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[1] A new Global Positioning System (GPS)-derived velocity field for the Andes mountains (26°-36°S) allows analysis of instantaneous partitioning between elastic and anelastic deformation at the orogen's opposing sides. Adding an ''Andes'' microplate to the traditional description of Nazca-South America plate convergence provides the kinematic framework for nearly complete explanation of the observed velocity field. The results suggest the oceanic Nazca boundary is fully locked while the continental backarc boundary creeps continuously at $4.5 mm/yr. The excellent fit of model to data (1.7 mm/yr RMS velocity misfit), and the relative aseismicity of the upper crust in the interior Andean region in comparison with its boundaries, supports the notion that the mountains are not currently accruing significant permanent strains. Additionally, the model implies permanent deformation is not accumulating throughout the backarc contractional wedge, but rather that the deformation is accommodated only within a narrow deformational zone in the backarc.

[1] Temporary deformation in great earthquake cycles and permanent shear deformation associated with oblique plate convergence both provide critical clues for understanding geodynamics and earthquake hazard at subduction zones. In the region affected by the M w 9.5 great Chile earthquake of 1960, we have obtained GPS observations that provide information on both types of deformation. Our velocity solutions for the first time span the entire latitudinal range of the 1960 earthquake. The new observations revealed a pattern of opposing (roughly arc-normal) motion of coastal and inland sites, consistent with what was reported earlier for the northern part of this region. This finding supports the model of prolonged postseismic deformation as a result of viscoelastic stress relaxation in the mantle. The new observations also provide the first geodetic evidence for the dextral motion of an intravolcanic arc fault system and the consequent northward translation of a forearc sliver. The sliver motion can be modeled using a rate of 6.5 mm/a, accommodating about 75% of the margin-parallel component of Nazca-South America relative plate motion, with the rate diminishing to the north. Furthermore, the new GPS observations show a southward decrease in margin-normal velocities of the coastal area. We prefer explaining the southward decrease in terms of changes in the width or frictional properties of the megathrust seismogenic zone. Because of the much younger age of the subducting plate and warmer thermal regime in the south, the currently locked portion of the plate interface may be narrower. Using a three-dimensional viscoelastic finite element model of postseismic and interseismic deformation following the 1960 earthquake, we demonstrate that this explanation, although not unique, is consistent with the GPS observations to the first order.

[1] Global Positioning System (GPS) measurements provide the first direct measurement of plate motion and crustal deformation across the Scotia-South America transform plate boundary in Tierra del Fuego. This plate boundary accommodates a part of the overall motion between South America and Antarctica. The subaerial section of the plate boundary in Tierra del Fuego, about 160 km in length, is modeled as a two dimensional, strike-slip plate boundary with east-west strike. Along the Magallanes-Fagnano fault system, the principal fault of this portion of the plate boundary, relative plate motion is left-lateral strikeslip on a vertical fault at 6.6 ± 1.3 mm/year based on an assumed locking depth of 15 km. The site velocities on the Scotia Plate side are faster than the relative velocity by an additional 1-2 mm/yr, suggesting there may be a wider region of diffuse left-lateral deformation in southern Patagonia. The northsouth components of the velocities, however, do not support the existence of active, large-scale transpression or transtension between the South America and Scotia plates along this section of the plate boundary.Components: 9235 words, 7 figures, 2 tables.

GPS crustal velocity data from the Scotia and South Sandwich plates, transform azimuths, spreading data, and an updated earthquake slip vector catalog provide the first Scotia and South Sandwich plate Euler vector estimates not dependent on closure as the GPS data tie them to the global plate circuit. Neither the GPS data, which sample limited portions of the plates, nor the geologic data, which are not tied to the global spreading circuit, are sufficient individually to define the Euler vectors. As Scotia plate GPS measurements do not sample the stable plate interior, plate boundary deformation field modeling is necessary for Euler vector estimation. Our South America‐Antarctic and Scotia‐South Sandwich Euler pole estimates agree with previous estimates from either GPS or geologic data. Our South America‐Scotia Euler vector, however, is significantly different and near the South America‐Antarctic Euler vector producing an approximately coaxial motion of Scotia between South America and Antarctica.