Late Pleistocene to Holocene activity of Alchichica maar volcano, eastern Trans-Mexican Volcanic Belt

Abstract

Detailed volcano-stratigraphy and 40Ar/39Ar dating are used for the first time to reconstruct the eruptive evolution of the Alchichica maar and to contribute to the understanding of maar volcanism within the eastern Trans-Mexican Volcanic Belt (TMVB). The maar excavated in an area where previous eruptions included Hawaiian, Strombolian, and phreatomagmatic types. Alchichica is characterized by an irregular shape tephra ring with a bulk volume of ∼0.088 km3 based on high-resolution digital terrain model calculations. The maar deposits comprises two stratigraphic units (lower and upper) with contrasting lithofacies characteristics suggesting different depositional conditions and variations in eruption behavior. These units correspond to: (1) vent-clearing stage with explosions producing a succession of massive tuff-breccia intercalated with massive lapilli-tuffs; (2) pulsating phreatomagmatic eruptions producing a series of accretionary lapilli-rich tuffs that alternate with dominantly well-stratified tuffs and lapilli-tuffs deposited from pyroclastic density currents. The geochronological data provide first constraints about the young age (Late Pleistocene-Holocene) of the Alchichica maar. The presence of the maar and some of the other pre-maar eruptions suggest long-lasting dispersed volcanism in a well-defined, but small area, where rejuvenated monogenetic vents cut into earlier ones, having significant implications for hazard assessments in the region.