[SCG61-P12] History of fault activity considered from the brittle shear zone and the geological structure near the Cape Otachi, Kumamoto Prefecture
Keywords:Hinagu fault zone
[Ground Surface Survey] As a result of the ground survey around the Cape Otachi, small faults constituting the fracture zone were observed in the lower Cretaceous Hinagu Formation, and it was roughly classified into 4 strains from the strike of the fault, the sense, and the cutting relationship. F1 is NNE-SSW trend sinistral slip fault, F2 is NNE-SSW trend dextral slip fault, F3 is NNW-SSE trend sinistral slip fault, F4 is E-W trend dextral slip fault.
F2 is a small fault group found in the brittle shear zone consisting of cataclasite. F1 is seen outside the brittle shear zone and part forms a planar cataclasite zone with a width of 5 cm. F3 and F4 are small fault groups which mutually cut. F3 deform the dike group described later. F4 cuts the brittle shear zone. Apart from the fault, we identified a folding group with a hinge in the ENE-WSW direction on the southeast side of the brittle shear zone. In addition, an andesitic dykes group with a dominant ENE-WSW strike was observed on the east side of the brittle shear zone. We confirmed the cutting and bending of the dike by F3, the injection of dike along the F3. This dike is believed to be related to the Neogene Igneous rocks in Amakusa area (Matsumoto · Kanmera, 1964).
[Small Fault Analysis by Multiple Inverse Method (Yamaji, 1999)] Compressive Stress Field in NW-SE direction from F1, Compressive Stress Field in ENE-WSW direction from F2, Compressive Stress Field in NW-SE direction from F3 and F4 Was detected.
[Stage Segmentation] The time of formation of the fault, dike and fold observed in the vicinity of the Cape Otachi is divided into three stages, D1, D2, D3 from the stress field obtained from the cutting relation.
In D1, F1 was activated in the compressive stress field in the NNW-SSE direction. Considering the compression axis direction, fold group with fold axis in NE-SW direction is thought to be formed in D1.
In D2, F2 was activated in the compressive stress field in the ENE-WSW direction, forming a wide brittle shear zone.
In D3, F3 and F4 were active in the compressive stress field in the NW-SE direction.
We thought that the bending and the injection of dike along F3 indicate that the dikes were deformed by F3 before consolidation. Considering the above-mentioned thing, we thought that the dike group had formed between the terminal stage of D2 and the beginning of D3.