Breaking the Ring of Fire: How Ridge Collision, Slab Age, and Convergence Rate Narrowed and Terminated the Antarctic Continental Arc

The geometry of the Antarctic-Phoenix Plate system, with the Antarctic Plate forming both the overriding plate and the conjugate to the subducting oceanic plate, allows quantification of slab age and convergence rate back to the Paleocene and direct comparison with the associated magmatic arc. New Ar-Ar data from Cape Melville (South Shetland Islands, SSI) and collated geochronology shows Antarctic arc magmatism ceased at similar to 19 Ma. Since the Cretaceous, the arc front remained similar to 100 km from the trench whilst its rear migrated trenchward at 6 km/Myr. South of the SSI, arc magmatism ceased similar to 8-5 Myr prior to each ridge-trench collision, whilst on the SSI (where no collision occurred) the end of arc magmatism predates the end of subduction by similar to 16 Myr. Despite the narrowing and successive cessation of the arc, geochemical and dyke orientation data shows the arc remained in a consistently transitional state of compressional continental arc and extensional backarc tectonics. Numerically relating slab age, convergence rate, and slab dip to the Antarctic-Phoenix Plate system, we conclude that the narrowing of the arc and the cessation of magmatism south of the SSI was primarily in response to the subduction of progressively younger oceanic crust, and secondarily to the decreasing convergence rate. Increased slab dip beneath the SSI migrated the final magmatism offshore. Comparable changes in the geometry and composition are observed on the Andean arc, suggesting slab age and convergence rate may affect magmatic arc geometry and composition in settings currently attributed to slab dip variation.

Automated summary

The geometry and evolution of the Antarctic-Phoenix Plate system is studied using new Ar-Ar dating and geochronology, revealing the cessation of arc magmatism at around 19 million years ago and the migration of the arc front and rear. The narrowing of the arc and the cessation of magmatism are primarily attributed to the subduction of younger oceanic crust and decreasing convergence rate. Similar changes in geometry and composition are observed on the Andean arc, suggesting a potential correlation between slab age, convergence rate, and magmatic arc geometry and composition.