ANDEAN GEODYNAMICS - CONTRIBUTIONS FROM GPS

The Central Andes GPS Project (CAP) uses campaign and continuous GPS, spanning the width of the Andean orogen from Bolivia to Patagonia, to examine crustal deformation associated with various active geodynamic processes in the Andes. The network provides sampling of Andean deformation from orogen to individual mountain block or structure scale. During the past decade, as GPS technology improved and the network expanded (from 80 campaign to >250 campaign and a dozen continuous sites), the CAP network observed a number of horizontal and vertical signals.

Horizontal, generally tectonic signals include those associated with plate convergence along the locked Nazca- and Antarctic- S. America subduction boundary, coseismic signals associated with subduction plate boundary earthquakes, long term postseismic signals associated with the great 1960 Chilean earthquake, strike-slip motion across the S. America-Scotia plate boundary, along and across strike variations in orogenic strain rates in the back arc (small zones with high strain rates separating “blocks”, such as the thin-skinned Precordillera, with lower internal strain rates), and strain field rotation due to subduction obliquity and oroclinal bending. Geodynamic vertical signals include those associated with elastic loading and glacial isostatic adjustment (GIA) due to changes in river, lake and glacial loading.

Deformation is relatively uniform along strike between the Arica Bend and the northern end of the 1960 rupture where it is dominated by interseismic strain accumulation associated with locked subduction. Deformation in the epicentral area of the 1995, M8, Antofagasta earthquake returned to normal within 10 years. This contrasts with the region of the great 1960, M9.5, earthquake. There, a long term postseismic response swamps and reverses the deformation from the steady, at least over a 10 year time span, interseismic contribution from plate motion. The southern end of the 1960 rupture zone coincides with the triple junction between the subducting oceanic Nazca and Antarctica plates beneath continental S. America. Subduction of the South Chile Ridge there produces a slab window with anomalous low viscosity upper mantle that affects the GIA properties and where time varying loads from the Patagonian glacier fields drive vertical deformations.