Interpretation of heat-flow density in the Central Andes

Abstract

This study focuses on the interpretation of surface heat-flow density data in the central Andes to examine possible lithospheric thermal structures. Along a profile at about 21°S from the Peru–Chile trench in the west to the Andean foreland in the east, surface heat-flow density is used to constrain quantitative models that are employed to investigate the influence of various effects on the thermal field. Negligible changes in surface heat-flow density along strike of the Andean orogen, in contrast to major variations across the orogen, seem to justify the application of two-dimensional (2-D) cross-section models. The region of the Andean orogen between the trench and the volcanic front is described by 2-D subduction models, the back-arc region, characterised by crustal doubling, is analysed by 2-D models describing simple underthrust phenomena and the Andean foreland is investigated by one-dimensional temperature estimates. The influence of various geometrical and petrophysical parameters on the thermal structure is investigated. Model calculations show that the low heat-flow density values in the fore-arc region suggest low shear stresses on the order of 15 MPa along the plate contact. Variations of the subduction angle influence mainly the W–E extent of the surface heat-flow density anomaly. The distribution of the crustal radiogenic heat-production rate influences the surface heat-flow density significantly, but has only a minor influence on crustal temperatures. Modelling results show that temperatures at the maximum depth of seismic coupling between the oceanic Nazca plate and the overriding South American continental plate (∼45 km) are on the order of 250 to 300°C, whereas at the maximum depth of the plate contact (∼60 km) temperatures are on the order of 300 to 350°C. The lithospheric temperature structure in the active magmatic arc region is strongly sensitive to temperature changes caused by the occurrence of an assumed asthenospheric mantle wedge at shallow depth (∼70 km). To match conditions for melting at subcrustal levels, the minimum extent of an asthenospheric wedge towards the west has to coincide with the position of the volcanic front. The thermally thinned lithosphere results in a surface heat-flow density on the order of 60–70 mW m −2. Locally higher heat-flow density in the volcanic arc is interpreted as the effects of shallow magma chambers. The doubling of the crust in the back-arc region affects the lithospheric thermal structure considerably, but models do not show reverse temperature–depth distributions in the continental crust. In the back-arc region the transition from a thermally thinned lithosphere to a `normal' lithospheric thickness for continental shields may occur. The observed surface heat-flow density in the Andean foreland suggests lithospheric thermal conditions as typical for shield areas.