Reconciling Geodesy and Geology at Subduction Plate Boundaries

It has long been recognized that elastic processes associated with the seismic cycle dominate the surface velocity or strain field at subduction zones. However, it may also be true that some fraction of geodetically measured displacement involves permanent deformation of the upper plate, for example, generation of Andean-style mountain belts via crustal shortening in the direction parallel to plate convergence or translation of forearc blocks at high angles to plate convergence. We explore the potential for reconciling geodetic versus geologic estimates of deformation at subduction zones, utilizing new results from the Central America convergent margin.

The interseismic, GPS derived velocity field in Central America has been interpreted in terms of two major processes: elastic strain accumulation due to locking on the shallow (less than 50 km down-dip depth) part of the dipping plate boundary and trench-parallel translation of the forearc due to oblique subduction. Our new velocity field also indicates significant strain partitioning in southern Costa Rica where tectonic shortening and uplift of a young mountain belt takes place. To a first approximation, the former may be regarded as a purely elastic process, generating no net deformation of overriding plate lithosphere, while the latter two processes are capable of generating considerable long-term lateral displacement of forearc terrains and upper plate shortening. We present a model of these processes assuming elastic half space rheology, and draw inferences from the model results regarding the driving forces for both trench-parallel forearc motion and shortening. We then compare our results with estimates of upper plate deformation based on geologic mapping and structural analysis.