Relative motions between plates in the east Pacific region during the past 150 m.y. have produced a variety of structural and magmatic features along the western North American plate margin. Compilation of age and composition for the magmatic activity along this margin have been used to reconstruct the shifting patterns of magmatic belts and to relate them to changing geometry of subduction and depth of related magma generation. However, due to the distribution and quality of these data more than one interpretation is possible. This research will test between the present hypotheses by using age data acquired along a single corridor across the Cretaceous and Tertiary magmatic belts in western Mexico. Results are expected to determine whether previously proposed regular migrations in western Mexico are adequately supported by chronology, how far inland these patterns can be followed, whether there are discontinuities due to lateral translations and whether resulting patterns can be related to subduction events.