Paleomagnetic constraints on spatial/temporal volcanic activity in the Santa Catarina-Chalco region, southeastern basin of Mexico

Abstract

Three groups of sites in the Sierra Santa Catarina and Chalco Lake region can be distinguished on the basis of their mean paleomagnetic directions. Group I(12 sites) of reversed polarity, shallow inclination and "normal' paleosecular variation (ie overall VGP angular dispersion in agreement with latitude-dependent global models). This group corresponds to andesitic and basaltic units from cinder cones and medium-size volcanoes and is assigned to the Matuyama Chron (0.78 to 2.64 Ma). Results for Cerro Estrella (normal polarity and intermediate inclination) and Tlapacoya (reverse polarity and intermediate inclination) are also assigned to the Matuyama Chron, as sub-Group I-B. Group II (7 sites) presents normal polarity, mean inclinations close to the dipolar value and low paleosecular variation. This corresponds to mainly cinder cones of basaltic composition and is assigned to the Brunhes Chron (0 to 0.78 Ma). Group III (4 sites) is characterized by intermediate polarity and includes the morphologically young cones of Sierra Santa Catarina. It probably represents a transitional period during the Brunhes Chron (Late Pleistocene?). Phreato-magmatic activity is observed for structures in the Matuyama (eg Xochiaca volcano) and Brunhes (eg Xico and Tetecon volcanoes) groups, indicating the presence of water bodies and a shallow water table. Volcanic structures show apparent preferred alignments which supports previous interpretations of a structural control for magma emplacement within a dominant stress regime. Severa! major fractures can be identified from aerial and satellite images. The paleomagnetic directions do not indicate any significan! rotations or displacements. The results for the Brunhes volcanic units (group II) have an overall mean direction close to the dipolar direction for the area.