Abstract
We present paleomagnetic and geochronological data of twenty-five stacked lava flows from São Miguel Island (Azores) that were emplaced during the last geomagnetic reversal (Matuyama-Brunhes, M-B). The characteristic direction of natural remanent magnetization was isolated through alternating field and/or thermal demagnetization. From bottom to top, the directional changes display a first phase with reverse, transitional and normal directions which is followed by a second phase with transitional and normal directions. Dating by Potassium-Argon (K-Ar) of fresh separated groundmass provide ages of 768 ± 5 ka and 765 ± 5 ka (1σ) for the first and second transitional episodes, respectively. We infer that this sequence recorded the last reversal and reveal the presence of a rebound that occurred soon after the transition. The age of the transition is 5-15 kyr younger than at other volcanic localities, but in agreement with astronomical ages that were recently proposed for the transitional interval. The reversal is characterized by weak field intensity during the transition, a recovery phase and a subsequent decrease during the rebound. The two transitional virtual geomagnetic poles (VGPs) that characterize the transition are found above the South Atlantic and the North Pacific. When compared to other M-B records, no VGP is found above preferred locations.
Highlights
After about 50 years of paleomagnetic studies, it is clear that the processes governing the geomagnetic reversals can only be constrained from sedimentary records with very high deposition rates and sequences of stacked volcanic lava flows
The transitional virtual geomagnetic poles (VGPs) from the Nordeste section switch from reverse to normal polarity across the South Atlantic Ocean, the North Pacific Ocean, and eastern Russia
The small set that emerges from this work is shown in Figure 6 and incorporates VGPs from SM11, SM12, and SM23, MT06 from Martinique (Tanty et al, 2015), flows 42 to 59 from Maui (Coe et al, 2004) and R1T to TT from Tahiti (Chauvin et al, 1990)
Summary
After about 50 years of paleomagnetic studies, it is clear that the processes governing the geomagnetic reversals can only be constrained from sedimentary records with very high deposition rates and sequences of stacked volcanic lava flows. The most recent 40Ar/39Ar age estimates of 783.4 ± 0.6 ka were published from tephra layers within marine sections from the Indian Ocean (Mark et al, 2017) while ages of 786.0 and 786.1 ka were derived from exposed lacustrine sediments in central Italy (Sulmona section, Sagnotti et al, 2016, 2019). In this last study, the duration of the polarity switch was estimated to have lasted only a few decades, but this result has been questioned by Evans and Muxworthy (2018) due to significant remagnetization of the sequence. Recalculated 40Ar/39Ar ages spanning ∼795 to ∼773 ka from Tahiti, La Palma, Chile, Guadeloupe and Maui lava flows were proposed by Singer et al (2019) and interpreted as evidence of a complex reversal process and attributed to successive geomagnetic events
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