LATE CENOZOIC SUBDUCTION OF THE CHILE RIDGE BENEATH PATAGONIA: RESULTS FROM PASSIVE SEISMIC DEPLOYMENTS (Invited Presentation)

The active divergent plate boundary between the Nazca and Antarctic plates is currently subducting beneath Patagonia. The Chile Triple Junction (CTJ) at ~46.5°S, 75.8°W marks the intersection at the surface of an actively spreading segment of the Chile Ridge with the Chile Trench immediately west of the Taitao Peninsula, southern Chile. North of the CTJ, the Nazca plate subducts towards N79°E at ~6.6 cm/yr, whereas to the south the Antarctic plate subducts eastwards at ~1.8 cm/yr. Temporary passive seismic stations deployed onland in the vicinity of the CTJ between 2004 and present day have provided extensive data suitable for resolving the seismic structure of the CTJ region down to mantle transition zone depths. Results from studies of converted body waves, shear wave splitting, and ambient noise, body wave, and surface wave tomographies, show that the ridge subduction has had a profound effect on the crustal structure of overriding South America and has generated a well-developed slab window between subducted Nazca and Antarctic oceanic lithosphere. Shear wave splitting of S waves impinging from the core mantle boundary beneath the region shows that upper mantle flow is diverted into the slab window beneath South America. Studies of the crustal structure from teleseismic P-to-S wave conversions reveal crustal thinning of South American forearc crust above the slab window. Depth to Moho north of the CTJ, beneath the Patagonian Andes and the islands of the Chonos and Guaitecas archipelago, increases northwards from ~36-48 km. However, beneath the southern Taitao Peninsula and the Golfo de Penas, immediately above the slab window, South American crust is ~30 km thick. Thinning of South American crust in this region is a result of subduction erosion enhanced by subduction of the buoyant spreading ridge. Anisotropic ambient noise tomography shows that the lower crust in forearc areas affected by ridge subduction is characterized by consistently low isotropic velocities (< 3.1 km/s) and high azimuthal anisotropy (1.5-2%), with generally east-trending fast axes in the region of crust thinned by subduction erosion. In contrast, north of the CTJ isotropic crustal velocities are higher (> 3.1 km/s) and forearc crustal anisotropy generally parallels the Chile Trench, the Andes, and the Liquiñe-Ofqui strike slip fault.