Recent finds of coesite eclogites, Proterozoic blueschists, and 3.7-Ga granulites have stimulated world-wide attention for the studies of metamorphic rocks in China, which is now generally regarded as one of a few suitable areas to study deep structures of continental collision. The publication of Metamorphic Map of China in 1986 provided a sound basis for the recent successful researches. Recent introduction of quantitative phase petrology and the increase of radiometric age measurements enabled Chinese geologists to depict detailed pressuretemperature-time (PT-t) paths of a metamorphism. The presence or absence of Archean greenstone belts, igneous or sedimentary origin of some felsic gneisses, extent of “migmatization”, and nature of Archean regional granulite-facies metamorphism are unfixed debates, which will be solved in the next decade.
A forest limit marks a distinct change in landscape, but it includes a variety of elements. In this study, the forest limit altitudes all over Japan were compiled, the relationship between the specific features of their distribution and thermal conditions were discussed. The forest limit altitudes on 211 mountains ranged from below 1, 000 meters in Hokkaido to 2, 800 meters in Central Honshu, and difference in these altitudes extended about 2, 000 meters between 35° and 45° North Latitude. Horizontally, these altitudes are distributed concentrically, focusing on the Taisetsu Mountains and the Hidaka Mountains in Hokkaido and the Akaishi Mountains in Central Honshu. The zone ranging from the southern part of Northern Honshu to the northern part of Central Honshu is crowded with isopleths, forming a division between the sides of the Japan Sea and Pacific Ocean. Calculation the Warmth Indices (WI) of forest limit altitudes, we find that their frequency distribution ranges from 16.1°C·M to 50.4°CEM, and their modes are 25 to 35, 34 to 40, and 25 to 30°C·M in Hokkaido, Northern Honshu, and Central Honshu, respectively. Reviewing these data individually, we see that forest limits are seldom equivalent to WI 15°C·M, which has been set as a boundary between the alpine and subalpine zones. In reality, it becomes increasingly difficult for the forest limit to converge on a particular WI value due to a variety of causes. It is worth noting, however, that at least there are some mountains on which the forest limit altitude is extremely close to WI 15. In such a situation, the WI 15 is of great importance, because it indicates that forests are capable of growing at least to that extent in terms of thermal conditions. This is the reason why WI 15°C·M is worth notice, and therefore the relationship between its altitude distribution and the forest limit altitude was determined. Additionally, the relationship between mountaintop altitude and forest limit altitude was also investigated. The correlation coefficient of the latter is more closely related than the former. But we must emphasize that the latter is only a seeming relation. Why? It is true that the higher the mountain, the higher the limit altitude, but the fact is that the limit altitude will not rise endlessly in step with the height of a mountain. Then, another relationship was examined, concerning differences not only between WI 15 altitudes and forest limit altitudes but also between mountaintop altitudes and forest limit altitudes, using the variation coefficient to verify the dispersion in these differences. As a result, we found out that dispersions are smaller in the former (WI 15 and forest limit) th an in the latter and that forest limit altitudes are more closely related to thermal conditions. On the other hand, the differences between WI 15 altitudes and the forest limit altitudes are defined by mountaintop altitudes. Additionally, the depth of snow cover strongly affects the difference in forest limit altitudes, if mountaintop altitudes are the same. The role of thermal condition, mountaintop altitude and snow depth condition for determining the forest limit altitude should be easy to determine as Fig. 10. Namely, altitudes of forest limit are primarily decided by thermal conditions depended on their geographical situation. They are secondarily modified by altitude of mountain. When the mountain altitudes are equivalent, snow depth conditions affect the forest limit.
The Japanese Islands consist fundamentally of late Paleozoic to Cenozoic accretionary complexes that formed in situ in a subduction zone along the East Asian continental margin, i.e. 2.0 Ga Yangtze craton (South China) and 450 Ma fore-arc ophiolite. Recent research utilizing microfossil and radiometric dating has distinguished several major accretionary complexes, including high-P/T metamorphosed parts, and subordinate ophiolites. In particular, recognition of oceanic plate stratigraphy and age of subduction-related metamorphism for individual accretionary complex allows the geotectonic subdivision of the Japanese Islands be emended with a new definition of geotectonic units and their mutual boundaries. Removing the effect of arcrelated magmatism and secondary tectonic modification by microplate activities such as backarc basin opening, fore-arc sliver movement, and arc-collision, a remarkable oceanward younging polarity is recognized among the accretionary complexes. This polarity in growth is well observed in Southwest Japan where seven distinct units occur, i.e. from the Japan Sea side to the Pacific side: 400-300 Ma high-P/T schists, Permian (250 Ma) accretionary complex, 230-180 Ma high-P/T schists, Jurassic (180-140 Ma) accretionary complex, 100 Ma high-P/T schists, Late Cretaceous (80 Ma) accretionary complex, and Tertiary (50-20 Ma) accretionary complex. The sinuous surface trajectories of these geotectonic boundaries and occurrence of several tectonic outliers and windows indicate that all these complexes, including high-P/T schists, occur as subhorizontal (or gently northward dipping) thin tectonic unit, i. e. nappe. Thus the Japanese Islands form a huge pile of nappes that become younger structurally downward to the modern Nankai accretionary complex. What is remarkable in this subhorizontal orogen is that high-P/T units are tectonically intercalated between low-P units, e. g. the thin nappe of 100 Ma Sanbagawa blueschists between Jurassic and Late Cretaceous accretionary complexes of the prehnite-pumpellyite facies. Uplift of the Sanbagawa high-P/T unit appears to correlate with the arrival of the Kula/Pacific spreading ridge at the trench, suggesting that this high-P/T accretionary complex may have been extruded and uplifted into low-P domain in fore-arc by buoyant subduction of the spreading ridge at the trench. Evidence of ridge subduction at that time is supported by reconstructed paleoplate motion and the coeval climax of arc-related Ry-oke magmatism associated with low-P/T regional metamorphism. Formation of older high-P/T blueschist nappes sandwiched between low-P units can be explained likewise. Subduction of major spreading ridges seems most critical for the episodic oceanward development not only of subhorizontal high-P/T nappes but also of continent side granitic belts.
Intermediate-depth seismicity in the subducting Philippine Sea slab beneath Kyushu island shows a tendency to become active associated with the commencement of shallow seismic activity in the crust. Such a special kind of seismic correlation has not been ever known anywhere. It is proposed that this unusual correlation between shallow and intermediate-depth earthquakes is caused by a unique tectonic situation in Kyushu that continuation of the spreading axis of Okinawa trough behind the Ryukyu arc traverses in the central part of the island and the crust is splitting there to the N-S direction. There were three seismically active periods during recent 100 years in the central part of Kyushu, and it is conjectured that the next active period in Kyushu island will begin around the year of 2010, provided that the pattern of change of seismic activity is maintained in the future.