EarthRef.org Reference Database (ERR) -- Quidelleur & Valet 1996

Two adjacent sequences of 41 and 21 individual lava flows have been sampled in the island of La Palma. Although no detailed correlation can be established between the two sections, they both encompassed the Matuyama-Brumhes (M-B) reversal. Stepwise alternating field (AF) and thermal demagnetization in air and in vacuum accompanied by investigations of the rock magnetic properties allowed us to identify several lava flows with the same signature and thus most likely associated with very short episodes of intense volcanic activity. The intermediate virtual geomagnetic pole (VGP) positions do not cluster within any preferred geographic areas. The polarity change is followed by a rebound (or an excursion) with a cluster of VGPs over northeastern India. Whether or not this feature is truly associated with the transition or correlated with the Delta event (0.69 Ma) will hopefully be resolved by further dating. Paleointensity experiments were performed on 60 samples using the modified Thellier method [Coe, 1967]. Corrections for magnetomineralogical changes induced by heating were applied using the technique of partial thermoremanent magnetization (pTRM) checks [Valet et al., 1994b] with a significant increase in the success rate (from 38% to 58%). Twenty-four independent estimates of the paleofield range between 3.9 and 46.0 mu T. The field intensity was reduced to at least one tenth of the present-day value during the transition. Low (transitional) field intensities are also observed during the rebound (or excursion). The lava flows preceding the polarity change show evidence for a significant drop in the dipole field intensity, while the posttransitional flows exhibit high field intensities. These features are compatible with the asymmetrical saw-tooth pattern recently observed in sedimentary records [Meynadier et al., 1994]. The characteristics of the M-B transition emerging from a compilation of the four available volcanic records are not inconsistent with the hypothesis of rapidly time-varying nondipole transitional components favored by the decrease of the dipole and its subsequent recovery in the opposite polarity state.