Structural Responses to the Chile Ridge Subduction, Southern South America

The Nazca-Antarctic plate boundary, the Chile spreading ridge, subducts beneath South America, forming the northward-migrating Chile Triple Junction (CTJ), now at ~46.5°S, where an actively spreading segment is currently in the Nazca trench. Ridge subduction is associated with diachronously developed variable structure and magmatism of overriding South America. To assess the effects of ridge subduction, we deployed a network of 39 broadband seismometers in southern Chile between 43 - 49°S and 71 - 76°W from Dec. 2004 - Feb. 2007, recording 102 earthquakes suitable for receiver function analyses, i.e., M > 5.9, of various backazimuths, and at epicentral distances of 30 - 90°. The network encompassed onland portions of the current triple junction and ridge subduction, areas to the south of the CTJ where ridge segments subducted during the last 6 m.y., and regions north of the CTJ not yet affected by ridge subduction, allowing the assessment of the effects of ridge subduction on crustal structure of overriding South America. We constructed 551 teleseismic receiver functions to estimate crustal thicknesses, H, and average compressional to shear wave velocity ratios, Vp/Vs = k, using the iterative time deconvolution method of Ligorria and Ammon (1999). H and k were calculated using the grid search method of Zhu and Kanamori (2000). Beneath stations closest to the trench, where the Nazca plate subducts, we found Moho depths between 28 and 55 km, thickening northward. At the locus of current ridge subduction, in the Taitao Pennisula, thinner crust ranges from 27 - 36 km. H is 36-38 km where the Antarctic plate subducts and the Chile ridge recently subducted. The direct effect of the subducting ridge on South America can be seen in H differences between forearc regions that have sustained ridge subduction versus those that have not. South American forearc crust above the subducted Nazca plate is as much as 28 km thicker than forearc crust recently affected by ridge subduction, and is as much as 11 km thicker than forearc crust above the subducting Antarctic plate. The latter difference may reflect primary crustal thickness differences. Crustal thinning above the current ridge subduction is a result of enhanced subduction erosion due to the Chile ridge's greater buoyancy.