Physics
Scientific paper
May 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusmgp13b..11a&link_type=abstract
American Geophysical Union, Spring Meeting 2005, abstract #GP13B-11
Physics
1525 Paleomagnetism Applied To Tectonics (Regional, Global), 3040 Plate Tectonics (8150, 8155, 8157, 8158), 8157 Plate Motions: Past (3040), 8450 Planetary Volcanism (5480)
Scientific paper
It has long been suggested that the North American pole for the Cretaceous can be represented by a single "standstill" pole. Here we present new paleomagnetic data that can be used to test this model. The new data are from the Fort Hays Limestone Member of the Niobrara Formation sampled in Kansas and Colorado east of the Rocky Mountains. The Fort Hays Limestone is almost entirely Coniacian in age (85.9-89.3 Ma) and consists mainly of medium to thick beds of chalky limestone. Shive & Frerichs [1974] previously showed that this unit carried a weak but stable remanent magnetization. All specimens from the 38 reliable sites (8-10 samples per site) that we collected from eight localities were measured in a cryogenic magnetometer and subjected to either stepwise alternating field or thermal demagnetization followed by principal component analysis. Intensity of the natural remanent magnetizations from reliable sites typically exceeds 3 x 10-4 A/m. For specimens from these 38 sites, the demagnetization paths are generally linear and toward the origin above 20 mT or 240 degrees C. The characteristic remanent magnetization direction obtained from these differs significantly from the present field direction. The demagnetization data, along with IRM results, indicate that the characteristic remanent magnetization is carried by magnetite. Twenty flat-lying sites at three localities in the stable interior of North America in Colorado and Kansas were sampled to determine a pole position for stable North America. Eighteen tilted sites from five localities along the western edge of stable North America in Colorado were sampled and used to construct a fold test of the magnetic stability of the Fort Hays Limestone. A highly significant positive classical fold test is obtained from either the 38 combined site mean poles or the 5 locality mean poles, consistent with the characteristic magnetization having been acquired at or near the time of deposition. The mean of the 20 site mean poles from the flat-lying sites is 75.97N, 188.16E with a A95 of 1.9 degrees (K = 300.9), which lies only 3.6 great circle degrees from the Early Cretaceous (ca. 120 Ma) pole, but differs significantly from it. Thus, apparent polar wander was slow, but the pole has not stood still. Instead, the Early Cretaceous, Coniacian, Paleocene, and Eocene poles define a consistent age progressive track spanning 11 great circle degrees. The Campanian pole for North America, based on paleomagnetic results from the Adel Mountain Volcanics, Elkhorn Mountain Volcanics, and the Maudlow Formation lies off the track suggesting, but not proving, that their sampling regions have undergone a small clockwise vertical axis rotation during Laramide deformation. Implications for inter-hotspot motion and true polar wander will be discussed.
Acton Gary
Gordon Roy G.
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