The stratigraphic record at the Tomochic volcanic center, Chihuahua, documents a change from andesitic to rhyolite-dominated volcanism, with late extrusion of mafic lavas, during the final ∼10 m.y. of subduction-related activity in the northern Sierra Madre Occidental. Andesitic volcanism, which started before 38 Ma and continued until ∼35 Ma, was followed by eruption of large-volume rhyolite ash flows from the Las Varas (34.1 Ma) and Tomochic (31.8-31.4 Ma) calderas. Basaltic andesite, the most mafic rock type in the area, was extruded near the end of calcalkalic volcanism at ∼30 Ma.
The change in style of volcanism at the Tomochic volcanic center also occurred in other parts of the northern Sierra Madre Occidental and is interpreted to reflect the evolution of waning, subduction-related magmatism, as convergence between the North American and Farallon plates slowed before it eventually ceased. Slowed plate convergence resulted in a decreased flux of mafic melts from the mantle to the crust, leading, in turn, to the stagnation and coalescence of intermediate-composition magmas similar to those that fed early andesitic eruptions. Subsequent differentiation led to growth of zoned magma reservoirs from which the rhyolite ash flows were extruded. Slowed convergence was accompanied by the onset of crustal extension, which possibly aided the coalescence of andesitic magmas and which allowed the ascent and eruption of mafic melts near the end of silicic volcanic activity. Although minor differences exist, the history of mid-Tertiary magmatism and tectonism in the northern Sierra Madre Occidental was broadly similar to that in parts of the south-western United States.