Paleomagnetic evidence from Cape Verde Islands basalts for fully reversed excursions in the Brunhes Chron

Abstract

In this study we present paleomagnetic data from two lava sequences on Santo Antão, Cape Verde Islands: the Tarrafal and Agua Nova profiles from which 63 and 43 lava flows were sampled, respectively. Previous 40Ar/39Ar ages have constrained the two profiles to the Brunhes Chron, which is in accordance with the normal polarity of the majority of the flows. Some individual lava flows as well as flow sequences with virtual geomagnetic poles deviating more than 45° from the geographic North Pole are interpreted to represent geomagnetic excursions. Based on interpretation of the directional data three excursions are recorded in the Tarrafal profile (T-I, T-II, and T-III) and four in the Agua Nova profile (AG-I, AG-II, AG-III, and AG-IV). Both 40Ar/39Ar results and paleomagnetic directional data indicate that an excursion defined by eight flows in the Tarrafal profile (T-I) is the same as one recorded in six flows in the Agua Nova profile (AG-I). This excursion, which passes the reversal test with classification C, is the first geomagnetic excursion in the Brunhes Chron to pass the reversal test. The 40Ar/39Ar ages suggest that this excursion could represent chryptochron C1n-1 (0.493–0.504 Ma), the only chryptochron or ‘tiny wiggle’ identified in the marine magnetic anomaly record for the Brunhes Chron. The remaining excursions may not yet be unambiguously correlated between the two profiles. Another excursion recorded by two flows in the top of the Tarrafal profile (T-III) also displays completely reversed polarity.