Early Pleistocene flexural folds and tectonic process of the Ashigara depositional basin, Southern Fossa Magna region, central Japan

Abstract

The deformation of the Ashigara region situated between the Izu Peninsula and the Fossa Magna has been ascribed to the regional horizontal compressional tectonic regime. However, contrary to this prevailing view, new field evidence shows that the deformation comes from local stress field in relation to diapiric intrusions and resultant uplift of the Ashigara block. The structural development of the region during the Early and Middle Pleistocene is summarized as follows: First stage-Ashigara monocline was formed in accordance with the subsidence of southwestern part of the Ashigara Basin. Second stage-Formation of small-scale folds in the southwestern flank of the Ashigara monocline and a box-shaped anticline on its axial part. These structures were formed by the plutonic emplacement which uplifted the block and produced strong local compressional force. Third stage-The active rise of the eastern part of the Ashigara basin steeply tilted the thick Shiozawa Formation.