Magnetostratigraphy of Cape Verde Islands Volcanics

During three field campaigns on the Cape Verde Islands (15N, 24W) in 1998, 2000, and 2003 paleomagnetic collections of several volcanic profiles from 5 of the 9 populated islands of the Cape Verde Archipelago were made. A summary of the paleomagnetic results obtained for some of the islands will be given in the presentation. On the island of Santo Antao paleomagnetic and magnetostratigraphic results from four lava sequences have been obtained: The Tarrafal, Agua Nova, Cha de Morte and Escabecada profiles. From the Tarrafal and Agua Nova profiles, 63 and 43 lava flows were investigated, respectively. Absolute Ar/Ar-ages indicate that the two profiles mainly correlate to the Brunhes Chron, which is in accordance with the normal polarity displayed by the majority of the flows. Some individual lava flows as well as flow sequences with virtual geomagnetic poles deviating more than 45 degrees from the geographic pole are interpreted as geomagnetic excursions, the number of which seem to be increasing these years. The most noticeable observations are the indications of Brunhes-aged reverse-polarity flows found within the Tarrafal and Agua Nova profiles. From the Cha de Morte and Escabecada lava sequences 21 and 24 lava flows were sampled, respectively. The uppermost flow in the Cha de Morte profile is constrained to the lower part of the Matuyama Chron by an Ar/Ar-age. Except for one flow of intermediate direction (ChM-I), the whole Cha de Morte sequence consists of reverse-polarity flows. The lowermost flow in the Escabecada profile is constrained to the upper part of the Gauss Chron by an Ar/Ar-age. Flows of both reverse and normal polarity are found in the Escabecada profile, corresponding to the lower part of Matuyama and upper part of Gauss, respectively. Two flows with anomalous behaviour, ESC-I and ESC-II, are found in the directional data of this sequence. The uppermost event (ESC-II) most likely recorded the Gauss-Matuyama transition, while the lowermost event (ESC-I) with fully reversed polarity may indicate the presence of a Gauss-Matuyama precursor. Paleomagnetic and magnetostratigraphic results were also obtained from three lava sequences on the island of Santiago, i.e. the Ribeira da Barca, Sao Gon‡alo, and Porto Formosa profiles. Samples were collected from 35 independent flow units (110 samples) distributed along the three volcanic lava sequences, which, by use of Ar/Ar-ages, may be correlated with the Geomagnetic Polarity Time Scale. The youngest profile, Ribeira da Barca, consists of normal-polarity flows corresponding to the lower Brunhes Chron. The lowest flow unit in the lava sequence possibly recorded the end of the Matuyama-Brunhes transition (MBT). The Sao Gon‡alo profile is dominated by Matuyama-aged flows of reverse polarity, but short sequences of flows with normal and intermediate directions were also recorded in the section. Based on ages available it seems most likely that the normal-polarity zone correlate with the Olduvai Subchron. The Porto Formosa profile is well-constrained to the lower Matuyama, which is consistent with all flows in this section displaying reverse polarity. We are pleased to acknowledge the collaboration with our colleagues in the Danish Working Group on the Geology of the Cape Verde Islands.