[S-SS06] Fault Rheology and Earthquake Physics

[SSS06-09] High temperature fluid flow through active décollement at the Nankai subduction zone, off Cape Muroto, Japan

★Invited Papers

*Nana Kamiya¹, Masataka Kinoshita², Takehiro Hirose³, Weiren Lin⁴, Man-Yin Tsang⁵, Yuzuru Yamamoto⁵, Stephen Alan Bowden⁶, Satoshi Tonai⁷ (1.Doshisha University, 2.University of Tokyo, 3.JAMSTEC, 4.Kyoto University, 5.Kobe University, 6.University of Aberdeen, 7.Kochi University)

Keywords:Nankai Trough, décollement, paleo-temperature anomaly, vitrinite reflectance

Thermal anomalies within subduction zones can be caused by frictional heating along faults and the movement of hot fluids through décollement horizons. Hot fluids are particularly important because they affect physical properties, chemical cycles and the deep biosphere. Despite these general associations being known, because subduction zones have highly dynamic subsurface conditions, identifying and measuring thermal anomalies can be complex. The Muroto area is part of the Nankai Trough seismogenic zone, where dynamic phenomena such as large-scale earthquakes can cause frictional heating along fault zones and localised changes in paleotemperature. Additionally, because of the subduction of a hot and active ridge (Yamano et al., 2003) heat flow is uncharacteristically high in the Nankai accretionary complex compared to adjacent or other accretionary complexes. Here we document evidence for anomalous heating along a décollement horizon located at the distal region of the Nankai accretionary prism, off Cape Muroto, drilled byInternational Ocean Discovery Program (IODP) Expedition 370.
We measured vitrinite reflectance, from which can be calculated an index of the maximum temperature experienced by sediments. To provide a context for the measured vitrinite reflectance and paleotemperature within the décollement, we first evaluated the general paleotemperature by calculating vitrinite reflectances assuming a range of constants heat flows; 140, 160 and 180 mW/m². A heat flow of 160 mW/m², was found to yield calculated reflectances that matched most of the measured reflectances outside of the décollement. Considering that present day heat flow for site C0023 is approximately 140 mW/m², site C0023 must have experienced a higher heat flow in the past.
Vitrinite reflectance values measured at the décollement horizon are significantly higher than can be accounted for by a basal heat flow of 160 mW/m², indicating the presence of a thermal anomaly centered around the décollement. We find that heating at 200°C for a duration of 500-1000 years can explain the vitrinite reflectance anomaly found at the décollement horizon.
This poses the question of the origin of the hot fluid. At Site C0023, a zone of high pore pressure is located below the décollement, indicating a path for fluids from deeper parts. Considering that in the Muroto area at the depth of the seismogenic zone the temperature is approximately 200°C, and that an equivalently hot region is not found in sediment directly below the décollement, hot fluids can only originate from a deep seismogenic zone. The advection of high-temperature fluids is thought to be intermittent, therefore, the high values of vitrinite reflectance at the décollement horizon are caused by the advection of hot fluids from deep shallow areas during past earthquakes.