Paleomagnetism of Holocene lava flows from Los Humeros caldera, eastern Mexico: Discrimination of volcanic eruptions and their age dating

Abstract

Los Humeros caldera in eastern Mexico has been formed since about 164 ka, with important effusive activity during the upper Holocene, which covered more than 100 km2 with lava. Two lava flows were 14C dated at 2692-1950 cal BCE and 1117-930 cal BCE, but the detailed succession of eruptions is yet unknown even though the interest for volcanic risk assessment. All Holocene lava flows were studied paleomagnetically in 22 sites to determine which of them have been contemporaneous and to apply the paleomagnetic dating method. For most sites well defined mean directions could be defined, resulting in an overall paleodirection of D = 7.7° E, I = 16.9°, α95 = 6.7°. This direction is discordant compared to the geocentric axial dipole field and indicates that secular variation has not averaged out. Paleointensities were obtained using the Thellier IZZI protocol, with only a small success rate of 27% due to thermally induced alterations, the presence of multidomain magnetic minerals, or unstable paleomagnetic directions as indicated by the checks during the experiments. Accepted paleointensities of 37 samples varied between 21 and 63 μT, with site mean paleointensity of 27–54 μT. The F-test was applied to mean directions of sites potentially belonging to the same lava flow, and if these were indistinguishable they were combined in flow mean paleomagnetic directions. Otherwise, these sites were assumed to represent independent eruptions. This way, thirteen independent flow mean directions were obtained and used for paleomagnetic dating by means of the SHA.DIF.14k reference model and the archaeo_dating software. The dating often provided several possible age ranges, and erosion state, soil cover, lava flow contacts, and the available 14C ages were used all together for selecting the most probable age range. Two well defined eruptive periods bracketed between 2500 and 1900 BCE, and 1300-600 BCE were defined, suggesting that all studied lava flows were emplaced during two main eruptive phases, which may indeed have been much shorter than 600 years. Paleomagnetic data support those eruptive periods and time among the different lava flow units can be inferred from those data. Early settlers occupied this region since about 2000 BCE and thus were certainly affected by these voluminous eruptions. At least another 12 nearby Holocene monogenetic volcanoes are known in the surrounding area, and all together these eruptions indicate that this is a highly active volcanic region, which might be affected by eruptions in the near future.