Heat‐flow determination in three DSDP boreholes near the Japan Trench

Abstract

The first deep borehole determinations of temperature gradients and heat flow on the landward wall of the Japan Trench and forearc were made on IPOD DSDP leg 57. These heat flow values are based on temperature logs corrected to equilibrium, using a detailed model of the drilling disturbance. Heat flow values on a deeply submerged marine terrace, landward of the trench slope break are 28 and 32 mW m−2. A measurement in the midslope terrace basin on the landward wall of the trench yielded a value of 22 mW m−2. The results are in good agreement with earlier seafloor measurements and indicate that most of the forearc area is characterized by heat flow about one half of that over oceanic lithosphere seaward of the trench. Our observations indicate only a small increase of heat flow from the trench to the volcanic arc, in agreement with thermal models, which suggests that the subduction of the relatively cold oceanic plate continues to dominate the temperature structure for distances of up to 250 km landward of the trench. The temperature profile in the borehole on the midslope terrace indicates possible vertical flow of pore waters. Hundreds of conductivity determinations were made using a new technique.

Helium anomalies suggest a fluid pathway from mantle to trench during the 2011 Tohoku-Oki earthquake

“…Estimated heat fluxes in the area are 0.6-2.5 mW m À 2 , which is markedly less than the terrestrial heat flow values of 22-32 mW m À 2 observed in the Japan Trench 33 . Helium may diffuse faster and farther than heat, which is well documented around the Ontake Volcano, Japan 34 .…”

An Estimate of the Heat Flow in the Western North Atlantic at Deep Sea Drilling Project Site 534

“…This similarity lends great credence to the technique of determining conductivity indirectly from porosity and mineralogy data, although the accuracy of the technique could have been better assessed if the temperature log had been made more carefully (cf., Burch and Langseth, 1981). Nevertheless, this method appears to provide a good estimate of the average conductivity, and it should be useful in obtaining heat-flow estimates at sites where no direct conductivity measurements are made, oil wells, for example.…”

Site 808

“…A possible explanation for the excess temperature measured near seafloor is that the upper part of the hole was warmed by the drilling fluid returning to seafloor from the hole bottom at a temperature significantly higher than the bottomwater temperature. Circulation rates lower than 25 L/s could produce this situation (Burch and Langseth, 1981). Circulation rates in holes of Site 808 were between 15 and 45 L/s.…”