ABSOLUTE GEOMAGNETIC PALEOINTENSITY AFTER THE CRETACEOUS NORMAL SUPERCHRON AND JUST PRIOR TO THE CRETACEOUS-TERTIARY TRANSITION

Abstract

We report a detailed rock magnetic and Thellier paleointensity study from Autlan volcanic succession in western Mexico. The whole rock K-Ar age obtained for these lavas, 67.4 ± 1.2 Ma, together with the magnetostratigraphy results indicates that the lava flows were erupted during chron 30n just prior to Cretaceous-Tertiary transition. Thirty-nine samples from eight cooling units were preselected for paleointensity experiments on the basis of their low-viscosity index, stable remanent magnetization, and close to reversible continuous thermomagnetic curves. High-quality determinations were obtained for 18 individual samples from four independent lava flows. The mean virtual dipole moment (VDM) obtained in this study is 4.9 ± 0.6 x 10 22 A m 2 , which is significantly lower than the present geomagnetic field strength but in reasonably good accord with other comparable quality determinations between 35 and 80 Ma. Our data are not significantly different from almost all published paleointensities between 5 and 90 Ma. Moreover, rigorously selected worldwide paleointensity data in the same interval better fit trend 2 of Heller et al. [2002]. A careful examination of high-quality paleointensity data from Cretaceous indicates that there is no simple relation between magnitude of Earth's magnetic field and reversal rate.