Paleomagnetic Study of the Sierra La Primavera: Preliminary Results

Abstract

A preliminary report is made of a paleomagnetic study of some units belonging to the Sierra La Primavera, a late Pleistocene rhyolitic complex just west of the city of Guadalajara, Mexico, and representing the volcanic activity that occurred in that place during the last 140,000 years. All the magnetic directions presented normal polarities, with exception of the direction corresponding to one unit, the Tala ruff, that presented intermediate polarities and a strong dispertion in the directions. These intermediate polarities could be related to the presence of a transitional even t corresponding to the Blake excursion (110,000 years). In this case this possible relation would imply an older age for this formation than that estimated by the K-Ar method. This hypothesis can be substantiated by the presence of secondary components in the normal remnant magnetism of the Tala tuff, that could be related to hydrothermal phenomena giving rose to a loss of radiogenic argon.