INVESTIGATING LITHOSPHERIC COUPLING ACROSS THE NAZCA/ANTARCTIC AND SOUTH AMERICAN PLATE BOUNDARY IN THE VICINITY OF THE CHILE RISE TRIPLE JUNCTION USING FINITE-ELEMENT MODELS

Finite-element techniques are employed to investigate the nature of plate coupling in the Nazca/Antarctica/South America subduction zone. As the Nazca and Antarctic plates are subducted beneath the South American plate, stresses are created at the main thrust interface and are subsequently transmitted into the lithosphere of the overriding plate. Additionally, the subduction of the Chile Rise changes the coupling of the interface, as well as the thermal and rheologic profiles of the South American lithosphere, normal and parallel to the trench.

A simplified 2-D elastic earth model has been used in FEMLab to explore relationships between key mechanical and rheological properties that may be used to describe the coupling. Three 3000 by 400 km cross-sectional finite-element profiles were created to investigate processes responsible for the different deformation styles seen as a function of location along strike of the trench; steady state, thermally perturbed, and formerly perturbed but returning to steady state. Critical parameters investigated are the depth/extent of brittle failure (coefficient of friction and pore pressure) versus power law creep (activation energy and the power law exponent). The models are composed of five subdomains that permit large strain commensurate with actual subduction over 10-15 million years. Temperature profile changes in the subducting plate are integral in determining the type and extent of deformation seen in shallow mantle (30-100km) xenoliths brought to the surface in Quaternary volcanic flows located 400 km east of the trench. They can be divided into three distinct groups based on deformation style, which is believed to be a direct result of where they originated with respect to the subducting, and actively spreading, Chile Ridge. As an independent constraint on the degree and type of coupling, far-field stress and strain accumulation from the models is compared with directly measured values from mantle xenoliths.