Geological structure, composition and evolution of crustal layers of the Japan Sea

Abstract

Seismic refraction data have shown that the crustal thickness in the Japan Sea basins (Japan Basin and Yamato Basin) is 12–16 km, but the crustal thickness under the Yamato Rise has not been determined, although it seems to be considerably larger. The crustal thickness in the Asian continental slope and shelf reaches 26 km. All crustal structure studied has a similar seismic layering: a sedimentary layer, a layer 2 (or intermediate layer) and a layer 3 (or the lower crustal layer). The velocities and ranges of velocity variations in the layers are almost similar. The greatest difference in crustal thickness is between layer 2 and layer 3. The thickness of layer 2 in the deep-sea basins is 1.5–5.3 km, beneath the Yamato Rise it is 6.0–7.5 km and beneath the shelf and continental slope it is 9.8–11.7 km. The composition of layer 2 beneath the topographic highs and the continental slope has been studied by bottom sampling. Proterozoic to Paleogene granitic rocks and Archean-Proterozoic metamorphic rocks are the most common rock types. These rocks were formed at great and medium crustal depths, presumably in crust with a thickness of 30–40 km containing a layer 2 with a thickness of 10–15 km. The discrepancy between original and present crustal thickness suggests that the continental crust was thinned during the formation of the Japan Sea basin. Magmatic erosion of the intermediate layer during the ascent of mantle masses is proposed as a probable mechanism of this process. The result of this process could be a replacement of the sialic layer of the crust by basaltic rocks, which resulted in the formation of acid and alkahne melts erupted onto the surface during Oligocene-Early Miocene time.