EXPLOSIVE ERUPTION HISTORY AND STRUCTURE OF A LARGE SILICIC PIECEMEAL CALDERA: LOS HUMEROS, CENTRAL MEXICO

The evolution of Los Humeros caldera volcano involved four main explosive stages that alternated with minor effusive activity. The first stage was a c. 460 ka, caldera-forming high-silica rhyolite eruption that produced the widely distributed Xáltipan ignimbrite (c.115 km³ DRE) and the 21x15 km Los Humeros caldera. The second explosive stage began at ca. 360 ka with the emplacement of several high-silica rhyolite domes within and along the caldera rim, followed by a series of plinian and subplinian eruptions that produced >10 km³ (DRE) of rhyodacitic-andesitic pumice fallout layers grouped as the Faby Formation. No caldera subsidence associated with this stage has been identified. The third stage, at ca.140 ka was a 15 km³ (DRE) rhyodacitic Zaragoza eruption that emplaced a radial, compositionally zoned intraplinian ignimbrite with subsidence of the 9-km-diameter Los Potreros caldera, which is nested within the older Los Humeros caldera. The fourth stage involved several plinian and strombolian eruptions, including a >0.34 km³ (DRE) dacitic pumice fall deposit (Xoxoctic Member; >65 ka) and ~6 km³ of andesitic and basaltic andesite scoria cones and lavas (40 and 30 ka), followed by the eruption of about ~ 0.5 km³ (DRE) of rhyodacitic and andesitic tephra (Cuicuiltic Member). The latter interfinger with products of the most recent (c. 20 ka) volcanic activity of Los Humeros volcano including a ~0.25 km³ of olivine basalt lava erupted from the southern margin of the caldera. The Cuicuiltic eruption is interesting as it involved several simultaneously active vents of contrasting composition: at least one was a subplinian rhyodacite pumice eruption, and others were strombolian fissure eruptions from several vents, apparently cited on reactivated caldera faults. Aspects of the volcano that are important for geothermal exploration include the evidence for Holocene activity, and the piecemeal nature of the caldera, as revealed by detailed mapping of the complex structural pattern that shows how the geometries and structural configurations of faults, some of which may have acted as eruption conduits, evolved during successive subsidence events. A NW-trending fault system inferred from borehole data affected the Pliocene andesites and influenced the distribution of the present-day, productive geothermal field.