讃岐山脈南麓における中央構造線沿いの大規模岩盤すべりと第四紀断層運動  [in Japanese] Large-scale rock slides and Quaternary faulting along the the Median Tectonic Line on the southern foot of the Sanuki Rangein Shikoku, Southwest Japan  [in Japanese]

讃岐山脈南麓の中央構造線沿いに分布する和泉層群の土柱層への"衝上"岩体は, 背後の断層崖から重力によって滑動した巨大な地すべり岩体である。地すべり岩休は, 1 Ma前後の更新世前期〜中期に土柱層中に滑動し, 滑動後中央構造線によって2〜3 km右横ずれ変位を受けている。また, 地すべり岩体先端の土柱層の急傾斜構造は滑動に伴うプルドージングによって形成された。土柱層の層相変化は, 中央構造線の右横ずれ断層運動に伴う後背地の移動によって説明でき, 含花崗岩礫扇状地堆積物の分布から算定される中央構造線の約1 Ma以降の累積右横ずれ変位量は2〜3 kmに達する。しかし, この横ずれに伴う累積鉛直変位はせいぜい50 m程度である。讃岐山脈南麓では, 第四紀前半の街上断層もしくは逆断層運動はなく, 中央構造線は少なくとも更新世前期から右横ずれ断層運動をしていた。また, 更新世前期までに讃岐山脈南麓に中央構造線の顕著な断層崖が形成され, 岩盤すべりの場を提供した。

Large-scale rock mass slides of the Late Cretaceous Izumi Group occurred in the Early to Middle Pleistocene along the Median Tectonic Line (MTL) on the southern foot of the Sanuki Range in northeastern Shikoku, Southwest Japan. They slid southward from the fault scarp onto the unconsolidated Dochu Formation, across the MTL, and have been dislocated 2 to 3 km right-laterally by the MTL after sliding. They had been believed to be tectonic thrusting or tectonic superficial overthrusting of the MTL and its Quaternary faults, but the evidence that their source areas can be determined on the fault scarps strongly supports that they are landslide origin. The bulldozing of sliding masses also formed high-angled boundary planes and steeply dipping structure of the Dochu Formation in front of them. The facies change of the Dochu Formationis explained by lateral change of hinterland due to the right-lateral faulting of the MTL, Granite pebble bearing fan deposits of the Dochu Formation have been dislocated 2 to 3 km right-laterally by the MTL since about 1 Ma. On the contrary, the vertical component accompanying this horizontal displacement is estimated to be at most about 50 m. Both sliding rock masses and sedimentary process of the Dochu Formation indicate that the right-lateral faulting of the MTL have continued since the Early Pleistocene time in northeastern Shikoku and the MTL had formed remarkable fault scarp along the southern margin of the Sanuki Range until Early Pleistocene time. Thrusting of the Shobudani Phase of the MTL should be re-examined from the view point of landslide.