Czarnecki's Thesis Abstract

Paleomagnetism of Ascension Island Basalts

Jill Czarnecki

Ascension Island basalts were examined to determine if their paleomagnetic behavior can provide clues to the transitional behavior of the earth's magnetic field during the Matuyama-Brunhes reversal. Oriented samples were obtained from 22 different locations on the island during 1987 and 1988 by Dr. M. Lee Allison. The samples were given to Professor Valentine in 1995. Several cores were drilled from each block for paleomagnetic analysis. Using alternating field and thermal demagnetization, 21 of the 22 units sampled produced stable, apparently primary directions. Two of the 21 units exhibited reversed polarity with directions similar to those expected for Ascension Island. Ten units with normal polarity also showed anticipated directions. Results from three units having normal polarity yielded anomalous directions which are to the west of expected directions. Six samples exhibit anomalously steep negative inclinations between -29° and -73° and highly variable declinations. Due to a lack of sun compass orientations for the oriented blocks, the significance of the anomalous declinations are uncertain. However, the inclination of a samples is not be affected by the lack of sun compass orientation. There are no published dates for the units with anomalous results, but the age range of Ascension Island rocks as reported by Harris et. al. (1982) is between .12 My and 1.5 My. This age range suggests that the anomalous units could have recorded the Matuyama-Brunhes reversal. This preliminary study shows results which indicate Ascension Island would be a good location for further paleomagnetic study of field behavior during the Matuyama-Brunhes reversal.

University of Puget Sound