Interpretation of heat-flow density in the Central Andes

This study focuses on the interpretation of surface heat-flow density data in the central Andes to examine possible lithospheric thermal structures. A long a profile at about 21 degrees S from the Peru-Chile trench in the west to the Andean foreland in the east, surface heat-flow density is used to c onstrain quantitative models that are employed to investigate the influence of various effects on the thermal field. Negligible changes in surface hea t-flow density along strike of the Andean orogen, in contrast to major vari ations across the orogen, seem to justify the application of two-dimensiona l (2-D) cross-section models. The region of the Andean orogen between the t rench and the volcanic front is described by 2-D subduction models, the bac k-are region, characterised by crustal doubling, is analysed by 2-D models describing simple underthrust phenomena and the Andean foreland is investig ated by one-dimensional temperature estimates. The influence of various geo metrical and petrophysical parameters on the thermal structure is investiga ted. Model calculations show that the low heat-flow density values in the f ore-are 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 cr ustal radiogenic heat-production rate influences the surface heat-flow dens ity significantly, but has only a minor influence on crustal temperatures. Modelling results show that temperatures at the maximum depth of seismic co uplings between the oceanic Nazca plate and the overriding South American c ontinental plate (similar to 45 km) are on the order of 250 to 300 degrees C, whereas at the maximum depth of the plate contact (similar to 60 km) tem peratures are on the order of 300 to 350 degrees C. The lithospheric temper ature structure in the active magmatic are region is strongly sensitive to temperature changes caused by the occurrence of an assumed asthenospheric m antle wedge at shallow depth (similar to 70 km). To match conditions for me lting at subcrustal levels, the minimum extent of an asthenospheric wedge t owards the west has to coincide with the position of the volcanic front. Th e thermally thinned lithosphere results in a surface heat-flow density on t he order of 60-70 mW m(-2). Locally higher heat-flow density in the volcani c are is interpreted as the effects of shallow magma chambers. The doubling of the crust in the back-are region affects the lithospheric thermal struc ture considerably, but models do not show reverse temperature-depth distrib utions in the continental crust. In the back-are region the transition from a thermally thinned lithosphere to a 'normal' Lithospheric thickness for c ontinental shields may occur. The observed surface heat-how density in the Andean foreland suggests lithospheric thermal conditions as typical for shi eld areas. (C) 1999 Elsevier Science B.V. All rights reserved.