Long-Term Geological Evolution and Mass-Flow Balance of the South-Central Andes

Abstract

In south-central Chile (36--42? S), the western edge of South America has evolved as an active margin since the Pennsylvanian (‚Äöàº305 Ma). Active margins are considered as sites of both potential continental growth and continental destruction. Continental growth in a margin setting can proceed by accretionary offscraping of juvenile material from the oceanic plate and by magmatic additions, whereas net mass loss can be achieved by subducting continental material, delamination, and chemical weathering. In southcentral Chile, margin evolution was never interrupted by island-arc accretion or continental collision. Thus, the area provides an excellent field laboratory for studying mass flux through a long-term, persistent, convergent-margin system. Using new isotopic age data, we summarize the current knowledge of the geological evolution of the south-central Chilean margin, from subduction initiation to the ongoing Andean morphotectonic processes, with emphasis on mechanisms of mass transfer. It is inferred that net crustal growth and mass losses alternated in time and space, and that dominance of one of the other process might have even occurred contemporaneously within a short distance along the same margin, controlled by factors such as sediment availability in the trench, lower-plate morphology, upper-plate tectonics, and climate. In south-central Chile, the margin north of 38? S is characterized by a landward trench migration of ‚Äöàº100 km that occured mainly in the early Permian, whereas farther south, the modern and the late Paleozoic magmatic arcs are superimposed. For most of its lifetime, the margin evolved in a delicate balance between constructive and destructive processes. Over the long term, the south-central Chilean continental margin has not been a site of net growth, but, rather, a site of continental mass wasting, crustal recycling and crustal rejuvenation.