Introduction to Landforms and Geology of Japan


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Structures Found on Wave-cut Benches in the Southern Miura Peninsula

Duplex structure

 A duplex structure is a type of imbricate structure bounded by a roof thrust and a floor thrust in which lenticular or S-shaped masses (called horses) derived from a bed are imbricated by small thrusts. There are three types of duplexes: hinterland-dipping, antiformal stack, foreland-dipping.  Figure 3 shows the formation process of duplex. When a bed is compressed laterally, a fault parallel to the bed and a thrust fault crossing the bed (dashed line [future fault] in Figure 3) are produced at a point of the bed, and the cut portion (hanging wall) goes up on the bed along the thrust fault (Stage 1). Subsequently, a thrust fault forms ahead of the first horse (dashed line of Stage 1) to produce the second horse. The repetition of this process makes a duplex structure (piggy-back style). However, in another process (overstep style), the thrust sequence is opposite; a subsequent horse forms behind the antecedent horse. Photo 14 shows an excellent duplex structure like an illustration in a geological textbook. This duplex is a hinterland-dipping type.

Figure 3: Formation process of hinterland-dipping duplex and other types
S0 to S3 represent displacement along the faults. (Modified from Boyer and Elliot, 1982)


Photo 14: Duplex structure (Hamamoroiso) [Click to enlarge]

The roof side of the duplex in Photo 14 is the left (south) side, although it appears to be upside down. The duplex developed from the back to the front (piggy-back style). The sediment around the duplex was liquefied. In addition, the upper side of the duplex is the south side, but the upper side of the formation in the area is the north side. Therefore, it is thought that only this duplex rotated without liquefaction. (Ogawa, 2007a)


Photo 15: Horse [Click to enlarge]
The scale is 1 meter long.

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