IAG-IASPEI 2017

講演情報

Oral

IASPEI Symposia » S26. Exploring connections between heat flow and tectonics

[S26-1] Exploring connections between heat flow and tectonics I

2017年8月3日(木) 08:30 〜 10:00 Room 503 (Kobe International Conference Center 5F, Room 503)

Chairs: Valiya Hamza (National Observatory - ON/MCTI) , Masataka Kinoshita (University of Tokyo)

09:15 〜 09:30

[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

Masataka Kinoshita1, Eiichiro Araki2, Toshinori Kimura2, Achim Kopf3, Demian Saffer4, Sean Toczko2 (1.Earthquake Research Institute, Univ. Tokyo, 2.JAMSTEC, 3.MARUM, Univ. Bremen, 4.Pennsylvania State University)

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.