North America; Plate-tectonic setting and tectonic elements
This volume is about the evolution of the North America Plate. It also briefly discusses the evolution of the Pacific Ocean and the Caribbean as they relate to North America. We will briefly sketch the present outlines of the North America Plate, review the plate-tectonic development of North America in a global context, and offer an overview of the major tectonic (Fig. 1) and geomorphic elements of our continent.
Earthquakes are primary indicators of present plate boundaries and of plate motions that are related to these boundaries. The outline of the North America Plate is shown clearly on Figure 2, which is based only on the distribution of earthquakes occurring from 1977 to 1987. Note also the good definition of the Caribbean and Cocos Plates. In the Arctic regions (Fig. 3), earthquakes clearly outline the plate boundary along the mid-ocean ridge up to the north coast of Siberia. However, from there southward, across northeastern Siberia, the margin of the North America Plate is quite diffuse.
Substantial intraplate earthquakes (Fig. 2) have occurred particularly within the United States. The 1811 to 1812 New Madrid (Missouri) and the 1866 Charleston (South Carolina) earthquakes exceeded magnitude 7 and occurred in areas that are still active today (Seeber and Armbruster, 1988; Hinze and Braile, 1988). New England is another area of intraplate seismic activity. According to Zoback and others (1986), the midplate stress of the North America Plate is compressive, with a maximum horizontal principal stress oriented northeast to east northeast. The stress field extends from the Rocky Mountain front to within 230 km of the Mid-Atlantic Ridge.