ABSTRACT

In 2009, petit-spot submarine volcanoes were discovered off the oceanward slope of the central Chile trench, offshore from Valparaiso, Chile, at around 33°S. Ar-Ar dating of mugearite and alkali-basalt from the volcanoes yields ages of 10.11 ± 0.22 Ma and 6.69 ± 0.88 Ma, respectively. Back-calculations of plate motion along the present absolute movement direction of the Nazca Plate, conducted using the Ar-Ar age data, indicate that eruption occurred above a zone of plate flexure. The back-calculation results suggest that the mugearite was erupted at a flexural arch prior to arrival at the active site of the Juan Fernández hotspot. In contrast, the alkali-basalt was erupted on a plate flexure at a site of interaction between a flexural moat and an outer-rise area, where the source material was probably influenced by the Juan Fernández hotspot. The geochemistry of the lavas supports this interpretation because the concentration ratios of various rare earth and other trace elements in the mugearite are different from those of the alkali-basalt and Juan Fernández hotspot lavas. Consequently, petit-spot melts could reflect the composition of source materials below the plates at sites of plate flexure.