Displaced Terranes

7.3.1 Western North America

In the early 1970s it was noted that the paleomagnetic record from rocks along the western margin of North America showed consistent and systematic discordance with results of the same age from the stable Laurentian craton (Irving and Yole, 1972; Packer and Stone, 1972). Beck (1976, 1980) and Irving

(1979) reviewed all available paleomagnetic results from the western margin and then compared them with an established set of reference poles from the cratonic interior (e.g., Irving, 1977). Only a few pre-Miocene directions were found to be concordant with those from the craton. Most rock units exhibited more easterly paleomagnetic declinations and/or shallower inclinations than would have been expected from the reference poles. It was concluded that the likely cause of this systematic difference was northward translation and/or clockwise rotation of exotic terranes within or alongside the westernmost Cordillera.

These observations made it clear that most of the western edge of North America is made up of terranes (commonly referred to as displaced terranes) that have been displaced from their original positions and have been transported, rotated, and accreted to the Laurentian craton. Northward transport and clockwise rotation have been the prime elements in shaping the western Cordillera. For example, south of Cape Mendocino (40°N) and north of Vancouver Island (50°N) the main displacement has been northward transport of these displaced terranes, whereas between these locations in the Pacific northwest only clockwise rotations are found. This is consistent with the steady underthrusting of the coastal Pacific northwest by the Farallon plate for most of the Cenozoic, contrasting with the transform activity to the north or south (Beck, 1980). Batholith belts, such as in the Peninsular Range batholith of southern California and the Coast Plutonic Complex of British Columbia, also appear to have been involved in this northward transport and clockwise rotation. Although undetected tectonic tilts could also explain the results from batholiths, Beck

(1980) argues that postmagnetization tilts are probably small.

Some of the displaced terranes north of Vancouver Island were shown in Fig. 6.11. The most important of these in southern Alaska from a paleomagnetic viewpoint are the Alexander, Peninsular, and Wrangell terranes, the latter being named Wrangellia by Jones et al. (1977). Based on paleomagnetic data, Monger

Fig. 7.11(d-f). Global paleogeographic maps for (d) Mid-Cretaceous (94 Ma), (e) Latest Cretaceous (94 Ma), and (f) Eocene (50 Ma). Oceanic trenches are shown in red. From Scotese (1997) (see also http://www. scotese. com).

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