日高衝突帯前緑褶曲・衝上断層帯の地下構造:石油公団日高測線(H91-2,3)反射記録の再処理と再解釈

概要

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A foreland fold-and-thrust belt has been growing west of the Hidaka Collision Zone (HCZ) since middle Miocene, associated with the collision between the Kuril and the Northeast Japanese island arcs (Fig. 1). The active front of the belt reaches the eastern margin of the Ishikari lowland 70 km west of the HCZ. As the collisional process has been recorded as the structure of the belt, it is essential for analyzing the process to restore the cross-section of the belt. JNOC (Japan National Oil Corporation) seismic profiles H91-2 and H91-3 across the western part of the belt (including the Ishikari lowland) are highly useful data for contributing to the restoration. Therefore, JNOC data were re-processed using a F-X prediction filter and considerably improved profiles of H91-2 and H 91-3 were obtained as shown in Figs. 4 and 7, respectively. The most probable interpretation for H 91-2 is provided in Fig. 6, and that for H91-3 in Fig. 8. Shortening length in the western part of the fold-and-thrust belt is evluated to be about 60 km from the interpretation of H91-3 (Fig. 9). Further studies must revise the interpretations and the evaluation of the shortening length, because interpretations are never unique, and have unresolved problems.