Partitioning of deformation along the Andean margin: insights from elastic block modelling

Along the Andean margin, oblique subduction of the Nazca Plate is accommodated by slip on the subduction interface and deformation of the overriding South American Plate. Active plate boundary deformation, in particular due to strain partitioning, is analyzed using elastic block modelling constrained by compiled GPS velocities to estimate plate motions, fault slip rates, and spatially variable interplate coupling on the Nazca-South American subduction interface. In the block modelling approach, interseismic GPS velocities are assumed to be the sum of rigid block rotation and elastic strain accumulation on block-bounding faults. Therefore, the western South American margin is divided into smaller blocks, primarily based on active faults traces. The block model geometry is adjusted to minimize the misfit between observed and modeled velocities.

The preferred model shows strain partitioning of varying degrees along the Andean margin. In the North, the margin-parallel component of convergence is partially accommodated by right-lateral slip on a strike-slip system that extends from the Gulf of Guayaquil off southern Ecuador to western Venezuela. This results in the northeastward motion of the North Andean Block with respect to stable South America. In Peru, the model confirms the existence of the southeastward moving Inca Sliver, that is bounded by the trench and the Subandean fold-and-thrust belt. Along the central Chilean margin, oblique convergence is partially accommodated by minor right-lateral slip along the Subandean thrust fault. In southern Chile, right-lateral transpression along the intra-arc Liquiñe-Ofqui Fault Zone results in the northward translation of the Chiloé Sliver.

The separation of the North Andean Block, the Peruvian Inca Sliver, and the Chilean Andean orogen is related to the curvature of the Andean margin and the associated changes in the sense of convergence obliquity. The differing directions of movement of these blocks result in extension in the Gulf of Guayaquil, where the Andean margin is seaward convex. In contrast, on the Altiplano, at the concave bend of western South America, the rotational velocities of the Peruvian and Chilean blocks are converging into a similar direction.

The spatial distribution of interplate coupling as estimated by our block modelling shows that the Andean margin is segmented into strongly and weakly coupled zones. Epicenters of major thrust earthquakes correlate fairly well with areas of strong interplate coupling.