Abstract of paper presented at American Geophysical Union 1980 Spring Meeting, Toronto, Canada.
Trans. American Geophysical Union (Eos), v. 61, p. 219.

Paleomagnetism of the Crepicephalus-Aphelaspis zone boundary (Upper Cambrian) in North America – Divergent poles from isochronous strata

S. L. Gillett, Dept. of Earth and Space Sciences, State University of New York at Stony Brook
D. R. Van Alstine, Sierra Geophysics, Inc.

The scatter of paleomagnetic poles from the Cambrian of North America has been variously attributed to remagnetization, tectonic rotation, rapid apparent polar wander (APW), or nondipole behavior. Abrupt changes between successive trilobite faunal units ("biomeres" of Palmer, 1965) define several Cambrian timelines (duration perhaps 10⁴y); such horizons provide precise time control to test these hypotheses. The Crepicephalus-Aphelaspis zone boundary (middle Dresbachian), one such horizon, has been sampled at 2 sites in Tennessee (N[samples]=29 & 48), 2 in Nevada (N=28 & 47), and 1 in Utah (N=25); the upper Aphelaspis zone has also been sampled in Alberta at 1 site (N=51). All sites are in thrust belts; vertical-axis rotations of 15-25° are shown by between-site declination differences in both Nevada and Tennessee. Similar rotations can be demonstrated in Alberta by declination differences in Middle Cambrian strata. In turn, paleopoles from all middle Dresbachian sites diverge from cratonic poles from Texas (Watts et al., 1980) which also span this interval. These discrepancies can be explained by remagnetization or by tectonic rotations of up to 45° with respect to the craton. A divergent pole from Dresbachian strata in Arizona (Elston & Bressler, 1977) might also be rotated but probably reflects remagnetization; another pole from Missouri (Beales et al., 1974) is aberrant and may also reflect remagnetization. Probably much of the scatter in Lower Paleozoic poles from North America results from vertical-axis tectonic rotations and/or remagnetization. The extent of rotations and remagnetization must be established before inferences about APW or non-dipole behavior can be made.