Terrestrial Heat Flow Overlying the Andean Subduction Zone.

Abstract

I report the results of a geothermal investigation in which thirty-four new heat flow sites in the countries of Peru, Bolivia, and Brazil have been established. In central Peru the heat flow is low (20-40 mW m('-2)), in southern Peru and northern Bolivia it is intermediate (40-80 mW m('-2)), and in central and southern Bolivia it is high (80-100 mW m('-2)). In the sub-Andean petroleum fields of Bolivia the heat flow (50-60 mW m('-2)) is typical of continental platforms, and in Brazil it is typical of Precambrian terranes (30-60 mW m('-2)). All values of heat flow have been corrected for the local topography and the uplift and erosional history. I divided the and es into five regions based on their apparently different uplift and erosional histories. The heat flow in each of these regions can be interpreted in terms of their past tectonic and magmatic history, which in turn is most likely related to the angle of subduction. In central Peru the heat flow is low and the angle of subduction is low (10(DEGREES) - 15(DEGREES)), in southern Peru and northern Bolivia the heat flow is intermediate where the subducted plate is in transition from low to higher angle subduction, and directly east of the active and recent volcanism of central and southern Bolivia the heat flow is high and the angle of subduction reaches 30(DEGREES). The observation of low heat flow in central Peru was unexpected, because there was very active volcanism in the mid-Miocene (12-7 Myr). Explanations for the low heat flow include the possibility that magmatic injections in regions of compression yield a relatively small thermal perturbation, and the low angle of subduction may result in an underplating of the continent by the cold oceanic plate depriving the continent of its normal supply of mantle heat, as well as extracting some heat from the base of the continental lithosphere. The observed high values of heat flow are most likely associated with the recent and active (3 - 0 Myr) volcanism in the continental volcanic arc and with the Pliocene (9 - 3 Myr) intrusions in the back-arc.