The Late Paleogene to Neogene Volcanic Arc in the Southern Central Andes (28°-37°S)

Abstract

Evolution of arc magmatism along the Southern Central Andes (*28°?37° S)is strongly controlled by changes in the geometry of the downgoing slab (e.g., slab dipangle). This is particularly evident in the present-day Chilean-Pampean flat-slaband thelate Miocene Payenia shallow subduction segments. Typical Andean-volcanismwasestablished from the late Oligocene to late Miocene in the high 30″?30°30″S),with arc-related calc-alkaline volcanism having geochemical signatures that reflectchanges in the residual mineral assemblages related increased crustal thickness(>50 km). The increase in crustal thickness resulted increased compression alongthe Southern Central Andean margin due to the the Juan Fernández Ridge,and consequent shallowing of the downgoing during Mid to Late Miocene.Associated with the decrease in the slab dip the volcanic front migrated to theeast. Further south, magmas developed the present-day Payenia back-arc region(35°?37° S) show an increase in slab-components in the middle Miocene to early Pliocene times, which also suggests a progressive shallowing of the subducting slab atthese latitudes. However, trace element ratios indicate a low-to-intermediate pressureresidual mineral assemblage and no significant increase in crustal thickness is apparent,unlike further north in the Chilean-Pampean flat-slab segment. Although flat-slabgeometry still prevails in this latter segment, re-steepening of the slab during earlyPliocene times (*5?3 Ma) promoted an increase of arc and back-arcmagmatism at thesemore southerly latitudes of the Southern Central Andes. A dynamic link between slabgeometry, geochemistry, and volcanic activity is therefore observed in the Southern Central Andes.