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AbstractThe western retroarc of the Southern Andes between 38° and 40° S is formed by a NNW-elongated ridge not associated withstacked thrust sheets. On the contrary, during the last 43 Ma this ridge was affected by extensional deformation, regional upliftand related folding on a very broad scale. Receiver function analysis shows that the drainage divide area and adjacent retroarc lieover an attenuated crust. Expected crustal thickness at these latitudes is around 38 km, whereas in this part of the retroarc thethickness is less than 32 km. The causes for such attenuation have been linked to a moderate steepening of the subducted Nazcaplate beneath the South American plate, which is suggested by a westward shift and narrowing of the magmatic arc during the last 4to 5 Ma. Gravimetric studies show that the upper plate did not react homogeneously to slab steepening, but ancient sutures andlithospheric discontinuities deeply buried under Mesozoic to Cenozoic sequences in the retroarc were locally reactivated. Theseprocesses resulted in an asthenospheric anomaly that correlates at the surface with the area of Pliocene to Quaternary doming,widespread extension and three radial troughs. Two of the troughs have accommodated substantial amounts of extension, but thethird was probably aborted at an early stage. Moreover, the presence of an anomalous concentration of calderas and large volcaniccenters over the proposed asthenospheric anomaly, and their age distribution, may indicate minor migration of the asthenosphericanomaly between 4 and 2 Ma through the western South American plate.© 2007 Elsevier B.V. All rights reserved.Keywords: Andean gravity; Retroarc extension; Thermal uplift; Asthenospheric anomalies; Retroarc volcanism