High heat flow anomalies on an old oceanic plate observed seaward of the Japan Trench

Abstract

Thirty-three new measurements on the seaward slope and outer rise of the Japan Trench along a parallel of 38°45 0 N revealed the existence of high heat flow anomalies on the subducting Pacific plate, where the seafloor age is about 135 m.y. The most prominent anomaly with the highest value of 114 mW/m 2 is associated with a small mound on the outer rise, which was reported to be a kind of mud volcano. On the seaward slope of the trench, heat flow is variable: high (70-90 mW/m 2 ) at some locations and normal for the seafloor age (about 50 mW/m 2 ) at others. The spatial variation of heat flow may be related to development of normal faults and horst/graben structures due to bending of the Pacific plate before subduction, with fluid flow along the fault zones enhancing the vertical heat transfer. Possible heat sources of the high heat flow anomalies are intra-plate volcanism in the last several million years like that discovered recently on the Pacific plate east of the Japan Trench.

Heat flow anomaly on the seaward side of the Japan Trench associated with deformation of the incoming Pacific plate

“…As the line B is close to the data obtained by Yamano et al (2008) along 38 • 45 N ( Fig. 1), we focused mainly on the lines A and C rather than the line B.…”

“…We conducted heat flow measurements in the northern part of the Japan Trench area for further research on the heat flow anomaly on the incoming Pacific plate reported by Yamano et al (2008). Investigation of the extent and the origin of the heat flow anomaly should provide essential information for estimating the thermal structure of the interplate seismogenic zone.…”

“…Yamano et al (2008) conducted heat flow measurements on the seaward slope and the outer rise of the Japan Trench along a parallel of 38 • 45 N and found the existence of heat flow anomaly. High heat flow values, 70 to 110 mW/m 2 , were obtained at many stations, while values normal for the seafloor age, about 50 mW/m 2 , were also observed at some stations.…”

“…The devastating 2011 Tohoku-oki earthquake demonstrated that seismogenic process along this plate interface is even more complicated both in space and in time (e.g., Tajima et al, 2013). For better understanding of the complex process, information on the temperature structure along the plate interface is indispensable, since mechanical properties of the subduction plate interface are Yamano et al (2008), small circles: measurements before 1990 (Yamano, 2004), triangles: measurements in ODP/IODP holes (Shipboard Scientific Party, 2000;Fulton et al, 2013). A, B, and C are lines on which heat flow data are projected in Fig.…”

Methods

“…The downhole temperature (T) was calculated using a constant thermal conductivity (TC) of 1.1 W/(m·K) and a surface heat flow (HF) of 45 mW/m 2 as documented in Yamano et al (2008). The calculation is based on the following law:…”