Abstract of paper presented at American Geophysical Union 1986 Spring Meeting, Baltimore, Maryland.
Trans. American Geophysical Union (Eos), v. 67, p. 269.

Pervasive remagnetization in Alaska, Western U.S., and China: Examples and speculations

R. S. Coe, Earth Sciences, Univ. of California, Santa Cruz
D. R. Van Alstine, Applied Paleomagnetics, Inc.

Pervasive normal-polarity remagnetization of rocks throughout large regions of Alaska, western U.S., and China is commonly assumed to have taken place during the Cretaceous Long Normal Polarity Interval. Why else, the argument goes, would entire regions have their primary remanent magnetization completely wiped out by normal-polarity overprint unless the overprinting occurred when the field was normal for a very long time? One can equally well ask, however, why would remagnetization throughout so many large areas be expected to occur precisely during the limited window (about 35 m.y.) of the Cretaceous Long Normal Interval?

Part of the answer, we believe, is that pure Long Normal remagnetization is not nearly as widespread as reported. First, present-day, Brunhes-age, and Cretaceous normal overprints can easily be confused; their directions are commonly similar, low-temperature VRM can be more stable than previously thought, and surface weathering can produce CRM that is very difficult to remove. Second, CRM acquired over several polarity chrons will usually exhibit the dominant polarity obtained by integrating the field over that timespan.

We interpret the regional overprinting of normal polarity in the western and central Brooks Range and eastern China, of reversed polarity in the northeastern Brooks Range and British Columbia, and of mixed polarity in the western U.S. as CRM that reflects the polarity bias during the time of acquisition.