Page 293 - Global Tectonics

278 CHAPTER 9

Late Cretaceous Hidaka belt (low pressure)

Early Cenozoic Kamuikotu belt (high pressure) Hokkaido

N

Sangun belt (high pressure) Permian –
Hida belt (low pressure) Early Triassic
F Honshu 0 400
A
km

F

Median Shikoku
tectonic line
A
Kyushu Ryoke belt (low pressure/high temp) Late Jurassic –
Sanbagawa belt (high pressure/low temp) Early Cretaceous
Figure 9.28 Three paired metamorphic belts in Japan, F–F‘ is the Itoigawa–Shizuoka Line (from Miyashiro, 1972.
Copyright 1972 by American Journal of Science. Reproduced with permission of American Journal of Science in the
format Textbook via Copyright Clearance Center). Profile A–A′ is shown in Fig. 9.29.

paired metamorphic belts. On the Japanese islands of the arc, some 100–250 km away, where geothermal
Hokkaido, Honshu, and Shikoku (Fig. 9.28), Miyashiro gradients are high.

identified three pairs of metamorphic belts of dif- The application of the paired metamorphic belt
ferent age that approximately parallel the trend of the model to Japan has allowed some investigators to infer
modern Japanese subduction zone. Each of these belts the direction of subduction and plate motions at various
consists of an outer zone of high pressure/low tem- times in the past. At present, Pacific lithosphere is sub-

perature blueschist and an inner belt of low pressure/ ducted in a northwesterly direction beneath the Japan
high temperature rock. This spatial relationship and arc. The metamorphic polarity of the Sangun/Hida
the similar age of each outer and inner belt led him and Ryoke/Sanbagawa paired belts (Fig. 9.28) suggests
to conclude that the belts formed together as a pair. that they were formed similarly, by underthrusting in a
After the introduction of plate tectonics, these paired northwesterly direction. The Hidaka/Kamuikotu paired
belts were interpreted to be the result of underthrust- belt shows the opposite metamorphic polarity, and
ing of oceanic crust beneath an island arc or continen- therefore may have formed during a different phase of
tal crust (Uyeda & Miyashiro, 1974). The outer plate movements when the direction of subduction was
metamorphic belt was interpreted to develop near the from the west of Japan. However, there are some dis-
trench due to the low geothermal gradient caused by crepancies in this interpretation. For example, the
subduction. The inner belt was interpreted to form in Ryoke/Sanbagawa belts are much closer together than