Origin and age of the directions recorded during the Laschamp event in the Chaîne des Puys (France)

Excursions of the geomagnetic field are likely even more frequent than reversals and thus must be seen as a major characteristic of the geodynamo. The Laschamp event discovered in lava flows of Massif central (France) is the youngest and the most studied field excursion. Its geomagnetic origin has been controversial due to the existence of self-reversal processes. Taking advantage of new dated sites, the initial aim of this work was to find additional flows with intermediate or reverse directions but also to investigate the magnetic properties of normal polarity flows surrounding the event. This study combines thermal and alternating field demagnetization of 272 samples from 21 units including 12 new localities and 12 K–Ar datings of main units. No new site with intermediate or reverse polarity was found. Ten sites have a normal polarity and all sites studied at Olby, Louchadière and Royat display intermediate but scattered directions. Magnetic mineralogy is characterized by primary titanomagnetite, variable amounts of titanomaghemite and almost pure magnetite. We confirmed that reverse polarity flows are affected by self-reversals but we found that this is also the case for normal flows. A direct consequence is that self-reversals cannot be taken as responsible for the reverse directions but they likely contribute to generate dispersion. However the overall coherency of the directions indicates that this process was limited and despite complex magnetization, the geomagnetic origin of the Laschamp in the Chaîne des Puys is not questioned. All volcanic pole positions (VGPs) published so far show significant scatter but they remain consistent with each other. Interestingly, they do not coincide with the longitudinal loops seen in the sedimentary records and are compatible with a dominant non-dipolar field geometry. New K–Ar datings provide a coherent chronology of the successive polarities. The 37 ka old reverse directions of the Olby flow are chronologically consistent with the 41.9 kyr old normal polarity flows preceding the event. The Royat flow is only marginally intermediate and indicates that the end of the event was not younger than 33.3 ka in the Chaîne des Puys.