This paper describes active faults which dislocate late Quaternary marine terraces of Sado Island and discusses a tectonic implication of these faults for the deformation of terraces. Sado Island is located on the tectonically active Sado Submarine Ridge in the Sea of Japan, off Niigata of central Japan, and consists of three major topographic units ; Osado Range, Kosado Range and Kuninaka Plain in-between, all of which trend nearly northeastward. Both Osado and Kosado are fringed by well-defined marine terraces up to 300m in altitude, and up to 130m for the last interglacial terrace, indicating the maximum uplift rate of 1 m/1, 000 yrs.
Two major active fault systems are recognised ; the Kuninaka-minami fault system on the west of Kosado and the Sotokaifu fault system on the west of Osado. They are striking parallel to the trend of the mountain ranges. The Kuninaka-minami fault system dislocates the last interglacial marine terrace and younger marine and fluvial terraces, and is composd of three segments, KM, KD and IT. These segments have two different surface expressions. One is an extremely convex terrace profile steeply dipping toward Kuninaka Plain (westward), and is interpreted as a flexural scarp (KM1, KD1 and IT1), and the other is a range-facing fault scarplet (KM2, KD2 and IT2). Vertical slip-rate is 0.1-0.3 m/1, 000 yrs for KM1, KD1 and IT1 and only 0.01-0.07 m/1, 000yr. for KM2, KD2 with an exception of 0.1 m/1, 000yr. for KD2. A progressive deformation is recognized in both types, which have been formed by repeated activities of reverse faulting. KM1, KD1 and IT1 are interpreted as the major synthetic faults with upthrown side on the east (mountain-side) and KM2, KD2 and IT2 are associated subsidiary antithetic faults.
The Sotokaifu fault system has two segments, KO and IG. No major synthetic flexural scarp is recognized and only range-facing fault scarps (KO2 and IG2) dislocate the last interglacial and younger marine terraces. KO2 and IG2 are regarded as the antithetic faults, judged by their similarity of the morphological characteristics and slip rate to those of KM2, KD2 and IT2, and the presence of exposure of reverse faults. Strong coastal erosion on the west coast of Osado, facing an open sea, is a reason for the absence of main synthetic fault on Osado. Kuninaka Plain is a fault angle depression bunded by the Kuninaka-minami fault system on the eastern side only.
These faults are responsible for the late Quaternary southeastward tilting, which is estimated from the height distribution of former shorelines on Osado and Kosado, but is opposite to the older deformation pattern expressed by generally steeper eastern slope than the western slope of mountainland of Osado and Kosado. It is, therefore, suggested that the faulting of these two systems have started sometime in middle to late Quaternary with the commencement of new tectonic regime.
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