[S-CG57] New perspectives of subduction zone earthquake dynamics through experiments across-scales

[SCG57-03] Paleo-thermal history deduced from vitrinite reflectance within the décollement, off Cape Muroto, Japan

*Nana Kamiya¹、Masataka Kinoshita²、Weiren Lin³、Takehiro Hirose⁴、Yuzuru Yamamoto⁵、Stephen Alan Bowden⁶、Man-Yin Tsang⁵、Satoshi Tonai⁷、Expedition 370 Scientists Group⁸ (1.Graduate School of Engineering, Kyoto University、2.The University of Tokyo、3.Kyoto University、4.JAMSTEC、5.Kobe University、6.University of Aberdeen、7.Kochi University、8.IODP)

キーワード：Nankai Trough, paleo-temperature anomaly, vitrinite reflectance

Subsurface temperature can be perturbed by the passage of hot fluids or volcanic activities and this can in turn affect physical, chemical and microbial processes. Subduction zones are notably dynamic tectonic environments in terms of both fluid and sediment cycling and this gives rise to very complicated thermal histories. For example, at the toe of the Nankai accretionary prism, off Cape Muroto, heat flow is uncharacteristically high compared to adjacent and other accretionary complexes, and this is considered to be a consequence of the subduction of a hot and active ridge (e.g. Yamano et al., 2003). Furthermore, because the Muroto area is a part of the Nankai Trough seismogenic zone, paleo-temperature changes could be controlled by dynamic phenomena such as large-scale earthquakes, which in other settings have been shown to cause frictional heating along fault zones. To investigate paleo-temperatures at the toe of the Nankai accretionary prism, off Cape Muroto, we have measured vitrinite reflectance on cores collected during IODP Expedition 370.
IODP Site C0023 is located 120 km from Cape Muroto and is in the protothrust-zone of the Nankai accretionary prism. At Site C0023 a décollement is encountered at 758-795 mbsf, but does not contain any major thrusts. Present-day temperatures were measured directly down to ~350 mbsf and the estimated present-day heat flow is 140 mW/m2.
From 200 to 700 mbsf vitrinite reflectance increases from 0.2 to 0.6%; deeper than 750 mbsf, vitrinite reflectance remains constant at around 0.6%. These values of vitrinite reflectance are larger than would be predicated from present-day measurements of temperature or predictions from heat flow. Instead a heat flow of 160 mW/m2 is needed to match the measured vitrinite reflectance. However, even with this correction, the virtrinite reflectance observed around the décollement is still anomalously higher, and higher than would be predicted from a simple down-hole increase in temperature. To investigate the cause of this anomaly, we assumed a higher temperature be present at the décollement, and that there was an additional source of heating at this position. It was found that such a source of heating would have to be active for several thousand years, to match the observed distribution of vitrinite reflectance. Therefore, at the toe of the Nankai accretionary prism in Muroto area, two thermal events and heat sources are suggested: a generally high basal heat flow and at some point in the past thermal advection through the décollement that has thermally altered surrounding sediments. These two events are the result of the development of the accretionary prism and the situation of Site C0023 at the deformation front. Considering the scales of space and time of thermal advection, a high-temperature fluid originating from a deeper position within the décollement zone is our currently preferred explanation.