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[SCG50-P02] Magnitude of east-west contraction forces acting to Japanese Islands since 3Ma estimated from altitude changes
Keywords:Japanese Islands, East-west contraction, Quaternary, Tectonics, Lithospheric body force, Gravitational potential energy
Rapid east-west contraction accompanied by uplift of mountain ranges has been occurred on the Central and North-Eastern Japan since 3 Ma. Takahashi (2006, 2017) proposed a hypothesis that these are caused by the east-ward migration of the TTT-triple junction and the Japan Trench arising from the motion direction change of the Philippine Sea plate. This hypothesis is consistent with several geological observations, while no validation from a view point of mechanics has been done.
Therefore, this study attempts to estimate the magnitude and distribution of the east-west contraction forces acting to Japanese Island since 3 Ma. Based on the estimation, we validate the hypothesis from a view point of mechanics and discuss the origin of the forces.
Horizontal contraction forces that cause uplift of a mountain range and extensional forces arising from an increment of lithospheric body forces associated with the uplift are nearly equilibrium (e.g, Molnar and Lyon-Caen, 1988). Therefore, we can estimate a magnitude of an increment of contraction forces from altitude changes of mountain ranges. This study estimated the magnitude and distribution of the increment of the east-west contraction forces acting to Japanese Island since 3 Ma from the altitude change of the five mountain ranges: Kiso, Akaishi, Hida, Echigo, and Ou.
We obtain the following results. For Kiso, Akaishi, and Hida mountain ranges, ~2×1012 [N/m] of the contraction forces per unit length has been added along 200 km of the length of the mountain ranges. For Echigo and Ou mountain ranges, they are ~0.4×1012 [N/m] along 300 km. In total, 5.2×1017 [N] of the east-west contraction forces has been added to the Japanese Island since 3 Ma.
If the east-west contraction is caused by the east-ward migration of the triple junction and the Japan Trench arise from the motion direction change of the Philippine Sea plate, the magnitude of the contraction forces should be consistent with the magnitude of the force that caused the motion direction change of the Philippine Sea plate. The candidate of the force caused the motion direction change is the slab-pull arising from the west-ward subduction from the Philippine Trench initiated at 5-8 Ma. Seno (2000) estimated that the magnitude of the slab-pull force along the Philippine Trench is 3×1018 [N]. From the analysis of global plate motion, Forsyth and Uyeda [1975] estimated that around 90% of a slab-pull force balances to a slab viscous resistance force and the rest 10 % of it balances to the forces acting to the surface part of plates. Therefore, 5.2×1017 [N] of the east-west contraction forces estimated by this study are consistent with the slab-pull force although both of the magnitude of the forces are rough estimation. This result indicates that the east-west contraction of the Japanese Island can be explained by the motion direction change of the Philippine Sea plate from a view point of mechanics. In addition, the estimated east-west contraction forces take a maximum value for Central Japan. This implies that the largest east-west contraction force acting from around the TTT-triple junction. This is also consistent with the hypothesis.
Forsyth, D. and Uyeda, S. (1975), Geophys. J. Int., 43(1), 163-200.
Molnar, P. and Lyon-Caen, H. (1988), Geol. Soc. Am. Special Paper, 218.
Takahashi, M. (2006), Journal of Geography, 114, 116–123.
Takahashi, M. (2017), Bull. Geol. Surv. Japan, 68(4), 155–161.
Seno, T. (2000), J. Geol. Soc. Phillip., 55, 105–107.
Therefore, this study attempts to estimate the magnitude and distribution of the east-west contraction forces acting to Japanese Island since 3 Ma. Based on the estimation, we validate the hypothesis from a view point of mechanics and discuss the origin of the forces.
Horizontal contraction forces that cause uplift of a mountain range and extensional forces arising from an increment of lithospheric body forces associated with the uplift are nearly equilibrium (e.g, Molnar and Lyon-Caen, 1988). Therefore, we can estimate a magnitude of an increment of contraction forces from altitude changes of mountain ranges. This study estimated the magnitude and distribution of the increment of the east-west contraction forces acting to Japanese Island since 3 Ma from the altitude change of the five mountain ranges: Kiso, Akaishi, Hida, Echigo, and Ou.
We obtain the following results. For Kiso, Akaishi, and Hida mountain ranges, ~2×1012 [N/m] of the contraction forces per unit length has been added along 200 km of the length of the mountain ranges. For Echigo and Ou mountain ranges, they are ~0.4×1012 [N/m] along 300 km. In total, 5.2×1017 [N] of the east-west contraction forces has been added to the Japanese Island since 3 Ma.
If the east-west contraction is caused by the east-ward migration of the triple junction and the Japan Trench arise from the motion direction change of the Philippine Sea plate, the magnitude of the contraction forces should be consistent with the magnitude of the force that caused the motion direction change of the Philippine Sea plate. The candidate of the force caused the motion direction change is the slab-pull arising from the west-ward subduction from the Philippine Trench initiated at 5-8 Ma. Seno (2000) estimated that the magnitude of the slab-pull force along the Philippine Trench is 3×1018 [N]. From the analysis of global plate motion, Forsyth and Uyeda [1975] estimated that around 90% of a slab-pull force balances to a slab viscous resistance force and the rest 10 % of it balances to the forces acting to the surface part of plates. Therefore, 5.2×1017 [N] of the east-west contraction forces estimated by this study are consistent with the slab-pull force although both of the magnitude of the forces are rough estimation. This result indicates that the east-west contraction of the Japanese Island can be explained by the motion direction change of the Philippine Sea plate from a view point of mechanics. In addition, the estimated east-west contraction forces take a maximum value for Central Japan. This implies that the largest east-west contraction force acting from around the TTT-triple junction. This is also consistent with the hypothesis.
Forsyth, D. and Uyeda, S. (1975), Geophys. J. Int., 43(1), 163-200.
Molnar, P. and Lyon-Caen, H. (1988), Geol. Soc. Am. Special Paper, 218.
Takahashi, M. (2006), Journal of Geography, 114, 116–123.
Takahashi, M. (2017), Bull. Geol. Surv. Japan, 68(4), 155–161.
Seno, T. (2000), J. Geol. Soc. Phillip., 55, 105–107.