BoI.IG-USP, Publ.Esp., 8:89-108,1991. INFLUENCE OF TECTONICS ON THE EXTENT OF QUATERNARV GLACIATION IN THEANDES

The extent of Quaternary glaciations varies throughout the Andes. Although the lack of radiometric dating in many placas precludes precise assessment of these variations, certain spatial pattems are evident. In the N. Andes (N. of 15·S) glacial landforms and sediments of only the ultimate and penultimate glaciations (O-isotope stages 2, 4, 6) have been identified. In the Central Andes (15-35 o S) successive glaciations slnce at least the mid-Quatemary have been progressively less extensive. In the Southem Andes successive glaciations since the greatest glaciation at ca. 1.2 Ma have been progressively srnaller; also, the Islas Malvinas 800-1,000 km to the east became glaciated for the first time during the penultimate glaciation and developed an even greater number of nivo-glacial features during the last glaciation. The segrriented nature of the oceanic and continental crust along the Pacific-South American subduction zone promotes tectonic independence of parts of the Andean chain. It is argued that variations in the extent of glaciation may be largely explained by Quaternary up/ift and subsidence in different places and by precipltation-shadow effects related to elevation and air mass source. INTRODucnON The extent of actual glaciers in the Andes is determined by the altitude of the Equilibrium Une (ELA) which is controlled by mean annual temperature and precipltation. The ELA presentJy rises fro