Archeomagnetic study at low latitude - North Atlantic Ocean (Fogo Island, Cabo Verde Archipelago)

Abstract

The recovery of the secular variation of the Earth's magnetic field in places of low latitude and in volcanic oceanic islands is crucial for the development of more precise models. Fogo Island (Lat. N14º57’; Long. W24º20’) in the Cabo-Verde Archipelago (Atlantic Ocean) is formed by a major conical and asymmetrical Quaternary strato-volcano with a summit depression (Chã das Caldeiras). It presents itself as a good object of study given its geographical position and frequency of eruptive events, 28 since 1460 A.D.. Accordingly, around Chã das Caldeiras and the east flank of the island, were sampled for paleomagnetic purposes 40 historical and prehistorical lava flows, representing 55 sampling sites. Rock magnetic and petrographic analyses reveal as main magnetic carriers several phases of the titanomagnetite solid solution without visible effects of alteration, as expected for such young basaltic rocks and semi-arid climate. The anisotropy of magnetic susceptibility ellipsoid is low and no effect in the paleomagnetic directions is observed. Thermal and alternating field demagnetizations were successful, retrieving well clustered mean characteristic remanent magnetizations (ChRM) directions. In a first instance twelve ChRM of lava flows previously mapped as historical were compared with known models of secular variation, revealing that nine are in close agreement with the suspected ages while the remaining three failed that concordance, corresponding to prehistoric eruptions. Moreover, it is possible to verify that ChRM of prehistoric lava flows closely follow known secular variation curves showing a small and consistent angular difference. Therefore, such results contribute for a better constraint of the volcano stratigraphy of this island and supply new paleomagnetic directions needed to improve the accuracy of secular variation models. This research has been funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – (https://doi.org/10.54499/UIDB/50019/2020), UIDP/50019/2020 (https://doi.org/10.54499/UIDP/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020). This work is a contribution to projects REGENA (Ref. PTDC /GEO-FIQ/3648/2012) and GEMMA (Ref. PTDC/CTA-GEO/2083/2021).