South-Vergent Imbricate Fan in the Ikenodan Formation of the Shimanto Supergroup, Southern Kyushu, Japan

ABSTRACT

The Noda area in the Hokusatsu District, southwest Kyushu is underlain by the Imanokoba, the Ikenodan and the Ogakura Formations of the Cretaceous Shimanto Supergroup. The Imanokoba Formation comprises beds of sandstone and mudstone. The beds generally trend NE-SW and dip in a moderate to steep angles to the east or west. The Ikenodan Formation comprises beds of sandstone, mudstone, siliceous mudstone and chert and basaltic lavas. The beds generally strike between NE and NW, and dip toward the west in moderate to high angles with a westerly way-up. The chert and basaltic lava beds appear in a left-stepping en echelon arrangement. The contact between the Imanokoba and the Ikenodan Formations is in a N–S trending fault known as the Noda Fault. The Ogakura Formation is composed of alternating beds of sandstone and mudstone. The beds generally strike between NE and NNE and dip toward the west in moderate to high angles with westerly way-up. The contact relationship between the Ikenodan and the Ogakura Formations is not distinct. The westward disappearance of the basaltic lava marks the western limit of the Ikenodan Formation. The en echelon array in the Ikenodan Formation is interpreted as an imbricate fan with the Noda Fault on its bottom as a décollement and is called “the Ikenodan imbricate fan”. The Ikenodan imbricate fan is considered to have been formed in an accretionary complex developed in a northbound subduction zone. The primary attitude of the Ikenodan imbricate fan was probably south-vergent then turned over with its west side down. It was previously reported that the strata of the Shimanto Supergroup in the Hokusatsu area form a regional bending structure called the “the Hokustasu Bend”. The Noda area is situated in a hinge zone of the Hokusatsu Bend, although the formations in the Noda area have not been involved in the bending structure. The Ikenodan Formation and the Noda Fault presumably extend to the north without bending, otherwise the bending structure occurs anywhere outside the Noda area. In either case, the Hokusatsu Bend is not present as interpreted in the previous studies.

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