Paleogeography and stress field

Unfortunately tectonic situations of the regions other than Northeast Honshu of Neogene age are not well understood. However, it seems evident that even in the regions other than Northeast Honshu epithermal Au-Ag vein-type deposits formed when the uplift started and the area of land expanded. In addition to the paleontologic data, the country rocks of epithermal Au—Ag mine districts also suggest that epithermal Au-Ag vein-type deposits have formed under the subaeiial condition welded tuff occasionally occurs in the mine area (e.g., Sado, Nebazawa, Northeast Hokkaido) and in general submarine sedimentary rocks and volcanic rocks are poor or absent in the Au-Ag mine districts (e.g., epithermal An-Ag vein-type deposits in Kyushu). 

As already noted, most epithermal Au-Ag vein-type deposits are hosted by young (late Miocene-Pliocene) volcanic rocks and by sedimentary rocks, but dominant host and country rocks for base-metal vein-type deposits are submarine sedimentary and volcanic rocks. Submarine felsic tuff, tuff breccia, dacite lava, intrusive rocks and mudstone are dominant host and country rocks of Kuroko deposits. 

It is also noteworthy that major igneous and hydrothermal activities in the Japanese areas seems likely to have taken place almost at same times as the stress changes. The stress changes occurred at about 22 Ma, 15 Ma, 12 Ma and 8 Ma in the southern part of 

NORMAL FAULT REVERSE FAULT STRIKE SLIP FLT. STRESS TRAJECT. 

Northeast Japan and at about 15 Ma in the eastern part of Southwest Japan (Tsunakawa, 1986 Takeuchi, 1987) (Fig. 1.146). 

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