THE EVOLUTION OF AN ANDEAN MARGIN: A MAGMATIC AND TECTONIC VIEW FROM THE CENTRAL AND SOUTHERN ANDES

Andean margins are those in which the oceanic crust is subducted beneath continental crust with the South American margin being the type section as a place where oceanic plate subduction has been occurring beneath a continental margin for much of the time since at least the late Precambrian. The pre-late Jurassic history records periods of subduction interrupted by collisions of terranes like Cuyania, Patagonian and Chilenia terranes and ends with massive silicic magmatism on the margins of the supercontinent Pangea. Since the Mesozoic opening of the Atlantic Ocean, the Andean margin has been dominated by subduction of oceanic plates with the late Jurassic to early Tertiary being dominated by steep to relatively steep subduction with intermittent periods of backarc extension and foreland basins. This picture changed in the mid Tertiary as the relative rate of westward overriding of South America increased and subduction of aseismic oceanic ridges under the central Andes lead to intermittent shallowing of the subducting plate. This shallowing caused arc magmatism and crustal weakening to extend far into the central Andean foreland producing the conditions for plateau uplift, crustal thickening, magmatic flare-ups and crustal and lithospheric delamination. Extreme shallowing led to temporary magmatic quiescence in the affected segments. The eruption of the giant central Andean ignimbrite complexes follow the steepening of the shallow slabs with the erupted magmas being modeled as near 50:50 mixtures of mantle-derived basalts and deep crustal melts that evolved as they ascended into mid crustal magma chambers and then erupted from shallow levels. Mass balance calculations and seismic images suggest that crustal loss through delamination associated with magmatic flare-ups and forearc subduction erosion could exceed new crustal gain through magmatic addition making the Neogene Central Andes a region of net crustal loss rather than gain and an important contributor of continental crust that is recycled into the asthenospheric mantle. In contrast, the margin of the southern Andes has been a site of generally steeper subduction zones with extensive backarc mafic magmatic provinces being related to mantle thermal anomalies associated with the break-up of Pangea and the near approach and collision of oceanic spreading ridges.