Paleomagnetismo y magnetoestratigrafía de la Sierra de La Primavera, Jalisco

Abstract

A preliminary 'report is made of a paleomagnetic study of several units belonging to the Sierra La Primavera, Jalisco State, Mexico. Most paleomagnetic directions presented normal polarities, which very likely correspond to the Brunhes normal polarity epoch (last 690 000 - 700 000 years). The only exception corresponds to the Tala Tuff, which presented intermediate polarities and a higher directional dispersion. These intermediate polarities could be related to a transitional event corresponding to the Blake excursion (110 000- 116 000 years). In this case, this relation would imply a slightly older age for this formation than that estirnated by the K-Ar method (i.e. about 96 000 years). This hypothesis can be sustained by the presence of secondary components in the normal remanent magnetism of the Tala Tuff, that could be related to hydrothermal alterations giving rise to a los of radiogenic argon. This also apparently solves the stratigraphic inconsistency posed by the K-Ar date for the Tala Tuff. The paleomagnetic results in general are compatible with the volcanic stratigraphy and K-Ar dating studies reported by Mahood and co-workers, which indicate that the Sierra La Primavera is a young volcanic feature developed mainly during the last 140 000 years.