Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmgp51a0288k&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #GP51A-0288
Physics
1513 Geomagnetic Excursions, 1520 Magnetostratigraphy, 1535 Reversals (Process, Timescale, Magnetostratigraphy), 1555 Time Variations: Diurnal To Secular
Scientific paper
Samples from five hitherto fairly unexplored young volcanic islands in the Cape Verde Archipelago have been collected for palaeomagnetic studies. Magnetic results from lava flows in 5 profiles located on the island Santo Antão are reported. The Cape Verde Rise situated over the Mesozoic marine anomalies M2-M16, is the result of a slow eastward movement of the Atlantic oceanic lithosphere relative to a hotspot commencing around 20 Ma ago. The uplifted segment of deformed Jurassic oceanic crust seems to be unrelated to the bulk of the igneous activity, which took place during the Neogene. The oldest outcrops encountered on Santo Antão are deeply weathered basalts with an age of 7.57 +/- 0.56 Ma, whereas the investigated profiles cover a time interval from 2.67 +/- 0.05 Ma to 0.38 +/- 0.05 Ma. Investigation of some basic mineral magnetic properties reveal titanomagnetite composition as a general feature, and a pseudo-single-domain behaviour of all lava flows subjected to hysteresis-experiments are found. Flows of normal polarity characterise the Tarrafal Profile, being confined to the Brunhes Chron by 40Ar/39Ar-datings. Two events of reverse polarity are recorded in the stratigraphic column and appear to represent the Big Lost and Emperor Subchrons. The minimum eruption frequency is estimated to be one flow every 2.5 +/- 2.2 Ka. The Agua Nova Profile is likewise dominated by flows of normal polarity belonging to the Brunhes Chron and further contains a few events of reverse polarity. The relatively complex stratigraphy of accessible flows encountered in the Ribeira Grande Area makes the correlation with the global polarity time-scale tentative. The Escabecada Profile records the transition between the lower part of Matuyama and the upper part of the Gauss Chron, and a minimum eruption frequency is estimated to be one flow every 10 +/- 7.1 Ka for the lower part of this profile. The Chã de Morte Profile consists merely of reverse-polarity lava flows belonging to the Matuyama Chron. A tentative correlation between all 5 profiles has been attempted by the use of both radiometric datings and magnetic polarities. Polarities from the Escabecada and Agua Nova Profiles appear to indicate the existence of short events of fully reversed polarity not registered as subchrons in the global polarity time-scale. The presence of a reversed polarity event within the Brunhes Chron, as recorded by 8 lava flows in the Tarrafal Profile, may suggest that this geomagnetic event is of a more substantial and permanent nature than typically expected for large-amplitude secular variations.
Abrahamsen Niels
Faurschou Knudsen Mads
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