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[SCG61-P12] Coexistence of reverse and normal faults formed in horizontal compressive stress field
Keywords:multiple inverse method, paleostress, fault gouge, Torigoe fault, Niigata Prefecture
We describe mesoscale faults that are congruent or incongruent with the topographical and geological structures in a limited area, discuss the possibility that the both were formed at the same period.
The Western boundary fault zone of Nagaoka Plain, an active fault zone, extends NNE-SSW and is composed of the Torigoe fault and other faults. The Uonuma Fm of the Early to Middle Pleistocene is distributed to the west of the fault zone, has been folded with the lower formations. Part of the Uonuma Fm is overturned and is covered by the Oyama Fm of the Middle Pleistocene with an angular unconformity. Approximately 200 mesoscale faults were described, and stress analysis using multiple inverse method (Yamaji, 1999), XRD analysis was also carried out.
The mesoscale faults can be classified into four groups, 1: NW-dipping reverse faults, 2: NW-dipping normal faults, 3: horizontal to SE-dipping reverse faults, and 4: horizontal to SE-dipping normal faults. Group 1 includes fault with a 10 cm-wide gouge zone. Chlorite, which is not contained in the host rocks, was detected in the gray gouge. Flow injections were observed at the top and side of the gouge zone. Clear cross-cutting relationship between the groups is not observed. Most of the slip data from the Group 1 can be explained by a reverse fault type solution with σ1: WNW-ESE, σ3: nearly vertical, and stress ratio of 0.6 (plane strain). The remaining data can be explained by a smaller stress ratio of 0.2 (uniaxial shortening). On the other hand, normal fault type solutions with σ3: WNW-ESE were obtained from the Group 2.
Normal faults are commonly formed around the Torigoe fault, even though horizontal compression is progressing in the area. Since the σ3 axis is perpendicular to the Torigoe fault, the stress field thought to be due to bending associated with the development of the anticline. On the other hand, the NW-dipping gouge zone has been altered at a high-temperature, suggesting the upwelling of fluid along the gouge zone. The reverse faulting under the regional stress field reached shallow depths, and the viscosity of the gouge decreased due to the influence of the fluid, causing the fluidization. If σ1 value increase at the same time as earthquake, uniaxial shortening may be caused.
The Western boundary fault zone of Nagaoka Plain, an active fault zone, extends NNE-SSW and is composed of the Torigoe fault and other faults. The Uonuma Fm of the Early to Middle Pleistocene is distributed to the west of the fault zone, has been folded with the lower formations. Part of the Uonuma Fm is overturned and is covered by the Oyama Fm of the Middle Pleistocene with an angular unconformity. Approximately 200 mesoscale faults were described, and stress analysis using multiple inverse method (Yamaji, 1999), XRD analysis was also carried out.
The mesoscale faults can be classified into four groups, 1: NW-dipping reverse faults, 2: NW-dipping normal faults, 3: horizontal to SE-dipping reverse faults, and 4: horizontal to SE-dipping normal faults. Group 1 includes fault with a 10 cm-wide gouge zone. Chlorite, which is not contained in the host rocks, was detected in the gray gouge. Flow injections were observed at the top and side of the gouge zone. Clear cross-cutting relationship between the groups is not observed. Most of the slip data from the Group 1 can be explained by a reverse fault type solution with σ1: WNW-ESE, σ3: nearly vertical, and stress ratio of 0.6 (plane strain). The remaining data can be explained by a smaller stress ratio of 0.2 (uniaxial shortening). On the other hand, normal fault type solutions with σ3: WNW-ESE were obtained from the Group 2.
Normal faults are commonly formed around the Torigoe fault, even though horizontal compression is progressing in the area. Since the σ3 axis is perpendicular to the Torigoe fault, the stress field thought to be due to bending associated with the development of the anticline. On the other hand, the NW-dipping gouge zone has been altered at a high-temperature, suggesting the upwelling of fluid along the gouge zone. The reverse faulting under the regional stress field reached shallow depths, and the viscosity of the gouge decreased due to the influence of the fluid, causing the fluidization. If σ1 value increase at the same time as earthquake, uniaxial shortening may be caused.