Amplitude of Secular Geomagnetic Variation in Late Cretaceous Based on Paleomagnetic Studies of the Okhotsk–Chukotka Volcanic Belt from Upper Reaches of Malyi Anyui River, West Chukotkaстатья
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Аннотация:A paleomagnetic study of the Okhotsk–Chukotka belt volcanics exposed in the region of the Kupol field (~66.9° N, 170.1° E) has been carried out. The studied rocks were formed between 85 and 95 Ma during the Cretaceous Normal Superchron (CNS). In most of the studied samples, the characteristic magnetization component is isolated. Its primary origin is supported by the positive fold test, magnetic mineralogy data, and similarity of the calculated paleomagnetic pole with that of coeval rocks from the same region. It is shown that the studied rocks were formed in high polar latitudes close to 80° N. The amplitude of paleosecular variation (PSV) of the geomagnetic field during CNS has been estimated. Depending on the calculation method, the estimates (and their confidence intervals) for ~81° N are Sb = 22.1° (19.5°; 24.7°) in case of using a fixed cut-off angle of 45° and Sb = 23.1° (20.3°; 25.7°) in case of the cut-off angle calculated by the Vandamme method. The obtained estimates generally support the existing models of PSV latitudinal dependence for CNS. At the same time, the estimates are less consistent with the model (McFadden et al., 1991) where PSV values are low at the equator and sharply increase with latitude than with the model (Biggin et al., 2008) predicting higher PSV values at the equator and their more gentle increase towards polar latitudes. Irrespective of the calculation method, the obtained estimates almost perfectly fit the PSV latitudinal dependences for the last 5 Ma. This means that the amplitude of secular geomagnetic variation during CNS, at least at high latitudes, was the same as it was during the last 5 Ma. The obtained result does not support the hypothesis that geodynamo regimes change at the transition from the periods of frequent geomagnetic reversals to superchrons (e.g., (Gallet and Pavlov, 2016)), which suggests, in particular, that the modern numerical models are not fully adequate to the real processes of generation of the geomagnetic field.
