Magnetic anomaly interpretation across the southern central Andes (32°-34°S):: The role of the Juan Fernandez Ridge in the late Tertiary evolution of the margin

Marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of Chile between 32 degrees -34 degrees S. Three magnetic domains have been identified: oceanic, continental margin, and subaerial. The oceanic domain has seafloor spreading anomalies (16 (similar to 37 Ma) to 18 (similar to 39.5 Ma)) disturbed by anomalies of the Juan Fernandez hot spot chain. In the continental margin, the most prominent fabric are E-W anomalies in the upper slope corresponding to onshore E-W anomalies of large intrusive bodies. Onshore, a N-S lineament of short-wavelength anomalies defines the roots of a Cretaceous volcanic are. A resembling lineament offshore indicates a submerged older volcanic are and that continental basement extends to similar to 50 km landward of the trench. Absolute Cenozoic plate motion for Nazca and South American plates and dating of the Juan Fernandez chain provide a kinematic model of ridge-continent collision. The reconstruction indicates rapid southward migration of the collision point along similar to 1400 km of the margin from 20 to 11 Ma (similar to 20 cm yr(-1)). From 11 Ma to present the collison point has migrated at a slower rate along similar to 375 km of the margin (3.5 cm yr(-1)). The predicted location of the subducted portion of the Juan Fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship. In the last similar to 10 Myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates. These observations indicate that subduction of the Juan Fernandez chain plays a major role in arc-forearc tectonics.