An investigation of the Nazca/South American plate boundary zone deformation kinematics

I investigate convergence between the Nazca and South American plates in the Central Andes to study the partitioning of motion in the boundary zone. The deformation extends thousands of kilometers inland of the nominal plate interface. The partitioning causes locking at the subduction interface, eventually released in interplate thrust earthquakes; aseismic slip after such earthquakes; stable interseismic sliding at the trench; and mountain building in the foreland fold and thrust belt (FTB).; The evolution of shortening within the Andes is examined by comparing structural data for the past 25 Ma and GPS data. These data indicate an acceleration of mountain building since 10 Ma, as shortening extended eastward within the FTB. Earthquake-related shortening is an order of magnitude less, indicating that aseismic deformation occurs.; Using historical seismicity data to estimate the seismic coupling predicts that significant aseismic slip occurs along the trench, even given uncertainties from undersampling the seismicity and in the determination of historical earthquake magnitudes. Earthquake slip vectors indicate a slight (∼10°) trench parallel motion of the forearc region.; Coupling fractions and shortening rates within the plate boundary zone are estimated by modeling the geodetic data. For an assumed subduction zone geometry, two and three dimensional models representing various amounts of locking and shortening fractions were used to fit the GPS observations. The best-fitting models vary depending on the subduction zone dip, with steeper dips yielding higher coupling estimates. The best-fitting models indicate coupling between 22–66%. Improvements to this method could be made using observations from seafloor GPS and detailed seismic data to better estimate the geometry of the subduction zone.