九州の中軸帯および黒瀬川構造帯の白亜紀堆積盆の堆積作用とテクトニクス Sedimentation and Tectonics of the Cretaceous Sedimentary Basins of the Axial and Kurosegawa Tectonic Zones in Kyushu, SW Japan

九州の中軸帯および黒瀬川構造帯には白亜紀堆積盆が広く発達する。その層序学的・堆積学的ならびに構造地質学的諸特徴には東アジアの活動的縁辺域で生じた白亜紀のテクトニクス変化が記録されている。黒瀬川構造帯の下部白亜紀系は断層規制を受けた堆積盆に形成され, 非海成〜浅海相ならびにタービダイト相からなる二つの異なるタイプのシークェンスが識別できる。これらは, 多くの不整合を伴って顕著な岩相変化を示す。このような堆積盆は, 分断された黒瀬川古陸上に形成された横ずれ構造盆にあたり, 白亜紀前期にカリフォルニア型のトランスフォーム縁辺が推定される。このことは黒瀬川構造帯の堆積盆が北北西-南南東の左横ずれ断層を伴うタンル-断層システムに属していたことを示唆する。一方, 中軸帯の後期白亜紀堆積盆は非海成〜浅海性相(御所浦・御船層群)と浅海〜タービダイト相(大野川・姫浦層群)の二つの異なるシークェンスからなる。前者は先白亜紀の構造に対し, オーバーラップの層序関係を示し, ベンチ状あるいは陸棚性前弧海盆に比較できる。後者は軸流運搬のタービダイト相が卓越し, 伸長性堆積盆を示す。堆積作用と変形スタイルの特徴から, 大野川, 姫浦層群はそれぞれ横ずれ構造盆, 断層規制を受けた陸棚盆に相当する。黒瀬川構造帯堆積盆の北方への顕著な堆積盆の移動は, 縫合性島弧地塊と前期白亜紀に隣接していたイザナギ・プレートの間のトランスフォーム過程に関連したと推定される。引き続く中軸帯の堆積盆のタイプの時空的変化は, 後期白亜紀のクラ, 太平洋プレートの収れん運動の変化に対応するだろう。

Stratigraphic, sedimentological and structural features of the Cretaceous basins of the Axial and Kurosegawa Tectonic Zones in Kyushu, SW Japan, record a marked change of tectonic processes on the active margin of the East Asian continent. The Early Cretaceous formations in the Kurosegawa Tectonic Zone occur in some fault-bounded basins. Among them, two different types of sequences, consisting of the sediments of non-marine to shallow-marine facies or the deeper marine turbidite one, can be discriminated. They show a rapid facies change with many unconformable contacts. They are assigned to the sequences of strike-slip basins, having been generated on the disrupted Kurosegawa paleoland. The California-type transform margin can be inferred in that time. This may suggest that these basins are included in the Tan-Lu Basin System together with other contemporaneous continental basins, which are accompanied by sinistral transcurrent faults of the NNE-SSW trend. Besides, the Late Cretaceous basins of the Axial Zone include two differcnt type of sequences; the non-marine to shallow-marine sequence (Goshonoura-Mifune Basins) and the shallow-marine to turbidite one (Onogawa and Himenoura Basins). The former shows an overlap relation to the pre-Cretaceous tectonic framework, and is comparable to benched and shelved forearc basins. The latter represents a turbidite-dominant facies and an elongated trough profile with uni-directional axial flows. From the characteristics of sedimentation and style of deformation, the Onogawa Basin and the Himenoura Basin can be assigned to the transtensional strike-slip basin and the fault-controlled shelved forearc one, respectively. A marked basin shifting from south to north in the Kurosegawa Tectonic Zone may be related with the transform processes between the sutural are massif and the Izanagi Plate during Early Cretaceous time. Subsequent time and spatial variation of a basin type in the Axial Zone seems to correspond to the change of convergent motions of the Kula and Pacific Plates during Late Cretaceous time.