The Gilsá excursion and the Matuyama/Brunhes transition recorded in40Ar/39Ar dated lavas from Lanai and Maui, Hawaiian Islands

Abstract

SUMMARY Directional results, Thellier-type palaeointensity determinations and Ar/Ar ages are presented from volcanic units of Pleistocene age from the Hawaiian islands of Maui and Lanai. Twenty-nine non-contiguous lava flows were sampled on Lanai. The lava flows formed during the late Matuyama polarity chron about 1.6 Ma and recorded the Gilsa geomagnetic polarity excursion. The obtained ages of 1.6 Ma for the Lanai lavas are significantly older than ages previously obtained for this island. These new ages are, however, in agreement with the age progression of the Hawaiian hotspot. The palaeomagnetic record over the sampled succession is characterized by reversed and intermediate directions. palaeointensities are generally very low, about half the present-day field intensity, dropping to values of ∼5 μT during transitional field states. Transitional virtual geomagnetic poles (VGPs) from Lanai are situated first near the west coast of South America and then switch to offshore western Australia. Normal polarity is never reached in the sampled succession. Transitional VGPs located close to Australia accompanied by relatively low palaeointensities are found in most records of short geomagnetic excursions during the Matuyama polarity chron (e.g. the Cobb Mountain subchron, the Punaru event and the Matuyama/Brunhes precursor) suggesting very similar physical processes likely related to dipolar dominance during the onset of these excursions and influences of lower mantle heterogeneities. Approximately 800 kyr after the Gilsa excursion, lava flows on Maui recorded the last reversal, the Matuyama/Brunhes transition. Fourteen lava flows were sampled which recorded the pre- and post-transitional behaviour of the Earth's magnetic field. The geomagnetic field intensity is low prior to the reversal, approximately 8 μT, and increases strongly afterwards up to ∼63 μT. These values of about twice the present field intensity of Hawaii after the transition emphasize a strong asymmetry between pre- and post-transitional fields, which is particularly strong in the region of Hawaii.