Paleomagnetic Analysis Of Basalt Flows Of The Crescent Formation, Olympic Peninsula, Washington

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Abstracts With Programs - Geological Society Of America




Numerous attempts have been made to obtain paleomagnetic directions from basalt flows of the Crescent Formation on the Olympic Peninsula of Washington. Most of these studies have demonstrated the unstable magnetic character of these rocks and have resulted in ambiguous results. Prior studies of the Crescent Formation basalts, by students at the University of Puget Sound, indicate that poor paleomagnetic results are due to hydrothermal alteration of the magnetic mineralogy of these flows. This work also suggests that the primary magnetic mineralogy is best preserved in fine-grained portions of basalt pillows. Warnock et al. (1993) sampled fine-grained pillow rims in obtaining acceptable paleomagnetic directions from the Crescent Formation of the northern Olympic Mountains, but no satisfactory paleomagnetic data have been extracted from the southern Crescent Formation. Good paleomagnetic directions are also available for Coast Range basalts south of the Olympic Peninsula. The lack of paleomagnetic information on crustal rotations of the southern Crescent Formation leaves a significant hole in the data used to constrain models of the tectonics of this region. Previous studies used randomly located, standard, one-inch diameter core samples for paleomagnetic analysis. The current study examines the southern Crescent Formation utilizing smaller diameter cores taken exclusively from pillow rims, as in Warnock et al's. work in the northern Crescent Formation. We also make use of extensive stepwise thermal demagnetization of samples at small temperature steps. Preliminary results suggest that this method may help jog the memory of these flows with magnetic amnesia. Reflected light microscopy and SEM analyses of these rocks will reveal the nature of the magnetic mineralogy and help establish whether the magnetic signal is primary or secondary in nature.