Graben type calderas: The Bolanos case, Sierra Madre Occidental, Mexico

摘要

Graben calderas are volcano-tectonic structures that use the faults of a graben as main vents from which magma is massively erupted from a shallow magma chamber during collapse of intra-graben blocks. These eruptions are generally silicic and explosive and form large volume ignimbrite sheets without a previous Plinian major depressurization event. Graben calderas can be associated to tectonic settings ranging from pure extension to strike-slip transtension, including complete grabens, half-grabens or pull-apart grabens. The Sierra Madre Occidental (SMO) in western Mexico includes several of these graben type calderas with the corresponding large volume ignimbrite sheets that result from fissure type eruptions along the graben's border and interior faults. The Bolanos graben constitutes one of the best examples of these tectonically controlled collapse calderas and is the second largest caldera of the world after Toba caldera. It is a 90 x 25 km rectangular caldera and the vent of the 25.37 +/- 0.36 Ma silicic Alacran ignimbrite, with a minimum Dense Rock Equivalent volume of 2650 km(3) (3800 km(3) rock volume). Post-collapse silicic domes were emplaced just after the Alacran ignimbrite from 25.02 +/- 0.33 Ma to 23.94 +/- 0.33 Ma, with a total minimum lava volume of 171 km(3). Both ignimbrite and domes account for at least 2800 km(3) rhyolitic magma output from Bolanos graben caldera, without considering the distal deposits, and in particular the co-ignimbrite ash-cloud deposit. We describe the characteristics of the Bolanos graben caldera focusing on its major products, the Alacran ignimbrite and the post-collapse dome volcanism, providing the geological frame of the Bolanos graben area documented with Ar-40-Ar-39 ages. We finally propose a conceptual model to explain the dynamics of the Bolanos graben that can be applied to other similar volcanotectonic depressions of the Sierra Madre Occidental and elsewhere in the world with similar geological settings. (C) 2021 Elsevier B.V. All rights reserved.