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Basal slip plane of the Kurotaki unconformity in the Boranohana area along the Pacific coast of the Boso peninsula, Central Japan

The Kurotaki unconformity (Koike 1951) is well known as a major Neogene unconformity and can be traced from the west to the east coast of the Boso peninsula, central Japan (Fig. 1). This unconformity is recognized as evidence of a regional tectonic transition around the trench–trench–trench triple junction off the Boso peninsula (e.g. a transition of the convergent direction of the Philippine Sea Plate to the Eurasian Plate about 3 Ma). The Kurotaki unconformity is identified with submarine erosion associated with rapid subsidence in response to evolution of the Kazusa forearc basin (Katsura 1984). In the Boranohana area located along the Pacific coast of the central Boso peninsula, the Kurotaki unconformity can be clearly recognized as a contact between two different types of sediments (Fig. 2). In the present study, the authors focused on the basal plane of the Kurotaki unconformity in the Boranohana area. Most of the exposed planes in this area are characterized as sharp, planar shear zones with no gauge present (Fig. 3). The slip direction of the hanging wall on the shear plane (S50°E to S60°E, Fig. 3d) differs from the paleocurrent direction (north to northeast, Ito et al. 1992) which the large-scale trough cross-bedded sandstones in the Kurotaki Formation indicate (Fig. 4). There are two possible causes for the basal slip in the Boranohana area: (i) synsedimentary slipping; and (ii) an event later in