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Paleomagnetism of the Eocene Flat Creek pluton, Yukon: Tectonics of the Intermontane terranes and Mackenzie Mountains

By
D.T.A Symons
D.T.A Symons
Earth Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada
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C.J.R Hart
C.J.R Hart
Yukon Geology Program, Box 2703 [F-3], Whitehorse, Yukon Territory Y1A 2C6, Canada
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M.J Harris
M.J Harris
Earth Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada
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P.J.A McCausland
P.J.A McCausland
Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada
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Published:
January 01, 2004

The massive 53.6 Ma Flat Creek granitic pluton of the Nisling Plutonic Suite intrudes flat-lying volcanic rocks of the 70 Ma Carmacks Group in the Stikine Terrane of the Intermontane Belt in the Yukon. Specimens (n = 334) from 22 sites in granite from the ~100 km2 pluton plus 3 sites in cross-cutting 51.1 Ma Eocene andesitic dikes were tested using alternating field and thermal step demagnetization and magnetic susceptibility and saturation isothermal measurements. Magnetite was the sole important characteristic remanent magnetization (ChRM) carrier. Most granitic and andesitic specimens carried a lower temperature and coercivity normal-polarity AN ChRM and an antiparallel higher temperature and coercivity reversed AR component, but some specimens of both rock types carried just AR and some granitic specimens carried just AN components. Combining the AN and AR directions, the granite pluton yielded a mean direction of declination (D) = 165.2°, inclination (I) = −76.8°, (number of sites [N] = 31, radius of cone of 95% confidence [α95] = 2.2°, precision parameter [k] = 139) and the dikes a mean D = 158.0°, I = −79.5°, (N = 3, α95 = 6.0°, k = 423). Paleomagnetic contact tests proved inconclusive because of the contemporaneous and dual polarity remanence of the specimens. The pluton’s paleomagnetic pole indicates a nonsignificant northward displacement of 0.8° ± 4.6° and a significant clockwise rotation of 14° ± 10° for the Stikine Terrane relative to the North American craton after ca. 54 Ma. Regression analysis of ≤;54 Ma paleomagnetic motion estimates for all Intermontane terranes against time also shows nonsignificant translation with a significant rotation rate of 0.34° ± 0.11°/m.y. This implies that the Intermontane terranes since ca. 54 Ma have behaved as a quasi-coherent upper crustal plate that rotated atop a North American cratonic lower crust about a proximal near-vertical axis. It is speculated that the rotation was marked by westward extension in southern British Columbia and by eastward compression in the northern Cordillera, together amounting to ~550 ± 160 km of displacement orthogonal to the stable cratonic margin. The compression component in the north, driven by Pacific plate subduction and collision of the Yakutat terrane, is the suggested cause of arcuate orogenic uplift in the Mackenzie Mountains.

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GSA Special Papers

Orogenic curvature: Integrating paleomagnetic and structural analyses

Aviva J. Sussman
Aviva J. Sussman
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Arlo B. Weil
Arlo B. Weil
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Geological Society of America
Volume
383
ISBN print:
9780813723839
Publication date:
January 01, 2004

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