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

Abstract

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 epsilonNd(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]]>

Reply to Comment of Encinas et al. (2014) on: ‘Evidence for an Early-Middle Miocene age of the Navidad Formation (central Chile): Paleontological, climatic and tectonic implications’ of Gutiérrez et al. (2013, Andean Geology 40 (1): 66-78).

“…They propose that this occurred in south-central Chile between 25-24 Ma (after Muñoz et al, 2000). While concurring with plate roll-back, we do not agree on the duration of this event.…”

The topographic imprint of a transient climate episode: the western Andean flank between 15·5° and 41·5°S

“…(3) South of 31°S, total shortening is significantly reduced and south of 37° shortening has stopped at the end of Miocene (Vietor and Echtler, 2006). South of 33°S the western onshore margin shows a pronounced morphotectonic segmentation integrating the forearc Coastal Cordillera, the Central Depression, and the Main Cordillera (Muñoz et al, 2000). (4) South of ~37°S the Main Cordillera is called Patagonian Cordillera.…”

Forearc uplift rates deduced from sediment cores of two coastal lakes in south-central Chile

“…Oligocene-Miocene volcanic and sedimentary rocks, overlain by Pliocene-Quaternary fluvial-alluvial sediments (Muñoz et al, 2000;Jordan et al, 2001), and (4) the Main Cordillera, which constitutes a Mesozoic-Cenozoic magmatic arc with intra-arc sedimentary basins (Mpodozis and Ramos, 1989).…”