11:30 AM - 11:45 AM
[SGL24-04] Late Miocene paleo-stresses recorded in the Miocene forearc basins, southwest Japan
Keywords:forearc basin, stress inversion, Miocene, southwest Japan arc
The late Miocene relative motion of plates along the southwest Japan arc has not been clarified, and two different models has been proposed for that. One model supposes a continuous subduction with slower speed [1] and the other supposes a transform plate boundary with a cessation of subduction [2]. Although the stress field in forearc is expected to reflect the relationship between the plates, it has not been revealed. To elucidate the stress state dominating the whole late Miocene forearc region in southwest Japan, we investigated stress history recorded in the middle Miocene Tanabe and Kurami Groups, paleo-forearc basin sediments, by analyzing outcrop-scale deformation structures.
In the Tanabe Group in the southwest of the Kii Peninsula, we measured 1124 clastic dikes, 426 outcrop-scale faults, and 321 mineral veins. After analyzing the structures for each of the 9 areas, detected stresses were classified into 3 groups by a hierarchical clustering technique, normal faulting stress with NW-SE tension axis (Group α), E-W tension stress (Group β), and NW-SE compression stress (Group γ). Group α, consisting of stresses detected from areas in the lower Tanabe Group, is considered to have been exerted before the deposition of the upper Tanabe Group (about 16 Ma). Stresses of Group β are detected mainly from clastic dikes. Following the hypothesis that clastic dikes intruded at the same time of or soon after the deposition of the upper Tanabe Group (about 16-15 Ma), stresses of Group β are supposed to be almost syndepositional. Crosscutting relationships between measured structures show that stresses of Group γ have been exerted after Groups α and β.
In the Kurami Group in Shizuoka Prefecture, we measured 65 outcrop-scale faults and detected NE-SW axial tension stress (Stress δ). N-S trending sinistral faults, consistent with stress δ, tend to be accompanied by mineral veins implying a hydrothermal activity. Because no mineral vein is found in the late Miocene-Pliocene Sagara Group near the Kurami Group, Stress δ are considered to have worked before the deposition of Sagara Group (about 11 Ma).
In southwest Japan, igneous activities were activated between 17-12 Ma. N-S trending igneous dikes intruded between 15-14 Ma in the southernmost of the Kii Peninsula, although spatiotemporal uniformity of N-S compressional stress, accountable to the dikes, is still unknown. Group β detected in the Tanabe Group indicates the stress with N-S σHmax axis worked near the trench and the Tanabe Group. Group γ in the Tanabe Group and stress δ in the Kurami Group share the NW-SE trending σHmax axis and hydrothermal activity. Therefore it is suggested that σHmax axis direction change from N-S to NW-SE before the cessation of igneous activity, and that the later stress widely worked in forearc basin in southwest Japan.
References
[1] Kimura et al., 2005, Geol. Soc. Am. Bull., 117, 969-986.
[2] Kimura et al., 2014, Tectonics, 33, 1219-1238.
In the Tanabe Group in the southwest of the Kii Peninsula, we measured 1124 clastic dikes, 426 outcrop-scale faults, and 321 mineral veins. After analyzing the structures for each of the 9 areas, detected stresses were classified into 3 groups by a hierarchical clustering technique, normal faulting stress with NW-SE tension axis (Group α), E-W tension stress (Group β), and NW-SE compression stress (Group γ). Group α, consisting of stresses detected from areas in the lower Tanabe Group, is considered to have been exerted before the deposition of the upper Tanabe Group (about 16 Ma). Stresses of Group β are detected mainly from clastic dikes. Following the hypothesis that clastic dikes intruded at the same time of or soon after the deposition of the upper Tanabe Group (about 16-15 Ma), stresses of Group β are supposed to be almost syndepositional. Crosscutting relationships between measured structures show that stresses of Group γ have been exerted after Groups α and β.
In the Kurami Group in Shizuoka Prefecture, we measured 65 outcrop-scale faults and detected NE-SW axial tension stress (Stress δ). N-S trending sinistral faults, consistent with stress δ, tend to be accompanied by mineral veins implying a hydrothermal activity. Because no mineral vein is found in the late Miocene-Pliocene Sagara Group near the Kurami Group, Stress δ are considered to have worked before the deposition of Sagara Group (about 11 Ma).
In southwest Japan, igneous activities were activated between 17-12 Ma. N-S trending igneous dikes intruded between 15-14 Ma in the southernmost of the Kii Peninsula, although spatiotemporal uniformity of N-S compressional stress, accountable to the dikes, is still unknown. Group β detected in the Tanabe Group indicates the stress with N-S σHmax axis worked near the trench and the Tanabe Group. Group γ in the Tanabe Group and stress δ in the Kurami Group share the NW-SE trending σHmax axis and hydrothermal activity. Therefore it is suggested that σHmax axis direction change from N-S to NW-SE before the cessation of igneous activity, and that the later stress widely worked in forearc basin in southwest Japan.
References
[1] Kimura et al., 2005, Geol. Soc. Am. Bull., 117, 969-986.
[2] Kimura et al., 2014, Tectonics, 33, 1219-1238.