The relation of the mid-Tertiary coastal magmatic belt in south-central Chile to the late Oligocene increase in plate convergence rate

Abstract

The mid-Tertiary Coastal Magmatic Belt in south-central Chile, which crops out both in the Central Valley and, south of 41°S, in the Coastal Cordillera as far west as the Pacific coast, formed when the locus of Andean magmatic activity expanded, both to the west and to the east relative to its previous and current location in the Main Cordillera. This expansion of the magmatic arc occurred in conjunction with a regionally widespread episode of late Oligocene to Miocene extension which thinned the crust below the proto-Central Valley in south-central Chile and generated sedimentary basins west of, within, and east of the Main Cordillera. The extrusive rocks of the mid-Tertiary Coastal Magmatic Belt are interbedded with the late Oligocene to Miocene continental and marine sediments deposited in these basins, and forty-seven of the fifty new and previously published age determinations for these rocks are within the time period 29 Ma (late Oligocene) to 18.8 Ma (early Miocene). The initiation of extension, basin formation and the westward migration of magmatic activity coincides closely to the beginning, in the late Oligocene, of the current period of both high convergence rate (>10 cm/yr) and less oblique convergence, which together resulted in an approximately three-fold increase in trench-normal convergence rate between the Nazca and South American plates. Extension continued, along with a transient steepening of subduction angle as indicated by the westward migration of the volcanic front during the formation of the mid-Tertiary Coastal Magmatic Belt, during an approximately 10 million year period after the trench-normal convergence rate tripled across the Nazca and South American plate boundary. The mid-Tertiary Coastal Magmatic Belt includes igneous rocks chemically similar to modern Andean arc magmas, as well as rocks with ocean island basalt chemical affinities characterised by lower Ba/La (<19), La/Nb (<1.6) and initial ⁸⁶Sr/⁸⁷Sr ratios (<0.7035), and higher e_Nd(T) values (>+5). The latter formed by melting of mantle uncontaminated by components derived from the dehydration of subducted oceanic lithosphere. This suggests the formation of the mid-Tertiary Coastal Magmatic Belt may have involved upwelling of asthenospheric mantle, possibly through a slab-window, due to the transient episode of invigorated asthenospheric wedge circulation caused by the three-fold increase in late Oligocene trench-normal convergence rates between the Nazca and South Amercan plates. The change in subduction geometry and the transient period of invigorated asthenospheric circulation caused by this increase in convergence rate may have combined to produced moderate extension across the southern South American continental margin by inducing an episode of slab rollback of the subducting Nazca plate.