北部フォッサマグナ，中信高原・美ヶ原区での前〜中期更新世における塩嶺火山域の火山活動とテクトニクス ―大規模多重コールドロンの形成―

抄録

Based on the field studies of the stratigraphy and geologic structures of the Lower Pleistocene Enrei Formation (Fm), we have re-evaluated the volcanic activity and tectonics of the Enrei Volcanic Field (EVF) in and around the Utsukushigahara area, Chu-Shin Highland, North Fossa Magna, central Japan. As well as in the other areas of EVF, the Enrei Fm in this area is mainly composed of lavas and pyroclastic rocks that filled up a collapse basin, about 6 km in NW-SE length and 3–5 km in NE-SW width, fringed by high-angled and irregularly-shaped abut-type unconformity surfaces on the Miocene basement rocks. The unconformity surfaces were originated by normal faultings with about 800 m vertical displacements. The volcanism here is approximately ranging in age between 2.1 and 1.3 Ma and is correlatable to the volcanism of the Older Enrei Fm in the other areas of EVF. In contrast with the mainly unimodal two-pyroxene andesitic volcanism in the other areas, the Enrei Fm here is characterized by the bimodal volcanism of two-pyroxene andesite and hornblende-bearing dacite in composition.The change of lithofacies and voluminous effusions of lavas and pyroclastic rocks on dry environments suggest that the Utsukushigahara area was situated in one of the volcanic centers of EVF. After the Early Pleistocene volcanism, the overall original near-horizontal structures have mostly been preserved until the present, except for the small scale faulting and gentle up-warping of the Miocene basement rocks and the overlying Enrei Fm in and around this area. This up-warping, that occurred probably during the latest Early to Middle Pleistocene, has been responsible for the rapid upheaval of the southern part of Chikuma Mountains to form the present-day Chu-Shin Highland. The volcanic/tectonic process of EVF is regarded as an example to form large-scale multiple cauldrons (calderas) in a back-arc region.