Radioactive heat generation and its thermal effects in the Alps-Apennines boundary zone

Radioactive heat-production data of rocks cropping out at the Alps-Apennine s boundary zone (NW Italy) are presented. Samples were analysed in the labo ratory by means of a gamma-ray spectrometer. They are representative of the Savona Unit and the ophiolitic sequences, with metasediments and sediments , of the Voltri Massif, the Sestri-Voltaggio Zone and the Lavagna Nappe. Th e heat-production rate of sedimentary rocks ranges from 1.05 (limestones) t o 2.52 muW m(-3) (shales). In ophiolites, it varies from 0.04 (serpentinite s) to 0.24 muW m(-3) (metabasalts). Orthogneisses of the Savona Unit show t he highest value (2.92 muW m(-3)). Potassium contributes to the heat genera tion on average by 17%. In the metasedimentary and sedimentary rocks, thori um contribution accounts on average by 43%, except for dolomites in which 9 7% of the radioactive heat is produced by uranium. Gale-schists, shales, ra diolarites and phyllites give a Th/U ratio ranging from 3 to 4, that is ver y close to that of orthogneisses. This ratio is lower in limestones, almost all ophiolites and dolomites. Most rocks show a K/Th ratio ranging between 2 x 10(3) and 4 x 10(3). These data together with available thermal conduc tivity information are used in a finite element simulation to analyse the e ffects on the heat flux and temperature distribution due to lateral variati on of thermal parameters. Changes in structure and composition of the upper most part of the crust result in small wavelength variation of surface heat Aux. The low radioactive heat production of ophiolites produces a decrease of about 10 mW m(-2) in the surface heat-flux on the Voltri Massif. (C) 20 01 Elsevier Science B.V. All rights reserved.