The Zacapu lacustrine basin is located in the north-central part of the Michoacán-Guanajuato volcanic field (MGVF), which constitutes the west-central segment of the Trans-Mexican Volcanic Belt. Geological mapping of a 395 km 2 quadrangle encompassing the western margin of the basin, 40 Ar/ 39 Ar and 14 C 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 km 3 (~86 vol.%) followed by 4 km 3 of dacite (~8 vol.%), 1.4 km 3 of basaltic trachyandesite (~3 vol.%), 1 km 3 of basaltic andesite (~2 vol.%), and 0.14 km 3 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.Keywords Zacapu basin . Michoacán-Guanajuto volcanic field . Quaternary . Monogenetic cluster . Radiocarbon . 40 Ar/ 39 Ar dating . Malpaís * Nanci Reyes-Guzmán
Four monogenetic Holocene lava flows located within the Michoacán-Guanajuato Volcanic Field, Mexico, were sampled for paleomagnetic dating. These flows (namely, El Infiernillo, Malpaís Las Víboras, El Capaxtiro, and Malpaís Prieto) are within the heartland of the pre-Hispanic Tarascan civilization and were inhabited repeatedly since at least 100 BC, but no relation with the volcanic evolution has been proposed so far. The stratigraphically oldest lava flow, El Infiernillo, has a radiocarbon age of 3200 ± 30 yr BP (cal. 1525-1420 BC), and it was used to validate the method. Using full-vector paleomagnetic data from three sites as input for paleomagnetic dating applying the global paleosecular variation model SHA.DIF.14k, an age range of 1500-1370 BC was obtained. Two age ranges of 1340-1230 and 1030-940 BC were obtained for Malpaís Las Víboras. A younger age range of 200-80 BC was obtained for the Capaxtiro lava flow and, finally, the Malpaís Prieto lava flow erupted within the range of AD 830-960. The human occupation history of these flows started around 100 BC during the late Pre-Classic, probably shortly after the Capaxtiro eruption. Archeological records indicate an abandonment of the entire area around AD 900 (late to terminal Classic), which coincides with the paleomagnetic age of the Malpaís Prieto eruption. Interestingly, this area was heavily repopulated again only few hundred years later around AD 1250 and belongs to the core region in which the Tarascan civilization has its roots. The eruption recurrence interval of roughly 1000 years indicates that a new monogenetic eruption should be expected to occur again in the future and that this area deserves to be studied in greater detail with particular emphasis on the impact of past eruptions. This could help to better evaluate volcanic hazards and design preparedness strategies to minimize the impact of a future eruption.
Few monogenetic eruptions that produced lava flows have occurred in historical times, limiting the observations of their impact on human settlements. However, rheological models based on morphological and petrological datasets can contribute to decipher the eruptive dynamics and durations of ancient eruptions. The Malpaís de Zacapu, a temporal-spatial monogenetic volcano cluster at the western margin of the Zacapu lacustrine basin (Michoacán, Mexico), offers a good opportunity to apply such models because of the availability of a high-resolution LiDAR topography from which detailed morphological data was extracted. The Malpaís de Zacapu comprises late Holocene lava flow fields emplaced in the last 3200 years by four different low magnitude volcanic eruptions: Infiernillo, Malpaís Las Víboras, Capaxtiro, and Malpaís Prieto. Jointly these eruptions produced thick andesitic block lava flows covering an area of 38.3 km 2 with a volume of ~ 4.4 km 3 . The lava viscosities at eruption vents were estimated from petro-textural analyses and range between 10 3 and 10 6 Pa s, while the final flow apparent viscosities, obtained from dimensional analyses, vary from 10 8 to 10 10 Pa s. We estimated the mean effusion rate and lava flow emplacement duration for each flow field. Results revealed that the more viscous flows, Malpaís Las Víboras and Malpaís Prieto, could have been emplaced in less than 3 years, while the more fluid Infiernillo probably took less than 1 year. In stark contrast, the morphologically different and more voluminous Capaxtiro flow field could have been emplaced in ~ 27 years. These findings can help to evaluate the impact that these eruptions had on adjacent pre-Hispanic populations, known to have inhabited the region since at least 3000 years ago.
The Michoacán-Guanajuato volcanic field (MGVF) hosts >1,400 monogenetic structures younger than 5 Ma. Here we focus on the Malpaís de Zacapu Late Holocene cluster located in the western part of the Zacapu lacustrine basin, situated in the heart of native Purepecha province. The Malpaís de Zacapu comprises four distinct eruptions: the Infiernillo lava flow emitted at ~1450 b.c.; Malpaís Las Víboras, a purely effusive eruption at ~1000 b.c.; the Capaxtiro compound lava flow at ~150 b.c.; and the most recent eruption, the Malpaís Prieto lava flow at ~a.d. 900. Although these lava flows are not inhabited today, they were densely populated in pre-Hispanic times (before a.d. 1521), especially during the Milpillas phase (a.d. 1200–1450). Volcanological studies (geochemical studies and detailed mapping using high-resolution DEM from LiDAR) allowed us to characterize these eruptions in terms of their magma source (rock chemical composition, mineral assemblage), age (radiocarbon and paleomagnetic dating), magnitude and dynamics (volume, morphology of the deposits), as well as lava flow emplacement duration. The findings allow us to infer the potential impact that these eruptions had on the pre-Hispanic settlement history of the area.
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