Neogene and Quaternary Structures of the Area around Suruga Trough and Their Neotectonics

Abstract

Neogene and Quaternary structures of the area around the Suruga Trough in the South Fossa Magna and their neotectonics are examined from the viewpoint of microplanktonic biostratigraphy and biochronology.
Folds and N-S trending thrusts of the South Fossa Magna are considered to have been affected by the stress field with the σ H_max axes of NW-SE to WNW-ESE directions since the latest early Miocene time at 16Ma. The undulatory deformations of the terrace surfaces of the area have also been affected by the same stress field. A series of N-S trending and high-angle thrust-faults of the area has successively appeared from west to east, and an inferred active thrust-fault along the lower Fuji River is presumably a northern extention of an inferred active thrust-fault along the Suruga Trough which bears major dip- and probable strike-slip components. The above-mentioned aspects seem to be concordant with the northwest subduction of the Philippine Sea plate along the Suruga Trough.
On the basis of fossil evidences obtained from the Izu Peninsula, it is most likely that the peninsula, an ancient volcanic island in the southern tropical ocean in middle Miocene time at 11Ma, drifted with the Philippine Sea plate north- and/or northwestward, and came into collision with the Honshu arc. All of the N-S trend of the Suruga Trough, intensive folds and thrusts of the South Fossa Magna, and active Quaternary undulations of the area can be well explained by the still continuing collision of the Izu block.
The above-mentioned neotectonics of the area may closely be related to the mechanism of large earthquakes off the area.