SUPERERUPTIONS OF THE SIERRA MADRE OCCIDENTAL LARGE IGNEOUS PROVINCE: PROBABLE FACTOR IN THE EOCENE-OLIGOCENE-MIOCENE GLOBAL CLIMATE CHANGE

At the Eocene-Oligocene transition, global climate changed from a greenhouse to an icehouse condition, which effect continued at least until Miocene. This change may be responsible of large mass extinctions occurring in the Eocene-Oligocene and Oligocene-Miocene transitions, particularly in North America and in the benthic ocean fauna. It coincided with a continued and long period at 38-23 Ma of voluminous ignimbrite forming eruptions that developed the Sierra Madre Occidental large igneous province (SMO) of Mexico, which is reported as the biggest continuous ignimbrite province in the world. The SMO extends continuously from the US-Mexico border (31°N) to the Mexican Volcanic Belt (21°N). A conservative estimate of the physical volume of the SMO rhyolitic ignimbrites is 400,000 km³. This event, named as the “SMO Ignimbrite flare-up”, coincided with the global climate change that initiated in the late Eocene (38 Ma) and continued until Miocene (23 Ma). The SMO ignimbrites were produced by hundreds of caldera supereruptions that devastated northern Mexico and southwestern U.S. and probably affected the world paleoclimate by the corresponding emission of large quantities of fine ash, volcanic gases and aerosols to the atmosphere. A single classic caldera eruption (e.g., Yellowstone) could account for a complete devastation many kilometers around the caldera and a major climate effect around the world. SMO’s ignimbrites volume represents 250 to 500 calderas with sizes similar to that of Yellowstone, or even bigger considering that many SMO calderas are graben type calderas, some with sizes of ca. 100 x 30 km, such as Bolaños caldera. Mid-Tertiary calderas of the San Juan Mountains, Colorado, including La Garita, should have contributed to this worldwide climate change, too. At the other side of the world, the Ethiopian-Yemeni (Afro-Arabian) Traps produced extensive basaltic volcanism followed by massive rhyolitic ignimbrites at 31-27 Ma. The combination of these prolific volcanic episodes likely was an important factor in causing the multiple global cooling events and associated faunal transitions that spanned the Eocene-Miocene, and may explain why the extinctions and paleoclimate perturbations were apparently distributed over several million years.