Transition from Flexural—Flow Folding to Flexural—Slip Folding in the Sambagawa Belt

The styles and physical condition of the folding of the Hijikawa—Oboke phase (Dh phase) have been described and discussed in this paper. The physical condition has been analyzed on the basis of quartz microtextres such as c — axis fabrics and deformation lamellae and of homogenization temperature of fluid inclusion in quartz. When the Dh phase folding occurred in multilayered rock types with extremely thin incompetent layers (films), it shows a transformation from a flexural—flow type during the early stage to a flexural—slip type during the later stage. The flexural—flow folding formed the axial plane cleavage (quartz shape orientation) in competent layers (quartz—rich layers) converging toward the fold core, resulting in the Class 1C type thickness variation for all competent layers, though the thickness variation is as near to Class 1B in the outermost knee and to Class 2 in the fold core. While the flexural—slip folding resulted in the Class 1C thickness variation for the competent layers involved in the outer knee and the fold core and around the inflection points of these involved in the middle knee and in the Class 2—Class 3 thickness variation around the axial zones of these involved in the middle knee. The fault system consisting of R1 set, Y set and P set developed along the layer boundaries and in competent layers during the flexural—slip folding has been also described and discussed, clarifying the relationship between the thickness variation and its related fault system: The Class 2—Class 3 thickness variation is related with the P set (thrust set) which converges toward the top of fold, while the Class 1C thickness variation around the inflection points in the middle knee with the R1 set and Y set.