Study on the "Green-tuff" in the Tanzawa massif, Cetral Japan (Part 2)

<p>The southern part of the Tanzawa massif, Central Japan, is characterized by the existence of crystalline schist peculiar to the Neogene (partly Oligocene) System, the so-called "Green-tuff". Though the geologic problems especially the origin of the crystalline schist in this region have been discussed up to the present by many researchers, this have been unfortunately based on no reliable geological map. For the first time, the detailed stratigraphical study of this region (about 60 Km2) was carried out by the present writers. The Tertiary System in the surveyed area consists largely of subaqueous lava flows and pyroclastics. It can be divided into the following three formations in ascending order: the Kurokura formation, Yozuku formation and the Hirayama formation. The Kurokura formation is composed of basaltic lava flows and hyaloclastites, accompanyied by finegrained basaltic to dacitic tuff. The schistosity parallel to the bedding plane is distinct in this formation. In the northernmost part of the surveyed area, the formation is intruded by the quartzdiorite masses which gave contact metamorphism to the former. Though the lowermost limit of the formation is not confirmed, its thickness is more than 2,500m. The Yozuku formation is mainly composed of fine to coarse-grained tuff with andesitic to dacitic composition. The lava flows are scarcely accompanied in the formation. The schistosity develops weakly in the lower horizon of this formation, and the contact metamorphism by the quartz-diorite masses is negligible. The formation is intruded by many dolerite sheets, the large ones of which are observed in the Omata-zawa velley. The formation is about 1,400 m in thickness. The Hirayama formation consists largely of basaltic pyroclastics in the lower part and of basaltic lava flows in the upper part, respectively. No schistosity can be observed in this formation. The formation is intruded by many complexes composed of porphyrite and quartz-diorite in the western area. It is more than 900 m in thickness, though the upper limit is not yet confirmed. These three formations are surperposed without any remarkable unconformity. The general strike of these formation is E-W, though they are distubed by the block faulting. Apparently, these formations are northerly dipping in the greater part of the surveyed area, but sedimentary structures such as graded bedding and sole mark suggest that the southern strata represent the upper horizen in the stratigraphic sequence. Therefore, the greater part of the strata dipping northerly show reverse sequence. In the surveyed area, there are conspicuous three fault system : the high angle reverse fault system which partially parallel to the bedding plane and two normal faults with NE-SW and NW-SE directions. Of those, the NE-SW fault system, is particularly remarkable, dividing the surveyed area into many strip of fault blocks of 0.5-1.5 Km in width. Small-scaled folds with E-W axis seem to be developed in the southern side of River Yozukugawa, but their real form could not be clarified in the present study. Moreover, the distribution and the rock facies of intrusive bodies were also described in this paper. Based on the facts clarified by the present survey, some important geological problems in the surveyed area were discussed. Some conclusive remarks are summarized as follows; 1) The initial volcanic activity in the "Greentuff" basin of the Tanzawa massif was basic one. This may be common in the southern Fossa-Magna region, from the data in the Koma-massif, the Takakusayama region and so on. The initial basic volcanism is distinct from the initial intermediate volcanism in the " Green-tuff" basin of the North-East Japan. 2) Another phase of basic volcanic activity accompanyed by the intrusive one was also distinguished in the later stage of subsidence. 3) There were distinguished two phases in acid plutonic</p><p>(View PDF for the rest of the abstract.)</p>