics and peak temperatures of the Sanbagawa Metamorphic and Shimanto Accretionary complexes on the central Kii Peninsula, SW Japan

Highlights

• Deformation and temperature were examined in Cretaceous subduction complexes.

• The complex at 280–440 °C records exhumation-related deformation.

• The complex at 280–290 °C records accretion- and exhumation-related deformations.

• The two deformations occurred in different tectonic settings in the subduction zone.

• The generation of exhumation-related deformation involved ridge approach.

Abstract

Structural analyses based on field survey and peak-temperature estimations using Raman spectra of carbonaceous-material thermometry were conducted for the Cretaceous subduction complexes on the central Kii Peninsula, southwest Japan, where there is a direct contact region between the Sanbagawa high-P/low-T Metamorphic Complex (MC) and the Shimanto Accretionary Complex (AC). In the study area, the Kosoku, Iro, and Mugitani complexes are arranged in descending order of structural position. The Kosoku and Iro complexes record exhumation-related deformation, which produced schistosity, stretching lineation, and folds, and peak temperatures of 280–440 °C. In contrast, the Mugitani Complex records both earlier accretion-related deformation which produced block-in-matrix structures and later exhumation-related deformation, as well as peak temperatures of 280–290 °C. The Mugitani Complex shares deformation characteristics with both the Shimanto AC and the Sanbagawa MC, and covers the interval from the upper temperatures of the Shimanto AC to the lower temperatures of the Sanbagawa MC. In addition, the two types of deformation differ in their kinematic patterns, suggesting that they took place diachronously in different tectonic settings in the subduction zone. We propose that exhumation-related Sanbagawa deformation occurred in a warm tectonic environment, such as ridge approach to the subduction zone.

Introduction

The Japanese Islands have evolved within an oceanic plate subduction system that has existed along the eastern Asian continental margin for 500 Myr (e.g., Charvet, 2013, Isozaki et al., 2010, Maruyama et al., 1997, Taira et al., 2016). This subduction system has given rise to numerous tectonic events, including multiple periods of igneous activity with low-pressure/high-temperature (low-P/high-T) metamorphism, as well as the formation of high-pressure/low-temperature (high-P/low-T) metamorphic complexes, accretionary complexes, and forearc sedimentary basins. A high-P/low-T metamorphic complex and an accretionary complex have formed in the deep and shallow parts of the subduction zone, respectively. Investigation of deformation mechanisms and processes of the high-P/low-T metamorphic complex and the accretionary complex, and the relationships between the two, are key to understanding the large-scale evolution of deep to shallow subduction complexes (e.g., Aoya, 2002, Endo and Wallis, 2017, Hashimoto and Yamano, 2014, Kimura et al., 2012, Palazzin et al., 2016, Raimbourg et al., 2009, Toriumi, 1982, Tulley et al., 2020, Yamamoto, 2006).

The Sanbagawa Metamorphic Complex (MC) and the Shimanto Accretionary Complex (AC), which are located in the Outer Zone of southwest (SW) Japan, correspond to a high-P/low-T type metamorphic complex (Wallis and Okudaira, 2016) and a Cretaceous to Neogene accretionary complex (Kimura et al., 2016), respectively (Fig. 1a). Detrital zircon U–Pb ages obtained using laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) have recently been used to determine the depositional ages of metaclastic rocks in the Sanbagawa MC (e.g., Aoki et al., 2007, Aoki et al., 2019, Endo et al., 2018, Knittel et al., 2014, Knittel et al., 2019, Nagata et al., 2019), and of clastic rocks in the Shimanto AC (e.g., Hara and Hara, 2019, Hara et al., 2017, Shibata et al., 2008, Shimura et al., 2017, Shimura et al., 2020a, Tokiwa et al., 2017). These geochronological analyses have identified numerous detrital zircons of Late Cretaceous age in the Sanbagawa MC (Aoki et al., 2007, Aoki et al., 2019, Endo et al., 2018, Jia and Takeuchi, 2020, Knittel et al., 2014, Knittel et al., 2019, Nagata et al., 2019, Otoh et al., 2010, Shimura et al., 2019), for which protolith ages had previously been considered to be the Jurassic to Early Cretaceous, and the depositional ages for a large part of the Sanbagawa MC are the same as those for the Cretaceous part of the Shimanto AC. The Sanbagawa MC and the Shimanto AC are considered to have formed in the deep and shallow parts of the subduction zone, respectively, during the Cretaceous (e.g., Wallis and Okudaira, 2016).

The Sanbagawa MC underwent ductile deformation during exhumation (e.g., Aoya, 2002, Faure, 1985a, Faure, 1985b, Wallis, 1990, Wallis, 1998, Wallis et al., 1992, Yagi and Takeshita, 2002), whereas the Cretaceous part of the Shimanto AC exhibits brittle to brittle–ductile deformation structures that formed during accretion processes (e.g., Hashimoto and Yamano, 2014, Kimura et al., 2012, Needham, 1987, Raimbourg et al., 2009, Raimbourg et al., 2019, Ujiie et al., 2007). The mechanisms and processes of deformation clearly differ between the two complexes. However, previous structural studies of the Sanbagawa MC and the Shimanto AC have focused on either one or the other of the two complexes. In addition, little attention has been paid to nature of the change in deformation characteristics from the deep to shallow complexes in the Cretaceous subduction zone, which is essential for ascertaining the large-scale tectonic development of complexes. One reason for this uncertainty is that the Jurassic Chichibu AC generally covers the boundary between the Sanbagawa MC and the Shimanto AC on land (Ito et al., 2009) (Fig. 1a), preventing detailed investigation into the sequential structures from the Sanbagawa MC to the Shimanto AC and the physical relationship between the two.

On the central Kii Peninsula, the boundary between the two complexes is partly exposed, without being covered by the Jurassic Chichibu AC, owing to Miocene uplift (Kimura et al., 2014) (Fig. 1b), and this area is therefore one of the best localities for investigating the sequential structures from the Sanbagawa MC to the Shimanto AC on land. In the present study, structural analyses based on field survey were conducted for the two complexes on the central Kii Peninsula (Figs. 1b and 2). In addition, estimations of peak temperatures were also determined using Raman spectra of carbonaceous-material (RSCM) thermometry, as determination of peak temperature is critical for understanding the maximum burial depth in the subduction zone. Combining structural observations with thermal data, we aimed to elucidate the deformation and thermal characteristics of Cretaceous subduction complexes including the Sanbagawa MC and the Shimanto AC.