Stratigraphy, K-Ar ages, and magnetostratigraphy of the Acambay graben, central Mexican Volcanic Belt

Abstract

The Acambay graben is an east-west intraarc tectonic depression in the central sector of the Mexican Volcanic Belt. The graben is 80 km long and 15‐38 km wide, bounded at the south by the Pastores and Venta de Bravo faults, and at the north by the Acambay-Tixmadeje and Epitacio Huerta faults. Two representative sections were measured, one in the northern wall of the graben on the Epitacio Huerta fault (Amealco caldera section), and the other in the southern wall on the Venta de Bravo fault (Tlalpujahua section). These two sections were documented with K-Ar ages and paleomagnetic measurements. The units exposed in the northern section are all volcanic and within a range of 4.7 to 2.2 Ma. The southern section includes metasedimentary rocks that could be Cretaceous (no fossils have been found) and the Miocene-Quaternary volcanic succession. The southern section has a K-Ar age of 4.7 ± 0.2 Ma and reverse polarity for a major ignimbrite of Las Americas Formation. This formation is correlated with the Amealco tuff of the northern section, which is the only map unit widespread enough to be exposed on either side of the Acambay graben. Paleomagnetic studies concentrated on the widespread ignimbrites, such as those within the Amealco and Huichapan tuffs, because of their potential for magnetostratigraphy. The K-Ar and paleomagnetic data are consistent within analytical uncertainties. The Amealco tuff sequence includes three major ignimbrites. The oldest ignimbrite, Amealco I, has a reverse polarity and a K-Ar age of 4.7 Ma; Amealco II has a normal polarity and a K-Ar age of about 4.7 Ma; and Amealco III has a normal polarity and a K-Ar age of about 4.6 Ma. The ignimbrite of the Huichapan tuff has a reverse polarity and a K-Ar age of 3.5 Ma. There is a predominance of reverse polarity for units within the Acambay graben. Magnetostratigraphy is used as another line of evidence for the correlation of Amealco tuff and Las Americas Formation, and proved useful to demonstrate that the Amealco caldera was the source for the Amealco tuff and Las Americas ignimbrites.