Southern Patagonian plateau basalts and deformation: Backarc testimony of ridge collisions

Abstract

The distribution and volume of Tertiary Patagonian plateau basalts and the evolution of the Patagonian fold and thrust belt between 46° and 49°S appear to be closely tied with ridge-trench interactions along the continental margin to the west. Eocene plateau basalts south of 43°S are associated with a gap in the Eocene arc and can be related to a proposed collision of the Aluk-Farallon ridge against the trench at this time. Eruptions of Late Miocene to Recent plateau basalts between 46° and 49°S can be related to time-transgressive slab windows in the underlying mantle generated by the collision of segments of the Chile ridge at 10–14 Ma and 6 Ma. The principal deformation in the Patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with Late Miocene ridge collision. Comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment. Almost all Eocene to Recent plateau basalts between 46° and 49°S have OIB-like (ocean island basalt) trace element signatures showing that the mantle source region changed little with time. Plateau basalts with more arc-like signatures appear to be contaminated by continental crust. Relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows.