New chronological constraints on intense Holocene eruptions and landslide activity at Tacaná volcanic complex (Mexico)

Usual methods are unfortunately unsuitable to accurately date many Holocene deposits and landforms from active Mexican volcanoes. This is notably the case for the Tacaná Volcanic Complex (TVC), located in the State of Chiapas in southern Mexico, and the San Marcos Department in Guatemala. The complex consists of the Chichuj, Tacaná, and San Antonio volcanoes and the Ardillas dome. Tacaná volcano, the main summit of the TVC, collapsed 15 ± 5 ka ago (⁴⁰Ar/³⁹Ar), producing the Agua Caliente Debris Avalanche. The result was the formation of a horseshoe-shaped crater 600 m wide open to the northwest. Afterward, several undated effusive and explosive eruptions occurred. All these eruptions are likely Late Pleistocene to Holocene in age because the landforms derived from that activity overlap the horseshoe-shaped crater. To corroborate this hypothesis, we used surface exposure dating with in situ-produced cosmogenic ³⁶Cl combined with lichenometry and dendrochronology in the summit domes, the Ardillas dome, the horseshoe-shaped crater's cliff, and an andesitic lava. The ³⁶Cl exposure ages (9.3 ± 1.9 ka; 8.9 ± 0.9 ka; 8.6 ± 1.7 ka) of the summit domes are statistically indistinguishable within the associated uncertainties and suggest that the domes formed during Early Holocene. Instead, the southern part of the Late Pleistocene collapsed crater yielded significantly younger ages ranging from 5.3 ± 0.6 to 7.0 ± 0.8 ka. These ages are assumed to be minimum and might indicate later rockfall activity and instability, affecting the horseshoe-shaped crater. The ³⁶Cl exposure age of an andesitic lava (0.4 ± 0.1 ka), emplaced to the SW of Tacaná, is in good agreement with lichenometry (>347 yr). These ages could represent a gravitational collapse event associated with phreatic explosions vented close to a scar collapse. The ³⁶Cl exposure ages derived from the Ardillas lava dome (0.3 ± 0.1 ka) do not correspond to its emplacement age. The dome must be older than pyroclastic deposits dated at 760 ± 30 yr BP (¹⁴C) that cover its surface. Strikingly, the ³⁶Cl, lichenometrical and dendrochronological data of the “Andesitic” lava and the Ardillas dome are associated with NE-SW landslide scars, explosion craters, phreatic vents, fumarolic activity, and tensional fractures. All these features are perpendicular to the NW-SE minimum stress regime (σ3), affecting the TVC and generating gravitational activity. Nowadays, gravitational collapses represent a real threat to the surrounding populations that can be triggered not only by volcanic activity but also by seismicity and extraordinary rains.