Attempts to reconstruct even a portion of North America's past geography (paleogeography) require a healthy appetite for travel, tedious lab work, and acceptance of geologic changes that occur over vast stretches of time. This article provides a brief overview of how my colleagues and I have used ancient magnetism preserved in rocks to interpret the past geographic coordinates of North America and associated crustal fragments. This work required sampling rocks within the "stable" interior of the country (northern Arizona, southeastern Utah, and eastern New Mexico) and also rocks from the more complicated continental fringe (southeastern Alaska). This field work was followed by many hours of laboratory analysis at the University of Arizona, Tucson.
Ancient magnetism preserved in rocks (paleomagnetism) can be used to restore segments of the earth's lithosphere (crust and uppermost mantle) to the positions they occupied when the magnetism was acquired. Rocks can acquire their magnetism in several ways. Igneous rock acquire a magnetism during cooling when the growth of iron-bearing minerals is influenced by the earth's magnetic field. Sedimentary rocks can acquire a magnetism when small, iron-bearing minerals align to the earth's magnetic field during settlement in a water column. In addition to these primary processes, the magnetism of a rock can be realigned during metamorphic heating or due to new mineral growth during chemical weathering.
The study of paleomagnetism requires one to determine and record the present orientation of a rock prior to collection. Sensitive laboratory equipment such as cryogenic or spinner magnetometers are used to measure the paleomagnetism of a sample. This ancient magnetism is then reoriented to reflect the position of the ancient magnetism with respect to the current geographic coordinates of the sampling location.
My research has been directed toward establishing the position of the North American continent during the Mesozoic Era (245 million years to 65 million years ago) and toward determining the position of exotic blocks relative to North America during their time of formation. The position of the North American continent is described in terms of an apparent polar wander path (APW path). An APW path is a time sequence of magnetic pole positions recording the paleolatitude and azimuthal orientation of a continent within the dipolar geomagnetic field. Figure 4 shows the APW path for North America during the period from 240 million year ago to 65 million year ago. The North American continent is considered to be fixed to this path. Therefore, the latitude and azimuthal orientation of North America at the time of any of these poles can be derived by taking the pole (and the position of North America relative to this pole) and moving it to the geographic north pole shown.
My colleagues and I collected Mesozoic rock samples from Arizona, Utah, Colorado, and New Mexico to refine a portion of the APW path. Our work [1] suggests that a sharp cusp or "hairpin-turn" exists in the 210 to 180 million year old segment of North America's APW path. Furthermore, we have provided a more precise location of the 150 million year old pole position [2]. This information provides a more accurate reference for determining the past positions of outlying crustal fragments relative to the continental interior.
Figure 4. Mesozoic apparent polar wander path for North America. Areas around each paleomagnetic pole (black dot) are 95% confidence limits. Arrows on upper diagram show the age progression of poles from 240 million years ago to 100 million years ago.
Much of western North America is made up of crustal material that has been added to North America during the process of plate convergence (subduction). In addition, much of this material (as well as native rock) has been moved up and/or down the western margin of North America along faults. These mobile pieces of crust are called "terranes" (the spelling is intentional). Since an APW path provides the geographic location of a continent throughout geologic time, it can be used as a reference for determining the history of terrane motions relative to the continental interior. Analysis of a rock's paleomagnetism is one of the primary tools for evaluating the geologic history of these terranes. The paleomagnetic direction of terrane rock (similar to a compass needle direction) is compared with the magnetic direction one would expect from the same age rock from the continental interior. The degree of difference is used to interpret the motion history of the terrane.
My colleagues and I [3] used this method to evaluate the displacement history of the Alexander-Wrangellia terrane that underlies much of southeast Alaska and coastal British Columbia. This history has been difficult to resolve, in part, because Alexander-Wrangellia is separated from inboard terranes (to the east) by 150 to 100 million year old sedimentary and volcanic rocks of the Gravina terrane. The objective of our Alaska project was to resolve this ambiguity by using paleomagnetic data obtained from the Gravina rocks and compare them to our APW poles to determine the paleogeography of the Gravina terrane. The information from the Gravina terrane provides the position of the underlying terrane during the period of Gravina formation (150 million to 100 million years ago).
We focused our research on the paleomagnetism of magnetite-bearing volcanic rocks (pillow basalt). Paleomagnetic samples were collected from 31 sites on the mainland and small offshore islands north of Juneau, Alaska. Another 17 sites were collected on Douglas Island west of Juneau, Alaska. Site mean directions from 25 sites were used to determine a structurally-corrected mean direction and a paleomagnetic pole. The pole location is at 78.2 degrees north and 140.8 degrees east (with a cone of 95% confidence equal to 14 degrees). The specific age of the Gravina volcanic rock is uncertain; which when considered with the confidence limits of the paleomagnetic data allow estimates of 2 degrees to 14 degrees + or - 13 degrees of northward displacement relative to North America. However, even this large range of possible latitudes implies these terranes were much closer to their present position than has been proposed by previous workers. In addition, a recent age dating study of detritus shed from the east onto the Gravina terrane provides strong evidence that these terranes were closely associated with western North America between 150 million and 100 million years ago [4].
Acknowledgments. The research discussed in this article was funded by National Science Foundation grant EAR-9405463 awarded to David Bazard, and National Science Foundation grants EAR-881594 and EAR-9017382 awarded to Robert Butler (Geoscience Department, University of Arizona). Special thanks to Robert Butler, George Gehrels, and Paul Kapp at University of Arizona for their contributions to this project.
David Bazard is a full-time science instructor who is currently engaged in his second year of teaching at College of the Redwoods. David was employed as an Assistant Professor at the University of Mississippi from 1991 to 1995. He received a BA in Geology from Humboldt State University in 1982, a MS in Geology from Western Washington University in 1987, and a Ph.D. in Geology from University of Arizona in 1991.
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