Heat flow study at the Chinese Continental Scientific Drilling site: Borehole temperature, thermal conductivity, and radiogenic heat production

Abstract

The Chinese Continental Scientific Drilling (CCSD) Project offers a unique opportunity for studying the thermal regime of the Dabie‐Sulu ultrahigh‐pressure metamorphic belt. In this paper, we report measurements of borehole temperature, thermal conductivity, and radiogenic heat production from the 5158 m deep main hole (CCSD MH). We have obtained six continuous temperature profiles from this borehole so far. The temperature logs show a transient mean thermal gradient that has increased from 24.38 to 25.28 K km−1 over a period of about 1.5 years. We measured thermal conductivities and radiogenic heat productions on more than 400 core samples from CCSD MH. The measured thermal conductivities range between 1.71 and 3.60 W m−1 K−1, and the radiogenic heat productions vary from 0.01 μW m−3 to over 5.0 μW m−3, with a mean value of 1.23 ± 0.82 μW m−3 for the upper 5‐km layer of the crust. The heat productions in CCSD MH appear to be more rock‐type than depth‐dependent and, over the depth range of CCSD MH, do not fit the popular model of heat production decreasing exponentially with increasing depth. The measured heat flow decreases with depth from ∼75 mW m−2 near the surface to ∼66 mW m−2 at a depth of 4600 m. High heat flow anomalies occur at ∼1000 and ∼2300 m, and low anomalies occur at 3300–4000 m. A preliminary two‐dimensional numerical model suggests that both radiogenic heat production and thermal refraction due to structural heterogeneity are at least partially responsible for the vertical variation of heat flow in CCSD MH.