Vancouver's Trek Northward

Vancouver Island was once at the latitude of Baja California, thousands of kilometers to the south, argues a team of geologists and geophysicists in the current issue of Science. A few exquisitely preserved fossils have validated traces of ancient magnetism that suggest this piece of crust has traveled thousands of kilometers north during the last 70 million years.

The hypothesis of northward migration for Vancouver and other chunks of the Pacific Northwest originated 20 years ago. Because Earth's magnetic field is horizontal at the equator but vertical at the poles, the inclination of a rock's magnetism shows how far north it was when it formed and locked in Earth's field. Along the west coast of North America, researchers measured paleomagnetic inclinations smaller than they should be if that rock had formed in place, as part of North America. Many researchers took that to mean that these tracts of rock, or terranes, had slid up the coast from far to the south, much as California west of the San Andreas fault is sliding now.

But other geoscientists were dubious. They couldn't find the large faults that would have guided the rocks northward, and they argued that the shallow magnetic inclinations were misleading because most of these measurements came from great masses of frozen magma, which could easily have been tilted from their original orientations. Sedimentary rock would solve that problem because it is laid down in recognizable horizontal layers. But sedimentary rocks from the largest terrane--the Insular superterrane, which makes up much of the coastal crust from northern Washington state into Alaska--seemed to have been heated long after they formed, wiping them clean of their original magnetic signature.

Joseph Kirschvink of the California Institute of Technology realized that temperature-sensitive fossils could be a marker for rock that hadn't been heated and magnetically altered. Paleontologist Peter Ward of the University of Washington, in turn, knew of fossils from islands off the east coast of Vancouver that fit the bill. These fossils of extinct mollusks, called ammonites and inoceramids, retained the pearly luster of the living animals, implying that the paleomagnetic inclinations in the surrounding rock could be relied on. The magnetism was about 25° shallower than expected at Vancouver Island's current latitude, Kirschvink, Ward, and their colleagues report, implying that 70 million to 80 million years ago, when the rock formed, the terrane was 3500 kilometers to the south off Baja California.

After so many years, the dispute is not likely to be settled by a single finding. Indeed, critics are already identifying loopholes. But the study "could influence the fence sitters," says geologist Darrel Cowan of the University of Washington.

Posted in Earth