Geomagnetic field strength during late Miocene: First paleointensity results from Baja California

Abstract

Thellier paleointensity experiments were carried out on 73 selected samples from the late Miocene Baja California volcanic flows for the first time. The selected samples showed a stable remanent magnetization observed upon thermal treatment and reasonably reversible continuous susceptibility curves, compatible with Ti‐poor titanomagnetites. Judging from the ratios of hysteresis parameters, it seems that the samples fall in the small pseudosingle‐domain grain size region. Thirty‐three samples from 10 individual flows yielded acceptable paleointensity estimates. The natural remanent magnetization (NRM) fractions used for paleointensity determination range from 37 to 81% and the quality factor varies between 4.5 and 24.5, being normally >5. The mean paleointensity values per flow obtained in this study range from 18.1 ± 3.1 to 45.6 ± 4.3 μT and the corresponding virtual dipole moments (VDMs) range from 3.8 ± 0.6 to 8.8 ± 0.8 (10 22 A m 2 ). These values correspond to mean value of 6.6 ± 1.7 × 10 22 A m 2 , which is ∼85% of the present geomagnetic axial dipole. The average VDM value for late Miocene, according to all available, selected data, is 6.1 ± 1.5 (10 22 A m 2 ), which is higher than average paleofield for the period 5–160 Ma (4.2 ± 2.3 × 10 22 A m 2 after Juarez and Tauxe [2000] ). Given the large dispersion and the very poor distribution of reliable absolute intensity data, it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field. More high‐quality determinations are strongly needed.