Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmgp21a0777k&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #GP21A-0777
Physics
1513 Geomagnetic Excursions, 1521 Paleointensity, 1522 Paleomagnetic Secular Variation, 1560 Time Variations: Secular And Longer
Scientific paper
We present new 40Ar/39Ar ages from a 500 m thick sequence of basaltic lava flows on the island of Santo Antão, Cape Verde, and assess their implications for two previously reported, fully reversed excursions in the Brunhes chron (Knudsen et al., Earth Planet. Sci. Lett., 206, 199-214, 2003). Paleodirectional experiments on a sequence of 52 lava flows at Tarrafal, Santo Antão, coupled with two argon dates, one from the bottom and one from near the top of the sequence, suggested that the majority of the volcanic stratigraphy was emplaced between 0.54 ± 0.09 Ma and 0.38 ± 0.05 Ma and that the two excursions could be related to geomagnetic instabilities within the Brunhes Chron that had not previously been observed in terrestrial volcanic records. New 40Ar/39Ar isochron ages, determined using the incremental heating method at the University of Wisconsin-Madison for nine groundmass separates from hand samples collected by Knudsen et al. (2003), range from 5.04 ± 0.10 Ma and 2.57 ± 0.02 Ma and indicate that that the stratigraphy of the Tarrafal section is more complex than originally thought. This more complicated stratigraphy can be explained by infilling of a flank collapse structure with younger Brunhes age lava flows and subsequent erosion of the sequence. The lower "excursion" reflects sampling across two normally magnetized Brunhes age flows at the base of the section and an older sequence of flows encompassing both reversed and normal polarity. The older reversed flows correspond to part of the Gilbert chron between 5.04 ± 0.10 Ma and 4.53 ± 0.09 Ma; the normal flows could relate to a normal polarity chron or subchron between 4.53 ± 0.09 Ma and 2.8 ± 0.02 Ma. The upper "excursion" marks the change from a volcanic event of reversed polarity (3.56 ± 0.29 Ma) to the normal polarity flows of the Brunhes chron (0.41 ± 0.02 Ma). Our findings further highlight the need for precise age determinations and a high sampling density of dated flows when using long lava sequences to reconstruct paleofield behaviour.
Brown Charles Max
Faurschou Knudsen Mads
Feinberg Joshua M.
Singer Brad S.
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