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

Abstract

Paleomagnetism of the Miocene igneous rocks in the Uchiyama area, central Japan, have been determined in an effort to clarify the rotation of the Kanto Mountains located on the east wing of the sharp bent structure of the pre-Neogene basement rocks (Kanto Syntaxis). A mean tilt-corrected paleomagnetic direction of the Yaekubo Formation (D = -145.3°, I = -51.2°, a95 = 4.4°) shows eastward deflected declination indicating the clockwise rotation of the east wing of the Kanto Syntaxis. The radiometric ages of the andesite lava in the Yaekubo Formation (11.9 ± 0.9 Ma, 12.2 ± 0.3 Ma and 12.4 ± 0.4 Ma) signify that the about 35° clockwise rotation of the east wing of the Kanto Syntaxis occurred since 12 Ma. Along with the previous paleomagnetic studies in the Kanto Mountains and in its adjacent area into consideration, we can conclude that the lateral bending of the Kanto Syntaxis was formed in the Middle to Late Miocene.

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.…”

“…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. They also suggested a possibility that the collision of the Izu-Bonin arc inhibited clockwise rotation of the Nohi-Yatsuo area.…”

“…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.…”

Paleomagnetic direction of the Lower Miocene Hokusetsu Subgroup in the Shitara area, Aichi Prefecture, and implications for the formation of the cuspate geologic structure in central Honshu

“…, Takahashi and Saito, 1997;, 1999;Otofuji et al, 19991980Itoh, 1986, 1988Hyodo and Niistuma, 1986;Takahashi and Nomura, 1989;, 1992;Takahashi and Watanabe, 1993;Hayashida, 1994;Takahashi and Saito, 1997;Otofuji et al, 1999;Itoh et al, 2006 MTL 80 Niitsuma andAkiba, 1985;Niitsuma and Matsuda, 1985;Hyodo and Niitsuma, 1986;Niistuma, 1989 6 Ma MTL Niitsuma, 1989 Takahashi and Saito 1997 Hyodo and Niitsuma, 1986;Takahashi and Watanabe, 1993 16 Ma 6 Ma (Hayashida and Ito, 1984); 2 = lower part of the Morozaki Group (Hayashida, 1986); 3 = Kani Group (Hayashida et al, 1991); 4 = Mizunami Group ; 5 = Hokusetsu Subgroup (Torii, 1983); 6 = Tomikusa Group (Hayashida, 1992); 7 = Kurami and Saigo groups (Hayashida, 1994); 8 = Chichibu basin sediments (Hyodo and Niitsuma, 1986). Accreted volcano-sedimentary units (Amano et al, 1999;Aoike, 1999) that initially formed on the Izu-Bonin Arc are also shown: K = Kushigatayama or Koma unit; M = Misaka unit.…”

A High Density Sampling K-Ar Dating of the Kinpu-San Plutonic Body and the Initiation of the Philippine Sea Plate Subduction.

“…However, the deformation of the Honshu arc did occur as a result of an arc-arc collision between the Izu-Bonn arc and the Honshu arc. Paleomagnetic investigations in the adjascent region just to the north of the Kanto Mountains (Hyodo and Niitsuma, 198 6;Takahashi and Nomura,1989;Takahashi and Watanabe, 1993) have revealed that the region rotated 60° clockwise between 15 Ma and 12 Ma and an additional 30° between 12 Ma and 6 Ma (Takahashi and Saito, 1995;submitted to Island Arc). In the last 6 Ma, no significant rotation appears to have occurred.…”