NEOGENE SHALLOW SUBDUCTION SEGMENTS IN THE CHILEAN/ARGENTINE ANDES AND ANDEAN-TYPE MARGINS

Evidence for transient shallow subduction zones along the Neogene Andean margin suggests that the formation of shallowly dipping subduction zones could be a common process on ancient Andean-type margins. The best known shallow segment in the Chilean-Argentine Andes is the modern Chilean flatslab (28° to 33° S) which has shallowed since 18 Ma. A second shallow segment has been proposed to explain the Early to Middle Miocene evolution of the central Puna- Evidence for transient shallow subduction zones along the Neogene Andean margin suggests that Altiplano plateau (~ 20° to 23°S). Here, a third shallowly dipping segment is suggested for the Late Miocene beneath the Neuquén Basin (35° to 37°S). Evidence comes from evolutionary parallels with the Chilean flatslab in the eastward broadening of the arc and uplift of block-faulted ranges far east of the trench. Particularly significant is the presence of the ~ 7 to 5 Ma Chachahuén volcanic complex near 35° S which erupted mafic andesitic to dacitic lavas ~ 500 km east of the trench. Arc-like La/Ta ratios (to 52) and hydrous, oxidizing residual mineral assemblages in equilibrium with these magmas indicate conditions best associated with a subducting slab. The lack of these features in underlying Miocene and overlying Pliocene alkaline lavas supports a transient shallowly dipping segment. Overall, a common factor in the eastward arc broadening that signals shallowing of these segments is a coincidence in time with Early or Late Miocene Pacific plate reorganizations (e.g., Tebbins and Cande 1997). The transient nature of these shallowly dipping segments thus appears linked to local instabilities in intermediate age subducting oceanic crust particularly at times of plate reorganization. As long suggested, these perturbations are likely due to thickened portions of oceanic crust.