Andean flat slab subduction through time

Abstract

The analysis of magmatic distribution, basin formation, tectonic evolution and structural styles of different segments of the Andes shows that most of the Andes have experienced a stage of flat subduction. Evidence is presented here for a wide range of regions throughout the Andes, including the three present flat-slab segments (Pampean, Peruvian, Bucaramanga), threeincipient flat-slab segments (‘Carnegie’, Guan˜acos, ‘Tehuantepec’), three older and no longeractive Cenozoic flat-slab segments (Altiplano, Puna, Payenia), and an inferred Paleozoic flatslabsegment (Early Permian ‘San Rafael’). Based on the present characteristics of the Pampeanflat slab, combined with the Peruvian and Bucaramanga segments, a pattern of geological processescan be attributed to slab shallowing and steepening. This pattern permits recognition of other olderCenozoic subhorizontal subduction zones throughout the Andes. Based on crustal thickness, twodifferent settings of slab steepening are proposed. Slab steepening under thick crust leads to delamination, basaltic underplating, lower crustal melting, extension and widespread rhyolitic volcanism, as seen in the caldera formation and huge ignimbritic fields of the Altiplano and Punasegments. On the other hand, when steepening affects thin crust, extension and extensivewithin-plate basaltic flows reach the surface, forming large volcanic provinces, such as Payeniain the southern Andes. This last case has very limited crustal melt along the axial part of theAndean roots, which shows incipient delamination. Based on these cases, a Paleozoic flat slab isproposed with its subsequent steepening and widespread rhyolitic volcanism. The geologicalevolution of the Andes indicates that shallowing and steepening of the subduction zone are thusfrequent processes which can be recognized throughout the entire system.