山陰東部における地形景観とその起源

A geomorphic landscape is one of the fundamental components of a regional land-nature system, so that its right understanding is essential to regional designing. this paper aims to clarify the characters and origin of the geomorphic landscape in the eastern San'in district and to investigate its influence to the land-nature system of the district. The eastern San'in district is located on the backarc side of the central Southwest Japan arc. Framing the mountainous landscape of the district is the regional slope down backarc-ward. Late Miocene to Pliocene lava flows with basal fluvial deposits extensively cover the slope and dip downslope-wars at an angle ca.4 degrees. Averaged stream gradients in the district less than one degree indicate that the basal fluvial deposits and the overlying lava flows have tilted about 3 degrees after deposition The framework of the geomorphic landscape thus has been formed by regional backarc-ward tilting after the deposition of the Mio-Pliocene lava flows. Structural analysis of the Southwest Japan arc reconstructs that aa regional peneplain has existed over the arc belt around the Mio/Pliocene boundary and that the asymmetric arching of the island arc since some time in the Pliocene has produced the backarc slope of the arc. The geomorphic landscape of the district hence appears to be originated from the Mio/Pliocene regional peneplanation and the subsequent backslope-tilting driven by the asymmetric arching of the arc. The backslope brings various influences to the district, e.g., consequent drainage systems, a linear coast line with few good ports, and snowy winters. The traffic difficulty across the backslope has left the district on the outskirts of economic development and industrial environmental pollution. Comprehensive understanding of the land-nature system in the district will thus promise a large contribution to the regional designing for sustainable development.