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

The mid-Tertiary Coastal Magmatic Belt in south-central Chile, which crops out both in the Central Valley and, south of 41 degreesS, in the Coastal Co rdillera 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 it s previous and current location in the Main Cordillera. This expansion of t he magmatic arc occurred in conjunction with a regionally widespread episod e of late Oligocene to Miocene extension which thinned the crust below the proto-Central Valley in south-central Chile and generated sedimentary basin s west of, within, and east of the Main Cordillera. The extrusive rocks of the mid-Tertiary Coastal Magmatic Belt are interbedded with the late Oligoc ene to Miocene continental and marine sediments deposited in these basins, and forty-seven of the fifty new and previously published age determination s 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 w estward 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 app roximately three-fold increase in trench-normal convergence rate between th e Nazca and South American plates. Extension continued, along with a transi ent steepening of subduction angle as indicated by the westward migration o f the volcanic front during the formation of the mid-Tertiary Coastal Magma tic Belt, during an approximately 10 million year period after the trench-n ormal convergence rate tripled across the Nazca and South American plate bo undary. The mid-Tertiary Coastal Magmatic Belt includes igneous rocks chemi cally similar to modern Andean are magmas, as well as rocks with ocean isla nd basalt chemical affinities characterised by lower Ba/La (<19), La/Nb (<1 .6) and initial (SrSr)-Sr-86-Sr-187 ratios (<0.7035), and higher <epsilon>( 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 ci rculation caused by the three-fold increase in late Oligocene trench-normal convergence rates between the Nazca and South Amercan plates. The change i n subduction geometry and the transient period of invigorated asthenospheri c circulation caused by this increase in convergence rate may have combined to produced moderate extension across the southern South American continen tal margin by inducing an episode of slab rollback of the subducting Nazca plate.