Late Quaternary Terraces and Their Deformations on Tsugaru Peninsula, Northeastern Japan.

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  • 変動地形からみた津軽半島の地形発達史
  • ヘンドウ チケイ カラ ミタ ツガル ハントウ ノ チケイ ハッタツシ

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Abstract

Late Quaternary terraces on Tsugaru Peninsula, northernmost Honshu, are divided into five levels (Terrace I-V), and each terrace is of marine (m) and fluvial (f) origin. Terrace IIIm, which is distributed most widely and is overlain by the Toya ash (100-130ka), was formed during the last interglacial maximum of ca.124ka (oxygen isotope sub-stage 5e). The height of the former shoreline of Terrace IIIm reaches 40m above sea level (average uplift rate is ca.0.3m/ka), and its height distribution shows a warping with short wave-length (ca.3km) on the north and a warping with about 30km wave-length on the east. Two active fault systems, running north-south, bound the plains area from the central mountains and dislocate Terrace Im, IIf on the west and IIIm on the east. The fault system on the west is composed of a main synthetic fault, represented by a west-facing flexural scarp and an antithetic fault which has a range-facing scarplet. The degree of fault activity is class B-C (slip rate; 0.08-0.15m/ka) for the synthetic fault and C (<0.1m/ka) for the antithetic fault. The slip rate of flexural scarp on the east is larger (0.14-0.30m/ka) than on the west. These faults are estimated to be reverse faults which have been produced under a stress field with E-W compression. Late Quaternary terraces have been deformed significantly in the eastern part and the northern central part of the peninsula where Pliocene sediments are thickly deposited.

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