Chapter 4 The Ignimbrite Flare-Up and Graben Calderas of the Sierra Madre Occidental, Mexico

Abstract

Abstract The Sierra Madre Occidental (SMO) is the largest continuous ignimbrite province in the world. It covers the NW portion of Mexico and has a minimum estimated volcanic rock volume of about 400,000 km 3 . The southern part of the North American Basin and Range extensional province is in Mexico and was formed by NW- to NE-trending normal faults that bound many large grabens, which are particularly long and deep in the southern SMO. Graben formation and ignimbrite-forming pyroclastic flow eruptions coincided in space and time, particularly for the 38–23 Ma period, which has been referred to as the Ignimbrite Flare-up event. Geologic observations in the southern SMO indicate that the vents of several large-volume silicic ignimbrites were related to the graben master faults. We propose the new name “graben caldera” for this type of explosive-volcano-tectonic-collapse structure. The evidences for vent location of graben calderas include large pyroclastic dikes, co-ignimbrite lithic-lag breccias, and post-ignimbrite aligned rhyolitic domes and rhyolitic lava dikes. All these features are found along the graben caldera walls or on the graben's margins and thus were controlled by the main graben faults. Large-volume ignimbrite-forming eruptions occurred during graben collapse, usually through several vents along the graben caldera walls. Often, only part of the graben acted as a collapse caldera. In most cases, the downdropped blocks inside the collapsed segment of the graben have several distinct tilting directions, opposite to those caused by regular domino faulting (with tilting outwards from the graben's axis), indicating a chaotic collapse of blocks. This is interpreted as collapse of blocks during magma evacuation from an elongated and possibly batholith-sized magma chamber controlled by the regional extensional tectonics that existed under the collapsed segment of the graben. This collapse produced a piece-meal-like caldera confined within a graben. Generally, the pyroclastic dikes occur as discontinuous elongate lenses rather than regular tabular bodies. Usually, an elongated lava dome filled the vent and this totally covers the pre-existent pyroclastic dike, but in some cases the pyroclastic dike is exposed. Faulting and subsidence continued for several millions of years after the collapse and ignimbrite emplacement, displacing the intra-graben caldera products downward into the tectonic depression, but preserving or even intensifying the chaotic arrangement of the collapsed blocks. Fluvio-lacustrine continental deposits are generally found beneath and over the main and most voluminous ignimbrite, indicating that subsidence started before ignimbrite emplacement and graben collapse, and that continued after these events. In many cases the graben caldera vents are related to gold and silver hydrothermal mineralisation; thus understanding the relationship between ignimbrites, graben caldera vents, and fluvio-lacustrine deposits will be important for economic purposes.