Evolution of oceanic lithosphere: A driving force study of the Nazca Plate

The role of the evolution of oceanic lithosphere in driving oceanic plates is tested with elastic and viscous finite element modeling of the Nazca plate. Results indicate that ridge-push forces, when distributed over that portion of the oceanic lithosphere giving rise to the force, predict a stress state that varies more spatially than does previous modeling with ridge forces concentrated at the ridge crest. In particular, the component of stress normal to the ridge vanishes near the ridge for the new modeling. Predicted stresses for distributed ridge force models are still in qualitative agreement with the limited data on Nazca intraplate stress. Stress states have also been calculated for a variety of slab-pull models, which include possible dependence on age, rate, and angle of subduction. The predicted stresses vary substantially near the trench among these models, with less variation farther from the trench. Combinations of ridge-push and slab-pull models are consistent with the earlier result of ah upper limit of about a factor of 2 for net slab-pull over ridge-push. Simple viscous modeling with only distributed ridge forces predicts relative motion between the Nazca and South American plates that is in fairly poor agreement with the observed direction. The fit to the observed direction could probably be improved by either including slab forces or the effect of the largely transform boundary with the Antarctic plate.