The Late Jurassic oblique collisional orogen of SW Japan. New structural data and synthesis

The structural configuration of SW Japan mainly reflects a late Jurassic‐early Cretaceous orogeny. The region is divided into an inner belt and an outer belt, on the Japan sea and Pacific ocean sides respectively, by a strike‐slip fault, the Median Tectonic Line (MTL). Both consist of a series of stacked nappes. The inner belt is divided into a Jurassic olistostrome known as the Tanba zone and a hinterland area comprising continental Triassic‐Jurassic sediments. The Tanba zone is sliced into two units: a lower one with late Jurassic matrix and Triassic‐early Jurassic radiolarite olistoliths, tectonically overthrust by an upper unit comprising an olistostrome with middle Jurassic matrix and blocks which include late Paleozoic limestone, basic lava, and radiolarite. The Tanba zone is overthrust by a Paleozoic nappe complex derived from the hinterland. The basal sole of the nappe corresponds to a peculiar unit called the ultra‐Tanba zone. In the Chugoku area, the hinterland is divided into an upper nappe: the Oga nappe, formed by Permo‐Carboniferous limestone and Permian clastic rocks and a lower one: the Sangun‐Maizuru nappe, formed by Paleozoic high pressure (HP) metamorphics (the Sangun schists), the Permian Maizuru olistostrome, and the dismembered Paleozoic Yakuno ophiolites. In the northern part of SW Japan, the Tanba zone is in faulted contact with the circum‐Hida and the Hida zones. The former is interpreted as the equivalent of the Oga and Sangun‐Maizuru nappes of the Chugoku domain crushed by post Cretaceous tectonics. The latter consists of Paleozoic high temperature (HT) metamorphic rocks and late Triassic‐early Jurassic granite, locally mylonitized and covered by early Jurassic sandstone. The outer belt is formed by a superficial nappe similar to the Tanba zone thrust upon a "deep domain" characterized by a synmetamorphic ductile deformation. The "deep domain" is divided into a lower unit, the Oboke unit formed by continental derived arenites and a Green Schist nappe consisting of oceanic sediments and resedimented ophiolites.The Green Schist nappe overthrusts the Oboke unit under synmetamorphic conditions with an eastward displacement. The two belts are separated by the Ryoke zone which corresponds to the southern part of the Tanba zone affected by a Cretaceous HT metamorphism and sharply cut by the MTL. A geodynamic model is proposed for the Jurassic orogeny of SW Japan assuming that the evolution of the inner and outer belts are linked. In the Late Triassic‐Early Jurassic SW Japan is an active plate margin. The upper plate or South China block consisted of the hinterland and the Tanba belt, a forearc basin; the lower plate consisted of an oceanic area and the South Japan continent. The basic mechanism of the orogeny is ascribed to the oblique subduction and collision of the South Japan continent.