Paleomagnetism of Proterozoic Rocks of the Western U.S.

As part of a long interest in the Precambrian evolution of the western U.S., I have been working on the paleomagnetism and geochronology of Proterozoic rocks exposed in the basement-cored Laramide uplifts of Colorado, Wyoming, and Montana, as well as other areas of the western U.S. including Arizona and Colorado. The overarching goal of this work is better understand the Proterozoic geologic and tectonic evolution of western North America and to contribute well-defined paleomagnetic poles to the North American paleomagnetic data base for the construct apparent polar wander paths and evaluation of Proterozoic plate reconstructions. This work has involved paleomagnetic sampling and analysis of mafic dikes and intrusions in most of the Laramide basement-cored foreland uplifts of Wyoming and Montana, including the Laramie Range, Bighorn Mountains, Granite Mountains, Wind River Range, Teton Uplift, Beartooth Mountains, and the Tobacco Root, Ruby and Highland Ranges. In addition, I have been involved in paleomagnetic and geochronologic studies of intrusive rocks in Colorado and Arizona, including the Sherman Granite, Pikes Peak Granite, Electra Lake Gabbro, the dikes and sills of the central Arizona diabase province, and alkaline intrusive rocks at Mountain Pass, California. Collaborators in this various aspects of this research have included: John Geissman, Lawrence W. Snee, Wayne Premo, Dan Unruh, Larry Heaman, Tony LeCheminant, Richard Reynolds, Karl Karlstrom, and Christopher Schmidt.

Proterozoic mafic dike exposed at Mount Moran Teton, Range, Wyoming

Map showing the distribution of Precambrian mafic dikes and intrusions exposed in the Laramide-age, basement-cored uplifts of the Archean Wyoming craton. LB = Little Belt Mountains; TRM = Tobacco Root Mountains; RR = Ruby Range; BT = Beartooth Mountains; TR = Teton Range; BHM = Bighorn Mountains; WR = Wind River Range; GR = Granite Mountains; LR = Laramie Range; SM = Sierra Madre Mountains; LAC = Laramie anorthosite complex. Modified from Snyder et al. (1988).

Here is a list of some of the published research that has resulted from this work:

Harlan, S.S., Premo, W.P., Unruh, D., 2005, Isotopic Dating of Meso- and Neoproterozoic Mafic Magmatism in the Southern Tobacco Root Mountains, Southwestern Montana: Precambrian Research, v. 136, p.269-281. PDF Version

Harlan, S.S., Heaman, L.W., LeCheminant, A.N., and Premo, W.R., 2003, The Gunbarrel mafic magmatic event: A key 780 Ma time marker for Rodinia plate reconstructions: Geology, v. 13, p. 1053-1056. View Abstract and Figures

Harlan, S.S., Geissman, J.W., and Premo, W.R., 2003, Paleomagnetism and geochronology of an Early Proterozoic quartz diorite in the southern Wind River Range, Wyoming, USA: Tectonophysics, v. 362, p. 105-122.

Geissman, J.W. and Harlan, S.S., 2002, Late Paleozoic remagnetization of Precambrian crystalline rocks along the Precambrian/Carboniferous nonconformity, Rocky Mountains: a relationship among deformation, remagnetization, and fluid migration: Earth and Planetary Science Letters, v. 203, p. 905-924.

Karlstrom, K.E., Åhäll, K-I. Harlan, S.S., Williams, M.L., McClelland, J., and Geissman, J.W., 2001, Long-lived (1.8-0.8 Ga) Cordilleran-type orogen in southern Laurentia, its extensions to Australia and Baltica, and implications for refining Rodinia: Precambrian Research, v. 111, p. 5-30.

Karlstrom, K.E., Harlan, S.S., Williams, M.L., McClelland, J., Geissman, J.W., and Åhäll, K-I, 1999, Refining Rodinia: Geologic evidence for the Australia-western U.S. connection in the Proterozoic: GSA Today, v. 9, no. 10, p. 1-7. PDF Version

Harlan, S.S., and Geissman, J.W., 1998, Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado: Journal of Geophysical Research, v. 103, p. 15497-15507.

Harlan, S.S., Geissman, J.W., and Snee, L.W., 1997, Paleomagnetic and ⁴⁰Ar/³⁹Ar geochronologic data from Late Proterozoic mafic dikes and sills, Montana and Wyoming: U.S. Geological Survey Professional Paper 1580, 16 pp. Web Version PDF Version

Harlan, S.S., Geissman, J.W., Snee, L.W., and Reynolds, R.L., 1996, Late Cretaceous remagnetization of Proterozoic mafic dikes, southern Highland Range, southwestern Montana: A paleomagnetic and ⁴⁰Ar/³⁹Ar study: Geological Society of America Bulletin, v. 108, p. 653-668.

Park, J.K., Buchan, K.L., and Harlan, S., 1994, A proposed giant radiating dyke swarm fragmented by the separation of Laurentia and Australia based on paleomagnetism of ca. 780 Ma mafic intrusions in western North America: Earth and Planetary Science Letters, v. 132, p. 129-139.

Harlan, S.S., Snee, L.W., Geissman, J.W., and Brearley, A., 1994, Paleomagnetism of the Middle Proterozoic Laramie Anorthosite Complex and the Sherman Granite, southern Laramie Range, Wyoming and Colorado: Journal of Geophysical Research, v. 99, p. 17997-18020.

Harlan, S.S., 1993, Paleomagnetism of Middle Proterozoic diabase sheets from central Arizona: Canadian Journal of Earth Sciences, v. 30, p. 1415-1426.

Photographs of Dikes and Sills from the Western U.S.

Gunbarrel Magmatic Event

Mafic Dikes of the Beartooth Mountains

GMU Paleomagnetism Laboratory

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