Over 30 million years ago, normal subduction occurred along the Pacific coast of Mexico, producing active volcanoes along the coast.
About 29 million years ago, the subducted oceanic plate started to resist the downward pull, remaining up against the overlying continental crust. This extinguished the coastal volcanic arc. The cause of this increased buoyancy is under study.
About 19 million years ago, the far reaches of the subducted slab started to lose its buoyancy and began to sink. Volcanic activity then resumed, but much further inland, rolling back toward the Pacific coast as the extent of the flat slab decreased.
Seven million years ago, the subducted oceanic plate broke.
At present, flat subduction continues, though only reaching inland by about 250 km. Normal earthquakes occur in this region, as well as Non Volcanic Tremor (NVT).