[S-GL40] Regional geology and tectonics

[SGL40-05] Duplexes and other geologic structures in the Misaki Formation of the southernmost Miura Peninsula: Revisited

*Yujiro OGAWA¹, Shin'ichi MORI², Atsushi NOZAKI³ (1.No affiliation, 2.Hiratsuka City Museum, 3.Yokohama National University, Graduate School of Environmental and Information Science)

Keywords:Jogashima Island, Misaki Formation, duplex, thrust system, fault-propagation fold

The Jogashima Island in the southernmost Miura Peninsula, Miura City, Kanagawa Prefecture has been studied since Kodama (1968) and many others. We independently studied the geologic structures of the Misaki Formation for these 40 years (Ogawa & Taniguchi 1988; Hanamura & Ogawa, 1993; Stow et al., 1998; Yamamoto et al., 2009), and reached many interesting conclusions including duplex structures. They are summarized as follows; The Tsurugizaki anticline trends not westward but northwestward in an en echelon pattern, whose south wings dip steeply while north wings gently dip, similar to the Nankai prism toes (Kawamura et al., 2009; Anma et al., 2011). The Toriya anticline is overturned southward, displaying many thrust faults and duplex structures, as the western extension of the Sengen thrust as a fault-propagation fold (Yamamoto et al., 2005). Not all the thrust duplex structures have south-vergency but some north, also resembling the Nankai. Both thrust duplexes and conjugate thrust systems have northwest-trending P-axes in horizontal reference frame (Ogawa & Horiuchi, 1978). Those duplexes and thrust faults are either of tectonic origin or slump origin, the latter may relate to liquefaction and mud-diapirism. At Awazaki, easternmost tip of the Jogashima, the subsequent Misaki Formation is eroded more than 300 m with 30 degrees discordance, suggesting the anticline-syncline formation began during deposition of the superjacent Hatsuse Formation. N-S or N30E-trending left-lateral fault system that is overwhelmed in the peninsular and Tokyo Bay areas to the Sagami trough as discussed by Mori et al (2010). As a result both the Miura and Boso tectonics are characterized by the Izu forearc collision, but the internal structures are similar to those in the Nankai prism (Yamamoto, 2005; Yamamoto et al., 2009; Michiguchi and Ogawa, 201).
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