Geology and radiometric dating of Quaternary monogenetic volcanism in the western Zacapu lacustrine basin (Michoacán, México): implications for archeology and future hazard evaluations

Abstract

The Zacapu lacustrine basin is located in the north-central part of the Michoacan-Guanajuato volcanic field (MGVF), which constitutes the west-central segment of the Trans-Mexican Volcanic Belt. Geological mapping of a 395 km2 quadrangle encompassing the western margin of the basin, 40Ar/39Ar and 14C radiometric dating, whole-rock chemical and petrographic analyses of volcanic products provide information on the stratigraphy, erupted volumes, age, and composition of the volcanoes. Although volcanism in the MGVF initiated since at least 5 Ma ago, rocks in the western Zacapu lacustrine basin are all younger than ~ 2.1 Ma. A total of 47 volcanoes were identified and include 19 viscous lava flows (~ 40 vol.%), 17 scoria cones with associated lava flows (~ 36 vol.%), seven lava shields (~ 15 vol.%), three domes (~ 6 vol.%), and one maar (~ 2 vol.%). Erupted products are dominantly andesites with 42 km3 (~ 86 vol.%) followed by 4 km3 of dacite (~ 8 vol.%), 1.4 km3 of basaltic trachy-andesite (~ 3 vol.%), 1 km3 of basaltic andesite (~ 2 vol.%), and 0.14 km3 of rhyolite (~ 0.3 vol.%). Eruptive centers are commonly aligned ENE-WSW following the direction of the regional Cuitzeo Fault System. Over time, the high frequency of eruptions and consequent accumulation of lavas and pyroclastic materials pushed the lake’s shore stepwise toward the southeast. Eruptions appear to have clustered through time. One cluster occurred during the Late Pleistocene between ~ 27,000 and ~ 21,300 BC when four volcanoes erupted. A second cluster formed during the Late Holocene, between ~ 1500 BC and ~ AD 900, when four closely spaced monogenetic vents erupted forming thick viscous ‘a’a to blocky flows on the margin of the lacustrine flats. For still poorly understood reasons, these apparently inhospitable lava flows were attractive to human settlement and eventually became one of the most densely populated heartlands of the pre-Hispanic Tarascan civilization. With an average eruption recurrence interval of ~ 900 years during the Late Holocene the western Zacapu lacustrine basin is one of the most active areas in the MGVF and should hence be of focal interest for regional volcanic risk evaluations.