Tectonic boundary between the Sanbagawa belt and the Shimanto belt in central Shikoku, Japan

Abstract

Mode of occurrence of high-P/T regional metamorphic belt is a key to understand the tectonic evolution of deepseated accretionary complex along the subduction zone.Thermobaric structure of the high-pressure and/or ultrahigh-pressure metamorphic belts have been examined to clarify their exhumation process e.g. Terabayashi et al., 1996;Ota et al., 2000; Correlation with surrounding geologic units is also important to verify the exhumation tectonics, and high-P/T regional metamorphic belts are bounded by a subhorizontal thrust on the bottom and by a subhorizontal normal fault on the top e.g. . It has been well documented from SW Japan

Age Distribution of Detrital Zircons in the Psammitic Schist of the Sanbagawa Belt, Southwest Japan

“…120-110 Ma （Okamoto et al, 2004） , which occupy the uppermost portion of the Sanbagawa Belt. Although some previous studies suggested that the Sanbagawa Belt consists of metamorphosed Late Jurassic to Early Cretaceous accretionary complex, the present study shows that the belt is largely occupied by metamorphosed Late Cretaceous rocks : the Shimanto Metamorphic Rocks of Aoki et al （2007） . As a result, the Sanbagawa Belt consists of the following three units with different protolith ages : （1） Lower Unit of Shimanto Metamorphic Rocks with proto- …”

SHRIMP U-Pb dating of detrital zircons from the Sanbagawa Belt, Kanto Mountains, Japan: need to revise the framework of the belt

“…Recently, Aoki et al (2007) found Late Cretaceous detrital zircons present only in a sample from the Oboke nappe while they found only Proterozoic zircons in a sample from the Besshi nappe. They concluded that the Oboke nappe acts as a "window" in which the tectonically lower Late Cretaceous accretionary complex, i.e., Shimanto Belt, appears.…”

“…This indicates that the main provenances of clastic rocks in the Sanbagawa Belt on the Kanto Mountains are Cretaceous igneous rocks. Although Proterozoic to Jurassic detrital zircons are found in clastic rocks in Japan Tsutsumi et al, 2000Tsutsumi et al, , 2003Aoki et al, 2007), Proterozoic zircons are originally derived from the North China Craton and/or the Korean Peninsula. The ages of the basement rocks in the North China Craton have been found to be in the ranges of approximately 2600 -2400 Ma and 2000 -1750 Ma (Zhao et al, 2001).…”

“…In addition, they applied the previously proposed chronological history (e.g., Isozaki and Itaya, 1990) of the Sanbagawa Belt to the Besshi nappe. Thus, Aoki et al (2007) believed that at least the Besshi nappe was a metamorphosed phase of the Chichibu Belt. The difference in the results obtained by our study and those of the study conducted by Aoki et al (2007) implies that the estimated accretionary ages of the Sanbagawa Belt are different between the Kanto Mountains and Shikoku Island Figure 7.…”

“…The reasons for the lack of fossil evidence for the accretionary age of the Sanbagawa Belt are probably the strong deformation and occurrence of recrystallization in this belt. Recently, Aoki et al (2007) attempted at elucidating the detrital zircon ages by studying the Sanbagawa schists on Shikoku Island and concluded that a portion of the belt, i.e., Oboke nappe, consists of a Late Cretaceous accretionary complex corresponding to the Shimanto Belt. Wallis and Anczkiewicz (2005) reported up to 89 Ma of garnet and omphacite Lu -Hf ages of eclogite as the age of peak metamorphosis of the eclogite nappe on Shikoku Island.…”