A new approach to building 2-D models of the heat flow density and radiogenic heat production: A Northern Tien Shan case study

Abstract

We propose a new approach to building 2-D / 3-D models of the regional HFD from the ground geophysical data without prior knowledge of the radiogenic heat production (RHP). A vertical heat flow density (HFD) vector map built for the study area enabled us to distinguish different heat sources in the crust, particularly, cooling of solidified felsic magma upwelling from the mantle depth, and radiogenic heat production in granitoids. An approach to assessing the apparent regional HFD is suggested. It uses the thermal conductivity and temperature models determined from the electromagnetic sounding data and borehole measurements. Numerical experiments show that when it is estimated for the layers with a thickness ranging from 1 to 5 km (which is similar to the surface HFD assessment from borehole measurements), the relative misfits can reach 30–80 %. On the other hand, it becomes practically insensitive to the thickness of the virtual layer approaching its value for the whole section (with uncertainty less than 10 %) when its lower depth exceeds some threshold level, in particular, exceeding the effective depth of the RHP decay in the study area. An approach to building 2-D model of the RHP rates from the lithology model and their values determined at the surface is proposed. It offers a more realistic surface RHP assessment than estimates based on constant RHP rates determined at the surface. It is shown that effective surface RHP is affected mainly by granitic rocks’ spatial distribution and could be a main heat source in the upper crust.