5:15 PM - 7:15 PM
[SGL23-P04] Paleostress analysis of mineral veins developed along an out-of-sequence thrust: Example from the Haiyagawa Thrust in the Tamba Belt, Southwest Japan
Keywords:subduction zone, out-of-sequence thrust, Tamba Belt
Out-of-sequence thrusts (OSTs) or mega-splay faults branching from the plate boundary fault in subduction zones are considered seismo-tsunamigenic faults (e.g., Moore et al., 2007). It has been suggested that when a megathrust earthquake occurs in subduction zones, the stress field changes from a reverse faulting to a normal faulting regime due to stress drop (e.g., Lin et al., 2013). It is expected that we can detect stresses before and after megathrust earthquakes by analyzing the structures formed by the activity of OSTs. However, observing the deformation structures around present-day OSTs is difficult.
In the Shimanto Belt of Southwest Japan, paleostress analyses of structures around the Nobeoka Thrust, an exhumed OST, have been performed by Kawasaki et al. (2017) and Otsubo et al. (2016). They detected reverse and normal faulting stresses, which were considered stresses before and after a megathrust earthquake, respectively, from minor faults and mineral veins developed around the Nobeoka Thrust. However, paleostress analysis targeting exhumed OSTs other than the Nobeoka Thrust has not been conducted. In this study, we carried out stress inversions of mineral veins developed along the Haiyagawa Thrust, an OST exposed in the Tamba Belt of Southwest Japan.
The Haiyagawa Thrust divides the Tamba Belt into the structurally lower Type I Suite and the structurally upper Type II Suite (Kimura, 2000; Tamba Belt research group, 1980). The foot wall of the thrust consists of mélanges of the Type I Suite, and phyllites of the Type II Suite are exposed in the hanging wall. Quartz and calcite veins are developed around the fault.
In this study, paleostress analyses were conducted by the method of Yamaji and Sato (2011), which fits a mixed Bingham distribution to the orientation distribution of poles to mineral veins. The later stage deformations were corrected by rotating the estimated paleostresses, assuming that the original dip of the Haiyagawa Thrust was close to that of present-day OSTs in seismogenic zones and the Nobeoka Thrust (about 10°N). By analyzing the orientation data of 96 mineral veins, a strike-slip faulting stress with a small stress ratio (σ2-σ3)/(σ1-σ3) (σ1≧σ2≧σ3, compression positive) of 0.10, a normal faulting stress, and a strike-slip faulting stress with a moderate stress ratio (0.50) were obtained. The detection of a strike-slip faulting stress with a small stress ratio and a normal faulting stress is consistent with the results of Kawasaki et al. (2017) and Otsubo et al. (2016). The strike-slip faulting stress with a moderate stress ratio might represent a stress state after the Haiyagawa Thrust ceased its activity as an OST.
In the Shimanto Belt of Southwest Japan, paleostress analyses of structures around the Nobeoka Thrust, an exhumed OST, have been performed by Kawasaki et al. (2017) and Otsubo et al. (2016). They detected reverse and normal faulting stresses, which were considered stresses before and after a megathrust earthquake, respectively, from minor faults and mineral veins developed around the Nobeoka Thrust. However, paleostress analysis targeting exhumed OSTs other than the Nobeoka Thrust has not been conducted. In this study, we carried out stress inversions of mineral veins developed along the Haiyagawa Thrust, an OST exposed in the Tamba Belt of Southwest Japan.
The Haiyagawa Thrust divides the Tamba Belt into the structurally lower Type I Suite and the structurally upper Type II Suite (Kimura, 2000; Tamba Belt research group, 1980). The foot wall of the thrust consists of mélanges of the Type I Suite, and phyllites of the Type II Suite are exposed in the hanging wall. Quartz and calcite veins are developed around the fault.
In this study, paleostress analyses were conducted by the method of Yamaji and Sato (2011), which fits a mixed Bingham distribution to the orientation distribution of poles to mineral veins. The later stage deformations were corrected by rotating the estimated paleostresses, assuming that the original dip of the Haiyagawa Thrust was close to that of present-day OSTs in seismogenic zones and the Nobeoka Thrust (about 10°N). By analyzing the orientation data of 96 mineral veins, a strike-slip faulting stress with a small stress ratio (σ2-σ3)/(σ1-σ3) (σ1≧σ2≧σ3, compression positive) of 0.10, a normal faulting stress, and a strike-slip faulting stress with a moderate stress ratio (0.50) were obtained. The detection of a strike-slip faulting stress with a small stress ratio and a normal faulting stress is consistent with the results of Kawasaki et al. (2017) and Otsubo et al. (2016). The strike-slip faulting stress with a moderate stress ratio might represent a stress state after the Haiyagawa Thrust ceased its activity as an OST.