| Abstract |
The Nichinan area, southeast Kyushu, is occupied by two tectonically different groups of the Tertiary strata. Middle Miocene-Lower Pliocene Miyazaki group with gentle homoclinal dip is underlain uncon...formably by the intensely folded Nichinan group, which is subdivided into the Nichinan (s. s.) (lower) and the Sakatani (upper) subgroup on the basis of the sedimentary features. The Nichinan subgroup is characterized by the rhythmic repetition of sandy and shaly members, in which shale predominates over sandstone. The Sakatani subgroup, which conformably overlies the Nichinan, is lead by rigid arenites, showing a narrower distribution than the Nichinan. The Nichinan subgroup is correlated with Lower Oligocene to Oligo-Miocene formations of north Kyushu by means of the molluscan fossils and with the Globigerina ciperoensis zone to Catapsydrax dissimilis zone (Lattorfian to Chatti-Aquitanian) on the grounds of the planktonic foraminifera. The Sakatani subgroup, however, is almost barren of fossil except only a few molluscan species, which suggest Lower Miocene. The frame-work of the geological structure in the Nichinan area is controled basically by folding and modified by the succeeding faulting. The folding structure is featured by rather irregular patterns of several gentle and broad synclines and the steep and narrow anticlines of fewer number. This structure, likewise the Jura type, may therefore be classified into the intermediate type between the typical alpinotype and the cratonic one. It, however, somewhat differs from the Jura type in having the shorter axes and less uniform dipping of the axial planes of the folds than the latter. Herein I propose to call this type of folding the Nichinan type. The mega-and micro-sedimentary facies of the Nichinan group indicate that the area was under the control of the tectogenesis which became increasingly mobile as the time elapsed. It also verifies the pre-Sakatani tectonic phase of moderate intensity. This phase represents the second step of the Takachiho disturbance, embrionic and major phases of which are illustrated respectively by the paracontemporaneous cobbles of the basal conglomerate of the second formation and by the intense folding and faulting of the Nichinan group, igneous activities of acid extrusives and intrusives, and subaerial erosion before the deposition of the Miyazaki group. Investigation of three phases readily indicates an evident tendency that the tectonic movement in this area had become greater step by step from the embrionic phase to the maximum (second) one via the intermediate (first) one in the lapse of time from early Oligocene to early Miocene in the general compressional field. Thereafter the area was leaded by the tensile movements. Therefore the Takachiho disturbance represents a chief phase of an orogenic cycle. In the perspective point of view it is considered to represent a minor cycle of the major sedimento-orogenic one, which had accomplished the orogenic activities of the first order in the late Mesozoic time giving rise an essential change in the general tectonic condition. Comparison of the tectonic phenomena at various areas in our country indicates that the crustal movement of the first phase seems to be more important than others because it has close genetical ralation to the origin of the major Neogene basins in Japan including circum-Japan Sea basins and Fossa-magna, which cut across the Mesozoic and earlier structures. The movements of the second phase seems to be rather restricted to the Pacific side of southwest Japan. Thus a serious doubt arises whether or not we should discriminate the crustal movements, the Takachiho disturbance, in the outgrowth belt of the Mesozoic geosyncline and those in the green tuff region, which cut across the Mesozoic structure.show more
|
Export Mendeley
0602_p135