9:15 AM - 9:30 AM
[S26-1-04] Reconstruction of recent 6Ma thermal structure seaward of updip limit of Nankai seismogenic zone off Kumano inferred from IODP NanTroSEIZE geothermal data and time-dependent numerical model
Understanding the slip behavior of the seismogenic faults in subduction zones requires accurate estimates on the present & past thermal structures. To depict the thermal history, we need to know the tectonics and thermal regime of the Philippine Sea plate (PHS), which may have stopped subduction until ~6MaBP, and restarted afterwards. Our purpose here is to test if such subduction tectonics can affect the ‘present' thermal regime.
During the IODP NanTroSEIZE expeditions, we deployed 2 borehole observatories containing 5 thermistors. In 2012, the first one was deployed at Site C0002 above the updip limit portion of megathrust, and the temperature and heat flow at ~900 m below seafloor is determined as 38 degC and 57 mW/m2, respectively. In 2016, we deployed the second one at Sites C0010 across the shallow megasplay fault zone. Thermistors are located between ~400 mbsf (crossing the fault zone) and 562.72 mbsf, and the temperature at 562.7 mbsf is 26.7 degC. With core thermal conductivity data, average heat flow is determined as ~60 mW/m2. It is slightly higher than nearby Site C0004 (54 mW/m2). It may be attributed either to pore fluid flow or transient phenomena (e.g., burial due to thrust faulting).
Using these heat flow data, a thermal evolution model around the updip zone of Nankai seismogenic zone is constructed for 2 end-member models; subduction of 12Ma-old Shikoku Basin (SB) started at 6MaBP, vs. subduction of 5Ma-old SB continued since 13MaBP. We found that two end-member models (subduction has been continuous vs. it started 6MaBP) are very similar and impossible to discriminate to each other. We also found that the thermal structure at 6-3MyBP can be different from the present one. In that case the accretionary prism evolution, as reconstructed through paleo-thermometry, may need reconsideration.
During the IODP NanTroSEIZE expeditions, we deployed 2 borehole observatories containing 5 thermistors. In 2012, the first one was deployed at Site C0002 above the updip limit portion of megathrust, and the temperature and heat flow at ~900 m below seafloor is determined as 38 degC and 57 mW/m2, respectively. In 2016, we deployed the second one at Sites C0010 across the shallow megasplay fault zone. Thermistors are located between ~400 mbsf (crossing the fault zone) and 562.72 mbsf, and the temperature at 562.7 mbsf is 26.7 degC. With core thermal conductivity data, average heat flow is determined as ~60 mW/m2. It is slightly higher than nearby Site C0004 (54 mW/m2). It may be attributed either to pore fluid flow or transient phenomena (e.g., burial due to thrust faulting).
Using these heat flow data, a thermal evolution model around the updip zone of Nankai seismogenic zone is constructed for 2 end-member models; subduction of 12Ma-old Shikoku Basin (SB) started at 6MaBP, vs. subduction of 5Ma-old SB continued since 13MaBP. We found that two end-member models (subduction has been continuous vs. it started 6MaBP) are very similar and impossible to discriminate to each other. We also found that the thermal structure at 6-3MyBP can be different from the present one. In that case the accretionary prism evolution, as reconstructed through paleo-thermometry, may need reconsideration.