Tectonic Rotation of the Kanto Mountains, Related with the Opening of the Japan Sea and Collision of the Tanzawa Block since Middle Miocene

Paleomagnetism of the Miocene Igneous Rocks in the Uchiyama Area, Central Japan.

“…The paleomagnetic direction of the Chichibu Basin is D = 93.7° and I = 52.7° with the radius of the 95% confidence circle of 8.3°, after inversion of the reversed polarity to the normal polarity (HYODO and NIITSUMA, 1986). The upper part of the Neogene sedimentary rocks in the Chichibu Basin is correlated to the N8 (16.5-15.5 Ma) of the planktonic foraminiferal zone (BLOW, 1969) and to Chron C5B (HYODO and NIITSUMA, 1986). Therefore the Kanto Mountains including the Chichibu Basin rotated clockwise at about 94° relative to the geomagnetic field after 15.5 Ma.…”

“…While MATSUDA (1978 also placed the time of this cusp formation in the pre-Miocene, NIITSUMA (1982) set forth that the subduction of the Philippine Sea Plate initiated at 6-7 Ma in the Late Miocene caused the bending ofthe zonal structure. Furthermore, HYODO and NIITSUMA (1986) explained this cusp formation as a result of the collision of the Tanzawa Block with central Japan at 5 Ma. Recently, TAKAHASHI and NOMURA (1989) determined the paleomagnetic direction of the Late Miocene diorite body, which is located on the east wing of the Kanto Syntaxis, concluding that the Kanto Syntaxis was formed in the Late Miocene or earlier.…”

Paleomagnetism of Miocene Sedimentary Rocks in the Kakegawa Area and Implication for Tectonic Rotation in Central Japan.

“…Paleomagnetic data from the Kanto Mountains area (Hyodo and Niitsuma, 1986;Takahashi and Nomura, 1989;Takahashi and Watanabe, 1993) suggest that the eastern extension of the MTL and the Sanbagawa terrane were rotated through about 60° between 15.5 Ma and 12 Ma, and of about 35° between 12 Ma and 6 Ma, both in clockwise direction. Takahashi and Watanabe (1993) explained that the Kanto Mountains area was rotated along with Southwest Japan associated with the opening of the Japan Sea, followed by additional motion caused by the collision with the IzuBonin arc.…”

“…Our paleomagnetic data thus support the idea of Itoh (1988) that the bending of the MTL and of the zonal arrangement of the pre-Neogene terranes occurred after the Early Miocene. Several authors (e.g., Hyodo and Niitsuma, 1986;Sugiyama, 1989) argued that central Japan including the Kakegawa and Nohi-Yatsuo areas was rotated clockwise as a part of Southwest Japan through about 45° at 15 Ma, and again rotated counter-clockwise after the Middle Miocene by the indentation of the Izu-Bonin arc. Paleomagnetic evidence suggesting such repetetious rotations of central Japan is not available, however.…”

“…His results imply two possibilities that (1) the Nohi-Yatsuo area was translated southward with no significant rotation, while the main part of Southwest Japan was rotated clockwise (at about 15 Ma), or alternatively (2) the area suffered repetetious rotations in clockwise sense at first (at 15 Ma) and subsequently in counter-clockwise sense (between 15 and 12 Ma). Paleomagnetic data reported from the east of the ISTL (Hyodo and Niitsuma, 1986;Takahashi and Nomura, 1989;Takahashi and Watanabe, 1993) suggest that the area around the Kanto Mountains suffered clockwise rotations of about 60° between 15.5 Ma and 12 Ma, and of about 35° between 12 Ma and 6 Ma. These lines of evidence suggest that the northward cusp structure of pre-Neogene basement terranes in central Japan (Kanto Syntaxis) was formed in the Middle to Late Miocene, possibly by the southward migration of Southwest Japan around 15 Ma and collisional indentation of the Izu-Bonin Arc to the Honshu Island since then.…”