FINITE-ELEMENT MODEL RESULTS FOR CHILE RIDGE TRIPLE JUNCTION AND SOUTH AMERICAN EXTENSION: STRESS TRANSFER ACROSS A SUBDUCTION ZONE

The Southern Chile Ridge has been subducting beneath the western margin of South America and steadily migrating to the north due to oblique subduction for approximately 14 Ma. Compression and uplift in the South American plate have been observed ahead of the migrating ridge, while extension and volcanism follow at the approximate location of the triple junction and strike-slip faults run throughout. The compressional and strike-slip features, including transpressional and transtensional basins are readily explained by the oblique tectonics, however, the cause of the uplift, magmatism, and possible extension is still under consideration. Previously proposed mechanisms for the magmatism and uplift include a slab window, slab roll-back, and the development of slab induced convection and upwelling. However, others note that the extensional structures in the overriding continental plate, including the LGCBA lake, may be related to the disparity in plate motions in the underlying Nazca and Antarctic plates. The means of stress transfer between the subducting slabs/oceanic plates and the overlying continent, however, are not clear. We use finite element models to test the viability of disparate plate motions causing the observed extension at the latitude of the Chile Ridge Triple Junction and to test the mechanism of stress transfer.

Initial 2-D models were constructed with the oceanic-continent boundary modeled as both weak and strong, and varied forces were applied to the oceanic plates to take into account possible changes in the dominant stress depending on ridge proximity. A strong stress gradient was observed in the South American Continent inall models at the latitude of the Chile Ridge. Changing the applied stresses and the strength of the ocean-continent boundary resulted in magnitude and spatial changes in the stress state of South America, however, the strong gradient remained in all models. These results suggest that active upwelling may not be necessary to explain the magmatism and extension associated with the currently subducting segment of the Chile ridge.