MAGMATISM AND THE SHALLOWING AND STEEPENING OF THE NAZCA PLATE UNDER THE CENTRAL ANDES FROM 28°S TO 37°S LATITUDE

The Andean margin between 28°S and 37°S has experienced a Neogene subduction history that can be discussed relative to three segments. The first is the Chilean flat-slab (28°S to 33°S) where the Nazca plate has shallowed in the last 19 Ma with the most pronounced shallowing occurring after subduction of the E-W trending segment of the Juan Fernandez Ridge at ~ 12 Ma. Shallowing is accompanied by the eastward expansion of magmatic and contractional deformational fronts that reached up to 700 km east of the trench between ~ 8 to 4 Ma. Subsequently, volcanism ceased over the Chilean flat-slab. To the south, the segment between 35°S to 37°S experienced a 19 to 5 Ma eastward expansion of magmatism and contractional deformation that is consistent with a contemporaneous, but less pronounced shallowing of the Nazca plate. Shallowing began after ~ 19 Ma as arc-like components appeared in backarc magmas erupting in a contractional regime. As in the Chilean flat-slab region, the peak of shallowing would have been in the late Miocene to earliest Pliocene. This peak is coincident with the eruption of hornblende-bearing mafic andesitic to dacitic magmas with arc-like chemistry and the uplift of ranges on reverse faults at distance up to 500 km east of the trench. Support for a shallow slab comes from a subduction component in ~ 8 to 5 Ma Chachahuén volcanic rocks that erupted over early Miocene mafic flows lacking these components. The history of this segment then diverged from that of the Chilean flat-slab as voluminous backarc Plio-Quaternary alkali basalts erupted in response to melting of a thickening mantle wedge over a steeper slab. In the intervening region between 33°S and 35°S, the arc front migrated eastward with a surge at ~ 19 to 16 Ma as the slab initially shallowed to the north and south, and again at ~ 8 to 4 Ma during the peak of backarc slab shallowing in these regions. Unlike to the north and south, far backarc magmatism is absent. A possible explanation for this pattern is simultaneous subduction of variably sized thickened portions of the Nazca plate leading to variable degrees and periods of shallowing of the Nazca plate. A synchronicity of major tectonic events along the South American margin suggests that an additional factor could be changes in the westward rate of drift of South American over the Nazca plate producing periods of broad scale shallowing.