Heat flow and surface radioactivity measurements in the Precambrian shield of Western Australia

Abstract

A borehole for heat flow measurement has been drilled in a region of high radioactivity granitic rocks in the Archaean shield of Western Australia. Most previous shield heat flow determinations have been in low radioactivity basic and ultrabasic rocks which constitute less than 20% of the shield surface rocks. The heat flow of 1.23 μcal/cm 2sec measured in the granitic rocks is significantly higher than most previous shield values but is still much lower than the heat flows measured in the younger areas of eastern Australia. Two new measurements on the shield in basic and ultrabasic rocks give heat flows of 0.82 and 0.69. The mean surface radioactive element heat production in the Western Australian shield is found to be similar to that of the high heat flow regions of eastern Australia. This suggests that the radioactivity is concentrated in a very thin surface layer in the shield crust. The high radioactivity surface layer is probably largely absent in shield regions of basic and ultrabasic rocks where very low heat flows have been measured. Crustal radioactive element and temperature profiles have been estimated for the shield and for a high heat flow area.