Structurally Controlled Geomorphology on the Southern Boso Peninsula, Central Japan

Abstract

A graben is defined geologically as a topographic depression generated by dip-slip faulting. The Kamogawa Graben on the southern Boso Peninsula, central Japan, is considered to be a typical graben, although it has yet to be geomorphologically or geologically verified. In this study we use seismic reflection profiling and analyses of geomorphic and geologic data to reexamine the unsolved question of whether this structure is in fact a graben. We present an interpretation of the structure in terms of its structurally controlled geomorphic evolution. On the basis of our analysis, we conclude that the so-called Kamogawa Graben is not actually a graben, as graben structures are not observed in seismic images and there is no positive cumulative deformation recorded by active faults at the surface. The topographic lineaments that bound the depression between the Kazusa Hills and the Awa Hills constitute fault-line valleys and obsequent fault-line scarps. The northern lineament has developed along the geologic contact between fractured shale of the Hota Group and mudstone and sandstone of the Miura Group, while the southern lineament has developed between mudstone and sandstone formations within the Hota Group, along the Sorogawa Fault. These observations suggest that differential erosion of shale, mudstone, and sandstone strata produced the observed graben-like relief. Weathered shale is especially susceptible to erosion and is marked by areas of relatively low relief. The presence of paleo-river channels and obsequent fault-line scarps leads us to the proposal that an earlier river system developed on the pre-erosion mountainous topography that overlaid the present Mineoka Mountains. Such a geomorphic evolution of the southern Boso Peninsula is therefore structurally controlled, and the Kamogawa Graben is in fact a structural depression rather than a graben.