Abstract

Geohistory analysis was performed on Neogene deposits south and north of La Serena, central Chile, to determine the behavior of the crust since the middle Miocene. The tectonic history of both study areas was found to be similar, but the timing of uplift and downwarp events at the two localities differs by 1.3 and 0.8 m.y., respectively, which we attribute to the approach and passing of the Juan Fernández Ridge. From trigonometric relationships between the ridge trend, the plate vector, and the trend between the study areas, we calculated a plate-convergence rate of 6.2 cm/yr between 11.8 and 10.5 Ma and 10.1 cm/yr between 7.7 and 6.9 Ma. Rapid uplift commenced ca. 2.3 Ma, with a delay of 0.3 m.y. between the two study areas. Trigonometric constraints suggest the presence of a hitherto unsuspected oceanic plateau trending ∼014°, subducted below the present continental margin. Flat-plate subduction may thus be better developed where it is assisted by the combined buoyancy of ridge clusters instead of single subducting plateaus.

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