Abstract
The Holocene alluvial fans and flood plains formed by the Sho and Oyabe Rivers spread out in the Tonami plain from the central through the northern parts in the Toyama Prefecture, central Japan. Along the foot of the surrounding mountains and hills, higher, middle and lower terraces of late Pleistocene-Holocene in age are distributed. These terraces have been displaced, by the reverse dip-slip activities of Tonami-heiya fault zone in a sense of upheaval in the mountains side, even during the Holocene time. We examined stratigraphic cross section utilizing borehole data and geomorphologically analyzed 5 m-DEM data in order to elucidate the fault trace of the Isurugi fault which Tonami-heiya fault zone. As the results, the northern segment of Isurugi fault seems to run along the northeastern foot of Hodatsu Hills and extends underground through the lower-most Oyabe River into the Toyama Bay. Consequently, its total length reaches about 30 km. In the southern segment, a continuous fault scarplet was recognized to cut across the lower dissected fans. The slip-rate of Isurugi fault is estimated to be 0.31 - 0.64 m/kyr. In the Hokuriku region, reverse faulting and related folding with strike in a NE-SW direction have occurred during the late Quaternary. The hinge line of block movement due to the activities of the Tonami-heiya fault zone is revealed to have shifted from the mountain side into the plain side within the Holocene time. In conclusion, the Quaternary folding and faulting associated with the crustal warping at a wavelength of about 20 km is currently in progress, causing both the subsidence of Tonami plain and the upheaval of surrounding mountains and hills.
Highlights
The Toyama Plain is located on the Japan Sea side of central Japan
The relationships between the evolution of the landforms and geology of the Tonami Plain and the activity of the Tonami-heiya fault zone in the late Quaternary are summarized as follows: 1) The new estimate of the location of the Isurugi fault in the western part of the Tonami-heiya fault zone places it running from the southern part of Oyabe along the Oyabe River and on to Toyama Bay
2) With the beginning of the Quaternary, the direction of the compression axis on the Tonami Plain changed to NW-SE, and groups of NE-SW-trending active faults of the Tonami-heiya fault zone began to be active
Summary
The Toyama Plain is located on the Japan Sea side of central Japan. It lies between the 1000 m-deep Toyama Bay in the north and the 3000 m-high Hida Mountain range and the 1500 m-high Hida Highland in the south. The Toyama Plain was formed primarily by new and old fans created by a group of rivers flowing steeply down from the mountains behind it. Spread over the middle of the plain are many alluvial fans created by the two major rivers, the Sho and the Oyabe Rivers. Spread over the northern part of the plain, in the lower reaches of the two rivers, are such natural features as floodplains, natural levees, and back marshes. Distributed at the foot of the surrounding mountains are higher, middle, and lower fluvial terraces, dissected fans, and marine terraces
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