REVISITING THE PALEOMAGNETISM OF A PALEOCENE TRENCH-RIDGE-TRENCH TRIPLE JUNCTION; KODIAK ISLANDS, ALASKA

The Ghost Rocks Formation of the Kodiak Islands, Alaska is part of an accretionary complex and contains turbidites interlayered with pillow lavas. It is generally accepted that this anomalous near-trench magmatism is the result of a Paleocene spreading center subducted beneath the Kodiak Islands. However, it is unclear where this event occurred in relation to the North American craton. Paleomagnetic directions could reveal this location, and a study by Plumley et al. (1983) concluded that the Ghost Rocks Formation was formed at 40 degrees north latitude. However, discordant site mean directions between the two major bays studied (Alitak and Kiliuda) complicate the results. The Alitak Bay samples were further confused by a 2-stage structural correction. This collaborative structural and paleomagnetic study seeks to resolve the location of the Paleocene triple junction by providing modern methods and more detailed sampling and structural analysis to the problem. Preliminary results from some of our sites at Alitak Bay are reported.

Oriented core samples were taken at 67 sites to include a baked contact test, conglomerate test, and fold test to ensure reliable data. 13 sites from Alitak Bay have already been analyzed, and 95 of 114 specimens displayed one to two component vectors during thermal demagnetization. Second components were averaged at each site, and 6 of the 13 sites showed a95 values of less than 20. In-situ mean direction for these 6 sites (D=10.1º, I=-45.6º, N=6, R=4.7, k=3.83, and a95=39.5) are slightly improved when corrected for local stratigraphy (D=257.9º, I=-77.7º, N=6, R=5.32, k=7.31, and a95=26.6). Although structural correction does not provide a positive fold test, it suggests that the paleomagnetic signature was acquired before deformation in the accretionary prism. These Alitak Bay results are in partial agreement with the original data, showing weakly magnetized samples with unblocking temperatures from 400 to 500°C. However, the new data reveal a steeper inclination direction and both normal and reverse polarities (previously only reverse). Field evidence has not been found for regional folding used by Plumley et al., (1983). Continued study of remaining 54 sites should further constrain the data.