COMPARATIVE SUBDUCTION TECTONICS: NAPPE POLARITY, EXHUMATION OF DEEP-ROCKS AND PALEOGEOGRAPHIC IMPLICATION IN THE CORDILLERAN-PACIFIC TYPE OROGENIES OF NORTH AMERICA AND JAPAN

Currently debated problems of the subduction polarity associated with orogenic belts and its relationship to deep-seated rock exhumation of imbricated nappes had been first raised decades ago by Eldridge Moores in 1970. Recently we have gained new insight into this problem with detailed field and laboratory data by many young earth scientists of Japan, particularly from the Mesozoic and Cenozoic orogenic belts in the SW Honshu region of Japan. The orogenic structures predating the opening of the Japan Sea have been interpreted as differentially-exhumed parts of south-vergent, southward-younging accretionary prisms associated with northward-dipping (present coordinates) subudction. The Chichibu belt fits this general model poorly and its place in the orogen remains an enigma. It is situated between the Sanbagawa high-pressure metamorphic belt and the Northern Shimanto accretionary prism, both Cretaceous. However, the Chichibu belt is composed mainly of Permian and Jurassic accretionary prism rocks, older than the rocks flanking it. Detailed geologic mapping by many has identified many overlapping nappe slices that were subhorizontal before being unconformably covered with Lower and Upper Cretaceous forearc basin type marine and brackish deposits. Another enigmatic zone in the middle Chichibu is the Kurosegawa zone that consists of Permian accretionary prism and Silurian-Devonian Gondwanan island arc-coral reef strata with granitic rocks (possibly equivalent to the Wrangellia terrane) which are unconformably covered with Triassic, Jurassic and Cretaceous shallow to deep marine deposits. The depositional and structural contrasts of Cretaceous time in and around the Chichibu suggest a revised paleogeographic model in which Cretaceous forearc sliver zones served as back stops for large-scale sinistral oblique subduction of the Kula plate during several stages within Cretaceous. The down-dip equivalent of the Shimanto was exhumed as the Sanbagawa. Such large-scale oblique subduction may characterize the Pacific type subduction orogenies rather than the trench-normal ribbon continent subduction-collision proposed for parts of the the North America Cordillera, particularly in its eastern part.