Volcanic stratigraphy and geological evolution of the Apan region, east-central sector of the Trans-Mexican Volcanic Belt

Abstract

The Apan region, which straddles the central and eastern sectors of the Trans-Mexican Volcanic Belt (TMVB), was geologically shaped by interrelated, intermittent tectonism and volcanism. The oldest volcanic rocks in the area were erupted about 13.5 Ma ago, followed by a nearly 10-million year volcanic hiatus. Volcanism in the area then resumed ~3.0-1.5 Ma; the most recent volcanic activity occurred in the late Pleistocene (~42-31 ka). The volcanic rocks in the Apan region range in composition from basalt to rhyolite (50-75 wt.% SiO2) and exhibit calc-alkaline affinity; their geochemical characteristics suggest that they are subduction-zone eruptive products. Detailed geologic and stratigraphic studies using a new digital 1:100 000 scale base map for compilation of new and previous data, coupled with new K-Ar age determinations, demonstrate that the loci of volcanic centers in the region were controlled largely by a system of NE-SW-trending normal faults and associated horst-and- graben structures, resulting from a stress field with a least principal stress (ó3) oriented to the NW. Moreover, the results of this study, combined with a reexamination of geologic and age data from other investigations of the TMVB, make possible a refined correlation of the volcanic stratigraphy of the Apan region with the stratigraphic sequences of volcanic rocks in other areas of the TMVB. An important conclusion of our study is that volcanic activity in the TMVB was initiated during the Middle Miocene.