A Volcanic Record of the Laschamp Excursion from Mt Ruapehu, New Zealand

English Abstract:

Geomagnetic excursions are short-lived deviations of the geomagnetic field from the normal range of secular variation. Despite significant advances in geomagnetic excursion research over the past 20 years, fundamental questions remain concerning the global extent, typical duration and morphology of excursional geomagnetic fields. To answer such questions, more high-resolution, chronologically well-constrained excursion records are required, particularly from the Southern Hemisphere, from which there is currently a paucity of records. Here we present a paleomagnetic record of the Laschamp excursion from lavas of Mt Ruapehu, New Zealand. Fourteen lava flows on the western and southern flanks of Mt Ruapehu were sampled, for which ⁴⁰Ar-³⁹Ar dating yields ages between 39 and 46 ka, with corresponding 2 sigma errors mostly approaching 1 kyr. The youngest and older flows carry a normal polarity magnetization; however, six flows, dated between 41.8±1.8 and 45.4±2.0 ka, record excursional field directions. The paleomagnetic directions of three of these flows record a declination swing of close to 180^o, in agreement with a previously published Laschamp record from the Auckland Volcanic Field. A VGP path, determined from the whole sequence, swings from high northern latitudes to mid-southern latitudes before returning to stable normal polarity. ⁴⁰Ar-³⁹Ar ages for the Mt Ruapehu excursion suggests that the Laschamp excursion was manifest in New Zealand from as early as 45 ka, with a probable return to stable normal field by ca 40 ka.