Seismic Refraction and Reflection Evidence Supporting Plate Tectonics in Hokkaido

Abstract

Comparing the crustal section obtained from seismic-refraction measurements off Hidaka, Hokkaido, with those obtain ed off Northeast. Honshu, and considering the structural features in and around Hokkaido, the continental crustal layer in the western half of Ho k kaido is inferred to be an extension of that of the Honshu arc. T he thick 4.2 km/sec layer found off Shizunai is considered to be compose d of the depositional materials of the Yezo geosyncline of the Cretaceou s time. The structural contrast between this sedimentary fu rrow and the Hidaka mountain range, where the continental crustal la y er seems to be shallow in depth, suggests a large-scale thrust of the crust of eastern Hokkaido upon that of western Hokkaido. Th is structural trend can be traced from the southern end of Hokka ido to the northern part of the Tatary strait. it is suggested t hat this thrust and vast sedimentation in the foredeep on its western side a re the products of the Hidaka orogenesis, and the structural bounda r y between them marks a plate boundary in the Mesozoic e ra. The extension of this plate boundary, which separates the Okhotsk pl a te from the Eurasian plate, is pursued along the Northern trough and further eastward between the Kolyma and Koryak mountain rang e s. The tectonic movements of the Okhotsk plate in the Mesoz oic to Palaeogene time are estimated in relation to the Hidaka orogenesis. Many phenomena such as the igneous activity along the m argin of the Asian continent in Late Mesozoic, the geographical distributions of the so-called Green Tuff region and of the present heat flow values in Hokkaido, and the shallow seismic activity in Sakhalin can be explained by this hypothesis.