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[SCG52-P08] Development Process and Tectonics of Fault Zone on Yonaguni Island
Keywords:Yonaguni Island, active fault, tectonics, vitrinite reflectivity, Okinawa Trough, RockJock
The Ryukyu arc is located near the boundary between the Eurasian plate and the Philippine plate, and the Philippine plate subducts in the Ryukyu Trench. The Okinawa Trough continues to expand in the backarc basin of the Ryukyu Arc. Large-scale tectonics such as Ryukyu Trench and Okinawa Trough develop around Yonaguni Island. It was reported that geological structures such as multiple faults and joints had developed in the island, and these were subjected to tensile stress after being formed by compressive stress (SAKAI et al., 1978). However, the clear reason for the conversion from the compression field to the tensile field has not been clarified. The purpose of this study is to reexamine the geological structure which develops in Yonaguni Island, and to discuss the relationship with the tectonics around Yonaguni Island. In this study, the geological structure development history of Yonaguni Island is clarified by investigating Yonaguni Island in wide area (East, Southeast, West), and clarifying fault zone structure, paleothermal structure in the vicinity, and mineral combination, and the relationship with the tectonics around the island is discussed.
In order to examine fault zone structure, crack density, fault rock distribution, and strike slope of each surface structure were measured and described from the fault center. In order to examine the surrounding structure under heat, the authors collected rock samples containing vitrinite from the Yaeyama Group, and measured the vitrinite reflectivity (Ro%). EASY% Ro (Sweeney & Burnham, 1990) was used to estimate the maximum temperature to be heated. In order to examine the mineral combination of fault rock and host rock, XRD measurement and mineral qualitative and quantitative analysis using RockJock (Eberl, 2003) were carried out.
Results: From the descriptions of the eastern and southeastern fault zones, the combination of faulted rocks (Fault gouges, fault breccia, cataclysite) is consistent, and the width and size of the fault core and damage zone, which are separated by unconsolidated faulted rocks (fault gouges) and highly consolidated faulted rocks (cataclysite), are also similar. The distribution of damage zone and cataclysite with high crack density overlaps with each other, but the decreasing tendency of crack density is greatly different. In the eastern part, a rapid decrease in crack density is observed around the cataclysite distribution area. On the other hand, the crack density in the southeastern part tends to gradually decrease even beyond the distribution area of cataclysite. These results indicate that the eastern fault zone has two structures, a multiple fault core and a single fault core (Mitchell and Faulkner, 2009), and the southeastern fault zone has a single fault core structure. From the XRD measurement results, quartz and amorphous materials , illite, chlorite, smectite, vermiculite, plagioclase, and potash feldspar were recognized as minerals contained in the weakly deformed and non-deformed parts of fault rocks and host rocks of fault gouges and cataclysites. As a result of the vitrinite reflectance measurement, Ro (%) of the upper disk side of the eastern fault showed 0.8, and that of the lower disk side showed 0.97.
Discussion: Chlorite and illite are known to be formed by hydrothermal alteration above 200 °C, and it is considered that there was a high temperature event. The upper plate side Ro (%) of the eastern fault is 140 °C and the lower plate side Ro (%) is 163 °C when converted to the maximum heat receiving temperature, and if this temperature difference (23 °C) is caused by the displacement of the fault, the vertical displacement of the Yaeyama Group and the Ryukyu Group across the fault zone cannot be explained only by the displacement after the Late Pleistocene (Ryukyu Group Deposition). Therefore, it is possible that there was a dip in the upper disk before the Ryukyu Group deposition. Although the eastern and southeastern fault zones are located at the northeastern and southwestern ends of the same linear component, the difference in the structure of the multiple fault core and the single fault core suggests that the primary structure of the fault zone was formed in the eastern part and the formation of the fault zone began in the southeastern part after multiple fluctuations. It is concluded that the eastern and southeastern fault zones joined together with the growth of the fault zone due to at least multiple variations, leading to the present series of fault zones.
In order to examine fault zone structure, crack density, fault rock distribution, and strike slope of each surface structure were measured and described from the fault center. In order to examine the surrounding structure under heat, the authors collected rock samples containing vitrinite from the Yaeyama Group, and measured the vitrinite reflectivity (Ro%). EASY% Ro (Sweeney & Burnham, 1990) was used to estimate the maximum temperature to be heated. In order to examine the mineral combination of fault rock and host rock, XRD measurement and mineral qualitative and quantitative analysis using RockJock (Eberl, 2003) were carried out.
Results: From the descriptions of the eastern and southeastern fault zones, the combination of faulted rocks (Fault gouges, fault breccia, cataclysite) is consistent, and the width and size of the fault core and damage zone, which are separated by unconsolidated faulted rocks (fault gouges) and highly consolidated faulted rocks (cataclysite), are also similar. The distribution of damage zone and cataclysite with high crack density overlaps with each other, but the decreasing tendency of crack density is greatly different. In the eastern part, a rapid decrease in crack density is observed around the cataclysite distribution area. On the other hand, the crack density in the southeastern part tends to gradually decrease even beyond the distribution area of cataclysite. These results indicate that the eastern fault zone has two structures, a multiple fault core and a single fault core (Mitchell and Faulkner, 2009), and the southeastern fault zone has a single fault core structure. From the XRD measurement results, quartz and amorphous materials , illite, chlorite, smectite, vermiculite, plagioclase, and potash feldspar were recognized as minerals contained in the weakly deformed and non-deformed parts of fault rocks and host rocks of fault gouges and cataclysites. As a result of the vitrinite reflectance measurement, Ro (%) of the upper disk side of the eastern fault showed 0.8, and that of the lower disk side showed 0.97.
Discussion: Chlorite and illite are known to be formed by hydrothermal alteration above 200 °C, and it is considered that there was a high temperature event. The upper plate side Ro (%) of the eastern fault is 140 °C and the lower plate side Ro (%) is 163 °C when converted to the maximum heat receiving temperature, and if this temperature difference (23 °C) is caused by the displacement of the fault, the vertical displacement of the Yaeyama Group and the Ryukyu Group across the fault zone cannot be explained only by the displacement after the Late Pleistocene (Ryukyu Group Deposition). Therefore, it is possible that there was a dip in the upper disk before the Ryukyu Group deposition. Although the eastern and southeastern fault zones are located at the northeastern and southwestern ends of the same linear component, the difference in the structure of the multiple fault core and the single fault core suggests that the primary structure of the fault zone was formed in the eastern part and the formation of the fault zone began in the southeastern part after multiple fluctuations. It is concluded that the eastern and southeastern fault zones joined together with the growth of the fault zone due to at least multiple variations, leading to the present series of fault zones.