Paleomagnetism of the Acambay graben, central Trans-Mexican Volcanic Belt

Abstract

Paleomagnetic results for Miocene to Quaternary volcanic units of the Acambay graben are used to investigate the Neogene tectonic activity within the central sector of the Trans-Mexican Volcanic Belt (TMVB). Characteristic magnetization directions were obtained for 22 sites, with an overall mean direction (Dec=176.7°, Inc=−34.3°, k=28, α 95=6°) that is concordant with the expected direction calculated from North American reference data. Examination of site-mean directions shows an apparent geographic pattern related to the intersection of the Queretaro-Taxco fault zone with the E–W border fault system of the graben. This pattern is characterized by easterly and westerly declinations arranged into two broad groups. The mean direction for group A (easterly) is ( n=9): Dec=189.9°, Inc=−28.5°, k=53 and α 95=4.8 . The mean direction for group B (westerly) is ( n=13): Dec=167.6°, Inc=−36.2°, k=38 and α 95=4.3°. Group A sites cover two areas, one inside the graben and the other immediately to the south including the Pastores fault. Group B sites are distributed over broad NW and SE areas across the graben, which include the Amealco caldera and Epitacio Huerta and Acambay–Tixmadeje faults and also the Venta de Bravo fault. Eighteen units show reverse polarities, and four units show normal polarity. Absence of normal polarity Quaternary units suggests a pre-Brunhes age for units sampled. K–Ar dating in the Amealco caldera gives a range between 5.7 and 2.2 Ma. A comparison of group A and B mean directions with reference directions calculated from the North American 10 Ma pole and the geographic pole gives rotation parameters between 9.1±5.3° and 9.9±4.6° and between −12.4±5.4° and −12.4±4.3°, respectively. These may be interpreted in terms of vertical-axis rotations associated with regional left-lateral shear in a normal/strike-slip fault environment, which has characterized the Acambay graben during the Neogene.