Magmatism as a probe to the Neogene shallowing of the Nazca plate beneath the modern Chilean flat-slab

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 arc 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.