Late Cenozoic tectonism, collapse caldera and plateau formation in the central Andes

The evolution of Andean volcanism including the formation of late Miocene t o Recent collapse calderas on the Puna plateau is generally interpreted in terms of the kinematic framework of the Nazca and South American Plates. We present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first-order NW-SE-striking zones of left-lateral transtension on the local and regional scales. Consequentl y. ages of collapse calderas indicate activity of these fault zones which i nitiated at about 10 Ma on the Puna plateau. The onset of such faulting poi nts to a change in the deformation regime from dominantly vertical thickeni ng to orogen-parallel stretching upon reaching maximum crustal thickness an d critical surface elevation. Horizontal magma sheets that formed at midcru stal level possibly due to heat advection by volume increase of asthenosphe ric mantle below thickened crust were tapped by sub-vertical faults. This a ccounts well for the observed tectono-magmatic phenomena at surface. It fol lows that formation of collapse calderas and eruption of voluminous ignimbr ites appear to be related to the mechanical evolution of the Andean plateau rather than to changes in the geometry of the Wadati-Benioff zone or plate boundary kinematics. (C) 2001 Elsevier Science B.V. All rights reserved.