A comprehensive rock­magnetic, paleomagnetic, paleointensity and geochronologic study along the western Trans­Mexican Volcanic Belt: geodynamic and geomagnetic implications

Abstract

We present results from a comprehensive paleo–magnetic, rock magnetic, geochronologic, and paleointensity study on the Miocene volcanic rocks from the western Trans Mexican Volcanic Belt. A total of 58 sites composed of 7 to 21 consecutive volcanic cooling units from four separated stratigraphic sections have been collected. Our new radiometric dating results suggest that lava flows of the Jesus Maria sequence were erupted within a short time interval from 11.1±0.8 to 10±0.8 Ma during the C5n.2n normal polarity chron. The nearby Atotonilco section displays similar lithologic characteristics to the Jesus Maria sequence, with the top 3 lava flows probably belong to C5n.lr. Our geochronologic results from the Funicular section indicate that these lavas were formed in a longer time interval from 5.2±0.7 to 2.8±0.5 Ma. Based on rock–magnetic and microscopy observations the magnetic mineralogy of all samples is determined to be primary and unaltered Ti–poor titanomagnetites, resulting from oxy–exsolution of original titanomagnetite during the initial flow cooling. Stable components of magnetization are determined after detailed thermal and alternating–field demagnetization. Our new results combined with existing regional paleomagnetic data indicate that the sampled areas have not undergone significant N–S displacement and block rotations since at least last 11 Ma. Accepted paleointensity determinations are of good technical quality with positive standard partial thermoremanent magnetization (pTRM) checks. Obtained paleointensity data range from 17.5 to 34.3 mT, suggesting the existence of both relatively high geomagnetic field strength and low fluctuations in late Miocene. Our results suggest that the studied rocks preserve a magnetic memory of the late Miocene geomagnetic field and provide constraints for paleogeographic reconstructions. The new paleomagnetic data should help refine models for the tectonic evolution of the TMVB.