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Abstract

Underplating of mantle-derived basalt plays an important role in crustal growth and arc crustal thickening, yet some arcs lack evidence of basaltic lower crust in spite of more than 200 m.y. of magmatism. Andean examples of basaltic lower crust are associated with plateau-type orogenic crust and include the Cordillera Blanca batholith in Peru (60 km) and the Western Cordillera volcanic arc in the central Andes (70 km). In the Late Cretaceous-Eocene paleo-arc in northern Chile (21-26°S) tectonic shortening and mafic underplating led to significant thickening of the arc crust (from ~37 km to ~45 km), with a ratio of tectonic : magmatic crustal thickening in the order of 2 : 1. In contrast, southern Chile (37-42°S) has been the site of largely stationary arc magmatism for more than 200 m.y., yet crustal thicknesses do not exceed that of normal continental crust (30-35 km). Interestingly, South Chile lacks a plateau-type topography and indication of either tectonic shortening or mafic underplating. Assuming magmatic addition rates of 5-25 km3/km/Ma, southern Chile lacks a mafic crustal keel in the order of 800-4.000 km3/km. What prevented underplating and crustal thickening in South Chile ? We envisage several possible scenarios: (1) Underplating + delamination: Underplated mafic crust may delaminate when transformed into eclogite, yet this requires crustal thicknesses >50 km. Alternatively, cumulates of mid-upper crustal magmas may have formed the lower crust, before they delaminated and were recycled into the mantle. (2) No underplating: The mafic andesitic main-arc composition may prevent basaltic underplating due to lack of a significant density contrast. (3) Underplating + lower crustal flow: Differences in crustal thickness like that generated by mafic underplate creates buoyancy forces that act to equalize crustal thickness and smooth the Moho topography, such that a hot and viscous arc crustal keel may flow sideways to regions with lower crustal thicknesses. Such a mechanism is consistent with subsidence and development of Quaternary basins above a positive regional gravity anomaly in the South Chilean arc-forarc, which corresponds to high Vp-velocities interpreted as thinned continental crust. Each of these scenarios may have solely acted or interacted with each other and prevented mafic underplating.