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Gunderson & Sheriff 1991
Gunderson, J.A. and Sheriff, S.D. (1991). A new late cretaceous paleomagnetic pole from the Adel Mountains, West Central Montana. Journal of Geophysical Research 96: doi: 10.1029/90JB01963. issn: 0148-0227.

North America's apparent polar wander path has been poorly defined between the mid-Cretaceous and Paleocene reference pole positions. Existing data allowed 13¿ of apparent polar motion over about 22 m.y. (87--65 Ma) roughly coinciding with the beginning of Laramide deformation (~80 Ma). We report on a paleomagnetic study of the Adel Mountain Volcanic rocks to refine the North American apparent polar wander path for this interval. The shonkinite rocks of the Adel Mountain Volcanic field are on the eastern edge of the Cretaceous-Paleocene fold and thrust belt; some of these structures disturb the western edge of the volcanic pile. We obtained two new K-Ar dates from the Adel rocks. One data, on biotite (from a shonkinite dike that crosscuts most of the volcanic rocks, is 71.2¿2.7 Ma. The other, a whole rock date from a flow deep in the volcanic pile, is 81.1¿3.5 Ma. We collected six to nine paleomagnetic samples from each of 34 sites in roadcuts and natural outcrops of flows, dikes, and laccoliths. Positive fold and conglomerate tests, along with alternating field and thermal demagnetization, indicate that our characteristic remanent directions are primary magnetizations acquired before Late Cretaceous to Paleocene thrust belt deformation. Averaging the virtual geomegnetic poles from 26 reliable sites, all of normal polarity, yields a paleopole at 82.2¿N, 209.9¿E (α95=6.80¿, k=18.38). This pole is concordant with the Paleocene reference pole (82.0¿N, 170.2¿E, α95=3.5¿, k=18.6 (Diehl et al., 1983) and is 11.6¿ from the Globerman and Irving (1988) mid-Cretaceous pole at 71¿N, 196¿E. The youngest information in the Cretaceous stillstand pole is from the Niobrara Formation (Shive and Frerichs, 1974) at about 85--89 Ma. If we take the average age of the Adel Mountain Volcanics to be 76 Ma, then ~12¿ of apparent polar motion occurred between 87 Ma and 76 Ma. Thus, rapid apparent polar motion correlates well with the onset of Laramide deformation. ¿ American Geophysical Union 1991

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Keywords
Geomagnetism and Paleomagnetism, Paleomagnetism applied to tectonics (regional, global), Geomagnetism and Paleomagnetism, Rock and mineral magnetism, History of Geophysics, Geomagnetism and paleomagnetism, Tectonophysics, Plate motions—general
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Journal of Geophysical Research
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American Geophysical Union
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