Geothermal Structure of the Central Andean Crust — Implications for Heat Transport and Rheology

Abstract

This study aims to detennine the central Andean geothennal structure by downward continuation of the surfcace temperature field. The heat flow density values along a trans-Andean proftle show a minimum of 40 mW/m 1 in the region of the Coastal Cordillera and Chaco and a maximum of 80 mW/m 1 in the Western Cordillera and the western Altiplano. The necessary structural data are provided by deep seismic sounding measurements. The petrophysical parameters used for this study are taken from the literature. Steady state and one-dimensional conditions and pure conductive heat transport are assumed. For the region with heat flow values between 40 and 50 mW/m 2 the temperature at the base of the crust does not exceed 700 °C. But in the zones with high heat flow density values of 60-80 mW/m 2 the temperature in the lower crust reaches values exceeding 1000 °C. Such high temperatures seem to be unrealistic. In order to lower the temperature gradient an additional advective heat transfer is postulated. For the advecting material (fluids and volatiles) a Darcy velocity of about 1 mm/year results. For comparison the amount of upstreaming fluids and volatiles released by the downgoing oceanic crust is detennined. This rough estimation yields the amount of fluids pervading the hanging lithosphere and is in the order of 0.3 mm/year. Because most parameters used for these calculations are only estimates the result may vary by a factor of 0.2-5 Finally the rheological response has been detennined as the maximum possible stress as a function of depth for the different heat flux locations and a strain rate of7·1O· IS S·I .