Detailed full-vector record of a mid-Miocene geomagnetic reversal from lava flows of Gran Canaria

Abstract

Our Earth’s magnetic field reverses its polarity non-periodically. Despite the large number of reversals in the magnetographic record, the geometry of the Earth’s magnetic field and the mechanisms driving reversals still remain obscure. One reason for this is that the majority of reversal datasets are sedimentary, which lack temporal resolution for recording the rapid field changes during reversals. Another reason is that most of the available volcanic reversal datasets -that may have the necessary temporal resolution- describe a reversal in discrete steps and therefore often have insufficient transitional directions to decipher the geometry of the changing field. Leonhardt et al. (2002) and Leonhardt and Soffel (2002) discovered and described a detailed volcanic record of a ~14Ma reversal on Gran Canaria that formed during a highly eruptive shield-building phase of the island. This paleomagnetic record has sufficient temporal resolution to show the dynamic behavior of the field before and after the reversal. It could therefore illustrate the non-dipolar behavior of the Earth’s magnetic field during a reversal. We resampled the section and extended it by 600 m below Leonhardt et al.’s (2002) section to a total of 110 flows. These flows were measured extensively for both paleodirections and paleointensities. A total of 922 paleodirection measurements were performed, 239 thermal and 683 alternating field demagnetization measurements, which resulted in a paleodirection for 109 flows. A total of 994 paleointensity measurements were performed, 307 IZZI-Thellier and 687 pseudo-Thellier measurements. The IZZI-Thellier measurements produced an absolute paleointensity for 29 flows, based on five sets of selection criteria. The Pseudo-Thellier measurements produced a relative paleointensity for 108 flows, based on one set of selection criteria. Absolute paleointensities from pseudo-Thellier measurements were obtained by scaling their results to IZZI-Thellier results. The resulting paleomagnetic record shows new and interesting features. First of all, during this single reversal the declination reverses only once whilst the inclination reverses its polarity at least five times. In addition, the magnetic field intensity appears to pulsate coinciding with the pattern of inclination reversals, which is observed in the IZZI-Thellier results as well as the pseudo-Thellier results. To analyze this behavior we used a simplified spherical harmonic model with stringent boundary conditions. We show that both these observations can be explained by interaction of the higher order poles during this reversal.