丹波帯ジュラ紀付加コンプレックスのスラスト系と運動像  [in Japanese] Thrust system and kinematics of Jurassic accretionary complexes in the Tamba Belt, southwest Japan  [in Japanese]

丹波帯のジュラ紀付加コンプレックス(JAC)は, 下位のI型地層群と上位のII型地層群とに区分され, さらに両地層群は2つと3つのコンプレックスにそれぞれ細分される.これらの各コンプレックスは互いにout-of-sequence thrustによって断たれ, 構造的に累重したナップ構造を形成する.I型地層群の各コンプレックスは海洋プレート層序からなるスラストシート(OPSシート)が繰り返す覆瓦構造で特徴づけられる.これに対し, II型地層群の各コンプレックスでは2階建て構造をなし, 上部構造帯はOPSシートをホースとするデュープレックス構造, 下部は海洋性岩石のスラブと混在岩からなるデュープレックス構造を呈している.スラスト系の特徴に基づいて, I型地層群ははぎ取り付加過程で, II型地層群は底付け付加過程で形成されたとする付加構造モデルを提案した.

Jurassic accretionary complexes (JACs) in the Tamba Belt, southwest Japan represent different types of thrust systems. JACs consist of Middle Triassic to Late Jurassic terrigenous clastic rocks interbedded with older oceanic rocks including bedded chert, bedded siliceous claystone and greenstone, ranging in age from Carboniferous to Middle Jurassic. Previous studies have divided JACs into two major tectonostratigraphic units, named the structurally lower Type I Suite and the structurally upper Type II Suite, which have been further subdivided into 2 and 3 complexes, respectively. These complexes are separated by thrust faults, and are arranged subparallel to each other with younger units exposed at structurally lower level. Each complex forms a subnappe, 0.5 to 5 km in thickness and more than 40 km long, demonstrating two-type thrust systems with mixed rock including blocks of various rocks in argillaceous matrixes. The complexes of the Type I Suite are marked by a south-verging imbricate structure. Each thrust sheet preserves most of an oceanic plate stratigraphic sequence (OPS sheet), being further subdivided into an upper and a lower sheet bounded by a intrasheet decollement. On the other hand, the complexes of the Type II Suite are characterized by a two-story duplex structure. The upper structural unit forms a duplex structure of a series of OPS sheets, whereas the lower structural unit is marked by isolated oceanic rocks surrounded by sheared mixed rock. The matrixes of the mixed rock show asymmetric shear fabrics with dynamic recrystallization of quartz grains.The above mentioned different structural features of both suites suggest that the complexes of the Type I Suite were offscraped on the toe of an accretionary prism in the form of imbricate fan structure, while those of the Type II were underplated to the base of the prism in the form of duplex structure. These initial structures were truncated by out-of-sequence thrusting, resulting in the formation of subnappes stacked subparallel to each other. The nappe structure may correspond to thrust sheets bounded by major out-of-sequence thrusts in the Out-of-sequence thrust zone of the Nankai Trough prism.