北海道白亜紀前弧堆積盆の一部の大規模北方移動 : 浦河地域に分布する蝦夷累層群の古地磁気 A large latitudinal displacement of a part of Cretaceous forearc basin in Hokkaido, Japan : paleomagnetism of the Yezo Supergroup in the Urakawa area

ユーラシア東縁におけるテクトニクスを解明するため,北海道中央南部・浦河地域に分布する白亜系蝦夷累層群の古地磁気学的研究を行った.5地点から得られた特徴的残留磁化はマグネタイトによって担われ,傾動前に獲得したと考えられる.磁化率異方性ファブリックと異方的な等温残留磁化獲得パターンから,堆積および残留磁化固定後に有意な伏角浅化か生じたと考えられ,伏角誤差は4〜11°の範囲であった.浅化の補正後も伏角は異常に浅い値(31°)を示す.伏角値が地磁気極性層序のクロンC33r〜C32rに対比されることを考慮すると,浦河地域を含むブロックは3400kmの北方移動を被ったことになる.回転量も非常に大きいことから,北方移動は大陸縁辺の横ずれ断層運動によって引き起こされたのかもしれない.白亜系蝦夷累層群は,離れた大陸縁辺で発達し,斜め沈み込みによって北上した異地性の地塊を含むと考えられる.

Paleo-and rock magnetic experiments were carried out on the Cretaceous Yezo Supergroup in the Urakawa area (southern central Hokkaido) in order to detect tectonic episodes of the ancient eastern Eurasian margin. Characteristic remanent magnetization determined for five sites resides in magnetite, and has primary origin as indicated by positive result of tilting test. Bedding-parallel fabric of anisotropy of magnetic susceptibility and anisotropic acquisition of isothermal remanent magnetization (IRM) indicate significant inclination shallowing after deposition and lock-in of remanence. Shallowing ranges from 4° to 11° for the five sites on the basis of IRM anisotropy. The flattening-corrected mean inclination is still anomalously shallow (31°). Considering the correlation between the obtained inclination data and the geomagnetic polarity Chron C33r-C32r, a block containing the Urakawa area was located at a low latitude (16.7° N) during Campanian and then has experienced at largest 3400 km northward transportation. Transcurrent fault motions along the continental margin may be responsible for the transportation, as inferred from extremely large rotational motion (-161°). The Cretaceous Yezo forearc basin contains an exotic landmass that had evolved along a remote continental margin, then been transported as a result of oblique subduction.