5:15 PM - 6:30 PM
[SIT19-P02] Comparison of paleostress and paleotemperature analysis at Roof Thrust and Out-of-Sequence Thrust of subduction zone : The Minami Awa Fault and Mizoochi Fault, The Mugi area in the Shimanto Belt
Keywords:Calcite, Shimanto belt, Vitrinite, Accretionary complex, Roof thrust, Out of sequence thrust
The duplex structure and out-of-sequence thrust (OST) of the hypocentral region in the of the deep subduction zone are typical structures. Many structures stated above result from repeating the underplating accretion which the thrust sheet overlaps by the step-down of the decollment which is the plate boundary. The old upper decollment is called roof thrust and the new lower decollment is called floor thrust. The OST is a large thrust at late stage that intersects with these accretionary topographies. The roof thrust has been considered inactive because the decollment forms the floor thrust at the bottom of the duplex structure. However, a number of pseudotachylytes have been found in the roof thrust. In this study area, “Mugi mélange”, the roof thrust as an earthquake source fault and the OST have been reported (Onishi and Kimura., 1995; Hashimoto and Kimura., 1999; Ujiie et al., 2000, etc.), which is the most suitable for comparing 2 major faults at the same burial depth in the identical mélange. The Minami-Awa fault (roof thrust) and the Mizoochi fault (OST) are located close together and may have splay fault.
Mugi mélange is located in the southeastern part of Shikoku and belongs to the southern margin of the northern Shimanto Belt. Mugi mélange is considered to be a tectonic mélange, a plate-boundary rock that appeared on the surface (Onishi and Kimura., 1995; Hashimoto and Kimura., 1999; Ujiie et al., 2000, etc.). Minami-Awa fault is a boundary fault (roof thrust) between Hiwasa Formation (turbidite) and Mugi mélange. Minami-Awa fault is the roof thrust containing pseudotachylyte (Kitamura et al., 2005; Ujiie et al., 2007 b). The cummulative displacement of Mizoochi fault (OST) is large as the depositional age and paleotemperature are different. The purpose of this study is to examine the difference in the stress concentration between the roof thrust and the OST using a calcite paleostress piezometer, and to estimate the paleotemperature and thermal diffusion from the vicinity of the fault to a distance using the vitrinite reflectance, and to estimate the roles of the 2 faults from them.
・Calcite paleostress piezometer: A geologic paleostress gauge that calcite in rocks records as twins when they undergo strain. In this study, we used calcite that cracks in boudin.
・Vitrinite reflectance paleothermometer: Vitrinite, a type of maceral, records the maximum heat exposure during the process of thermal maturation and carbonization of plant fragments.
The paleotemperature of the host rock of the Mizoochi fault (OST) is approximately 200 -244 °C in the hanging wall and approximately 150 -185 °C in footwall. This large thermal difference may indicate accumulative great offset to have repeated many slips.
The paleostress was obtained from calcite twin analysis. The value of the stress near the Minami Awa fault (roof thrust) ranged from 146 ± 27 MPa to 183 ± 24 MPa. The paleostress around the Mizoochi fault was 135 ± 16 MPa to 179 ± 22 MPa, and the paleostress decreases from 123 ± 18 MPa with distance from the fault. This low value of the paleostress at distance from the fault may be background crustal stress. By contrast to this, the high shear stress around the faults may be due to the stress concentration during the rupture propagation from deep nucleation zone. The stress concentration at tip of the rupture front is the result of the resistance to failure of the locked fault. In this study, similar levels of the stress concentrations were found at both of the roof thrust of the Minami-Awa fault and the OST of the Mizoochi fault. If one fault has stronger resistance from fracturing than the other in bifurcated fault, the rupture will propagate to weak fault any times. Two faults in this study experienced similar stress concentration, and the rupture can propagate either fault. This idea can explain that the rupture propagates to the roof thrust to reactivate the fault.
Mugi mélange is located in the southeastern part of Shikoku and belongs to the southern margin of the northern Shimanto Belt. Mugi mélange is considered to be a tectonic mélange, a plate-boundary rock that appeared on the surface (Onishi and Kimura., 1995; Hashimoto and Kimura., 1999; Ujiie et al., 2000, etc.). Minami-Awa fault is a boundary fault (roof thrust) between Hiwasa Formation (turbidite) and Mugi mélange. Minami-Awa fault is the roof thrust containing pseudotachylyte (Kitamura et al., 2005; Ujiie et al., 2007 b). The cummulative displacement of Mizoochi fault (OST) is large as the depositional age and paleotemperature are different. The purpose of this study is to examine the difference in the stress concentration between the roof thrust and the OST using a calcite paleostress piezometer, and to estimate the paleotemperature and thermal diffusion from the vicinity of the fault to a distance using the vitrinite reflectance, and to estimate the roles of the 2 faults from them.
・Calcite paleostress piezometer: A geologic paleostress gauge that calcite in rocks records as twins when they undergo strain. In this study, we used calcite that cracks in boudin.
・Vitrinite reflectance paleothermometer: Vitrinite, a type of maceral, records the maximum heat exposure during the process of thermal maturation and carbonization of plant fragments.
The paleotemperature of the host rock of the Mizoochi fault (OST) is approximately 200 -244 °C in the hanging wall and approximately 150 -185 °C in footwall. This large thermal difference may indicate accumulative great offset to have repeated many slips.
The paleostress was obtained from calcite twin analysis. The value of the stress near the Minami Awa fault (roof thrust) ranged from 146 ± 27 MPa to 183 ± 24 MPa. The paleostress around the Mizoochi fault was 135 ± 16 MPa to 179 ± 22 MPa, and the paleostress decreases from 123 ± 18 MPa with distance from the fault. This low value of the paleostress at distance from the fault may be background crustal stress. By contrast to this, the high shear stress around the faults may be due to the stress concentration during the rupture propagation from deep nucleation zone. The stress concentration at tip of the rupture front is the result of the resistance to failure of the locked fault. In this study, similar levels of the stress concentrations were found at both of the roof thrust of the Minami-Awa fault and the OST of the Mizoochi fault. If one fault has stronger resistance from fracturing than the other in bifurcated fault, the rupture will propagate to weak fault any times. Two faults in this study experienced similar stress concentration, and the rupture can propagate either fault. This idea can explain that the rupture propagates to the roof thrust to reactivate the fault.