Abstract

Changes in the Neogene (Miocene to Recent) magmatic style of the modern Chilean flat-slab region and its margins (27-34°S) reflect both variations in Nazca-South American convergence parameters and the effects of subduction of the Juan Fernández Ridge hotspot track. These effects are superimposed on an initially thicker are crust in a more compressional tectonic regime in the north than in the south. Distinctive magmatic periods across the region reflect relatively steep subduction from ~27 to 20 Ma. frontal arc migration associated with forearc subduction erosion from ~20 to 16 Ma, compressional deformation and andesitic volcanism from ~15 to 9 Ma, a peak of slab shallowing associated with frontal arc migration and arc broadening from ~8 to 4 Ma, and termination of volcanism over the flat-slab by ~5 Ma. Changes in magmatic style and distinctive chemical signatures in ~20 to 16 Ma volcanic rocks coincide with a marginwide transition from a more extensional to a more compressional tectonic regime linked to changing Nazca-South American convergence parameters. North to south differences in ~15 to 9 Ma magmas partially reflect the southward migration of the northeast-trending arm of the subducting Juan Fernández Ridge, whose effect arrived in the flat-slab region at ~14 Ma. The termination of andesitic arc volcanism across most of the region at ~9 Ma and extreme shallowing of the slab beneath the central flat-slab coincides with the arrival and eastward propagation of the east-west-trending segment of the Juan Fernández Ridge. Overall, the subduction of the Juan Fernández Ridge is a perturbation that causes extreme shallowing in part of a regionally shallowing subduction zone. Chemical signatures of the erupted magmas reflect the changing mantle and crustal configuration above the shallowing subduction zone. Those with adakitic-like signatures are derived from subduction zone mafic magmas contaminated by garnet granulitic to eclogitic facies crust in the thickened crust or by crust incorporated into the mantle as a result of forearc subduction erosion. There is no evidence that any of these magmas were generated by melting of the subducted slab. © 2002 Elsevier Science Ltd. All rights reserved.