要　旨　秩父郡皆野町の荒川右岸で，三波川帯御荷鉾ユニット下部の泥質片岩と秩父盆地層群小鹿野町層下部の泥岩砂岩互層とが接する出牛−黒谷断層の断層面が確認された．断層面付近には，N17°W， 67°WやN15°W， 69°Wの走向・傾斜を示す鏡肌が観察される．前者はシャープで平滑な面をもち，後者にはS73°Wの方向に67°で沈下する明瞭な太いスリッケンラインと，これを切りS53°Wの方向に60°±で沈下する細いスリッケンラインとが認められる．御荷鉾ユニット（下盤）側の岩石には，東北東−西南西ないし東西方向の地殻の伸長を示す，断層ガウジ・小褶曲・南北性の高角正断層などが発達し，破砕・変形が及んでいる範囲は，断層面から北東へ水平距離で80ｍに達する．小鹿野町層（上盤）側の岩石には断層ガウジは見られないが，小鹿野町層側が断層面を滑り落ちる際に，摩擦に伴う引きずりによってできた考えられる開口型断層が認められる．断層面とスリッケンラインの姿勢や断層の両側の岩石の変形状態などから，本断層は，第四紀中期更新世に外秩父山地が南北に伸長軸をもつホルスト状に隆起した際に，地殻表層部で高角度の傾斜ずれ正断層もしくは高角度の斜めずれ正断層として運動したと考えられる．

キーワード：荒川右岸，出牛−黒谷断層，断層面，スリッケンライン，傾斜ずれ正断層，斜めずれ正断層

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図版１−Ｂ．断層面付近のスケッチ（Ａの写真に対応）．

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The Jushi-Kuroya Fault Cropped out at the Right Bank of the River Arakawa, Central Japan.

Takeshi HOMMA
Saitama Museum of Natural History. 1417 Nagatoro, Nagatoro-machi, Chichibu-gun, Saitama, 369-1305 Japan

Abstract
　　The fault plane of the Jushi-Kuroya Fault (JKF) which bounds the pelitic schist (the Lower part of the Mikabu Unit，Sambagawa Belt) and the alternating beds of mudstone and sandstone (the Lower part of the Oganomachi Formation, Chichibu Bonchi Group) cropped out at the right bank of the River Arakawa, Minano Town. Along the fault plane, two types of slickensides are observed. The strike and dip of one slickenside is N17°W, 67° W and those of the other is N15°W, 69°W. The former has a very flat and smooth surface but the latter has two types of slickenlines on it. One is a wide type slickenline plunges 67°in the direction of S73° W and the other is a narrow type one plunges 60°±in the direction of S53°W. The latter cuts the former. Fault gouges, minor folds and high angle normal faults in NS direction due to extention of the earth's crust in ENE-WSW or EW direction are developed in the Mikabu Unit (footwall) and the fracture zone amounts to 80m in width in NE-SW direction, but they are scarcely recognized in the Oganomachi Formation (hanging wall). The attitudes of the fault plane and slickenlines, the styles of deformation of the both side of rocks, suggest that JKF moved at a shallow level of the earth's crust as a high-angle dip-slip normal fault or a high-angle oblique-slip normal fault at the time of the uplifting of the Soto-chichibu Mountains in the Middle Pleistocene as a horst elongated in NS direction.