Abstract
The folding of Neogene-Quaternary strata on Boso Peninsula, central Japan, has been attributed to differential vertical movements of basement blocks. Previous researchers have suggested that the folding resulted from two-dimensional, thick-skinned deformations along a vertical cross-section perpendicular to the fold axes. To test this hypothesis, we investigated the geologic structures in an area with dimension of 3×3 km2 in the southern part of the peninsula, where the upper Awa Group (upper Miocene-Pliocene fore-arc basin fill) makes E-W trending folds. We found that lateral variations in stratal thicknesses record three-dimensional syn-depositional map-scale movements that are incompatible with the model of two-dimensional fold structures; moreover, outcrop-scale faults also record patterns of three-dimensional deformations. Bedding faults, including a map-scale flat-ramp structure and an outcrop-scale hinterland-dipping duplex, both show NNE vergence, indicating that the strata were subjected to thin-skinned rather than thick-skinned deformation.
