Tectonic Movement and its Relation to the Mineral Resources in the Green Tuff Region

Abstract

Tectonic movement in the Green Tuff region of the northern part of Northeast Japan is divided into two types, i. e., subsidence and formation of horst-graben structure at the latest Oligocene to late Miocene, and uplifting accompanying intense folding and thrusting at the latest Miocene to Recent. The former is interpreted to have taken place under tensile stress field, and the latter under compressional stress field.
Accompanied by the subsidence, vigorous volcanic activity and the Kuroko-type base metal mineralization took place in the inner part of Northeast Japan and coal-bearing sediments were deposited at early stages of the subsidence at the eastern margin of Kitakami massif and at some places of the Green Tuff region. On the other hand, volcanic activity and base metal mineralization (vein-type) related to the uplifting movement were rather weak, and deformation of sediments due to the compressional stress was responsible for the formation of oil-traps in the oil-bearing formations in Japan Sea Coast province.
In terms of the plate tectonics, it is suggested that the tensile tectonic movement and subsidence, which initiated at the latest Oligocene, might have been caused by dragging of the crustal layer by the Pacific plate, which changed its motion from NNW (subparallel to the trend of Northeast Japan) to WNW (perpendicular to the trend of Northeast Japan) about 40 m. y. ago. The intense volcanism during Miocene would have resulted from formation of a great amount of magma at the Benioff zone due to frictional heat generated between the plunging Pacific plate and the overlying mantle. The compressional stress field and the volcanism of the latest Miocene to Recent may probably be a result of convection induced in the overlying mantle by the plunging Pacific plate.