Eruptive, Magmatic and Structural Evolution of a Large Explosive Caldera Volcano: Los Humeros, Central Mexico

Little is known about how the internal structures of explosive calderas vary with time, and whether this can affect magma differentiation, eruption behaviour and periodicity. Some exhumed calderas underwent a single piecemeal collapse – e.g. Scafell caldera,UK (Branney and Kokelaar, 1994), however this study tests whether some calderas become more intensely fractured with time due to successive distinct caldera-collapse eruptions. This scenario might lead to more frequent, smaller eruptions over time (noncyclic); magma leakage through an increasingly fractured volcano might also lead to less evolved compositions with time due to shorter residence times (Ferriz and Mahood, 1984). This could have important implications for predicting future catastrophic eruptions. This study revisits the modern ~20 km diameter hydrothermally active Los Humeros caldera (México) where this hypothesis was first formulated, to see how well the structural evolution can be reconstructed, and whether changes in structure affected the styles and periodicity of eruptions. Detailed structural mapping, documentation of draping and cross-cutting field relations, together with logging, petrography, XRF major and trace element geochemistry, 40Ar-39Ar and radiocarbon dating of the pyroclastic stratigraphy has revealed that: (1) eruptions were less frequent after the second major caldera collapse (the ~10 km diameter nested Los Potreros caldera); (2) both calderaforming and smaller explosive eruption volumes decreased by an order of magnitude over ~460 ka; (3) changes in eruption periodicity and magnitude were driven by the rate of magma replenishment, possibly superimposed over caldera cyclicity; (4) the last ~230 ka records the repeated tapping, fractionation and replenishment/re-heating of an evolved magma body; (5) the arrival of exotic, primitive magma triggered the second caldera-forming eruption and caused extensive post-caldera collapse magma mixing; (6) mapping has revealed many structural features that conform to observations at ancient calderas and analogue models; (7) Los Potreros caldera subsided in two discrete stages.