Nappe structure of the Sambagawa belt

Abstract

The thermal structure of the Sambagawa belt, which is reflected by minerals produced during the highest temperature of metamorphism, generally is characterized by the occurrence of the highest grade schists in the middle of the structural pile. The origin of this structure has been analysed in the upper horizon of the Sambagawa schist sequence in Central Shikoku as an example for the structure of the whole belt. The upper horizon of the Sambagawa schist sequence in Central Shikoku consists of three nappes, the Saruta nappe 11, the Saruta nappe I and the Fuyunose nappe in descending order of structural level. The Saruta nappe Il shows a downward increase of metamorphic grade from the garnet zone, through the albite-biotite zone, to the oligoclase-biotite zone. The Saruta nappe I consists mainly of rocks in the albite-biotite zone and partly in the oligoclase-biotite zone, but the direction of the increase of metamorphic grade is not clear. The Fuyunose nappe shows an upward increase in metamorphic grade which changes from the glaucophane zone to the barroisite zone. It is concluded that the Saruta nappe I1 and Saruta nappe I were overturned and then mechanically coupled with the Fuyunose nappe. The Sambagawa metamorphic field, which is of the highest temperature phase of metamorphism, appears to have had an inverted thermal gradient and a thermal structure comparable with that expected in the deeper parts of a subduction complex.

Provenance and Dispersal Patterns of Sediments in the Turbidite Wedge of Nankai Trough

“…Thus, it is difficult to account for the documented influx of neovolcanic material in the turbidite sands. Third, the south side of the drainage divide on the island of Shikoku (as well as on the Kii Peninsula) includes high-pressure pelitic and basic schists of the Sambagawa Belt (Ernst et al, 1970;Banno, 1986;Hara et al, 1990;Higashino, 1990). Lying between the Sambagawa Belt and the Cretaceous Shimanto Belt is the Chichibu Belt (Fig.…”

Compositional zoning and inclusions of garnet in Sanbagawa metapelites from the Asemi-gawa route, central Shikoku, Japan

“…The northern albitebiotite zone is considered to be situated at a higher structural level than the southern albite -biotite zone. Based mainly on field observations, Hara et al (1990) interpreted the repetition of the albite -biotite zone as implying a pile -nappe structure, and suggested structural and thermal discontinuities between the mineral zone boundaries. However, the Mg -Fe 2+ partition coefficient between garnet and chlorite (K D ), and the continuous changes of the degree of graphitization, suggest no major discontinuity of metamorphic grade throughout the garnet and highergrade zones (Itaya, 1981;Banno and Sakai, 1989).…”

Coupled micro-faulting and pressure solution creep overprinted on quartz schist deformed by intracrystalline plasticity during exhumation of the Sambagawa metamorphic rocks, southwest Japan

“…The D 1 phase is characterized by ductile flow in an east-west direction, and during this phase of deformation the main foliation and lineation were formed. The lineation is defined by the shape-preferred orientation (SPO) of matrix amphiboles, which grew under retrograde conditions (Hara et al, 1990Wallis et al, 1992;Nakamura and Enami, 1994;Wintsch et al, 1999;Banno, 2000;Yagi and Takeshita, 2002;Toriumi, 2004, 2005), and by elongated quartz (Takeshita and Yagi, 2004), whereas the shape-preferred orientation of phengite and flattened quartz 8 define the foliation.…”