Abstract 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.

ABSTRACT Early diagenetic chert in the upper Kotuikan and Yusmastakh formations, northeastern Siberia, preserves an exceptional record of carbonate textures and microfossils in an early Mesoproterozoic peritidal carbonate platform. Silicified lithologies include carbonate precipitates that formed at or near the sediment-water interface, as well as micritic event laminae that appear to have lithified more slowly. Precipitated textures include (1) radial-fibrous laminae nucleated on organic horizons and locally forming botryoids that stack vertically to produce microdigitate structures; and (2) micron-scale carbonate laminae. Both radial-fibrous carbonates and microlaminites contain abundant microfossils, some of which are preserved as uncompressed casts and molds that retain cellular detail. These indicate that lithification preceded microbial decay; actualistic taphonomy experiments on filamentous cyanobacteria suggest that lithification occurred on a timescale of days to weeks. In other silicified textures, microfossils show evidence of extensive post-mortem decay and compression, suggesting less rapid lithification. Papier-mâché carbonate sedimentation, characterized by essentially instantaneous lithification, appears to have been locally common in restricted tidal-flat environments during the Mesoproterozoic and earlier eras but uncommon in Neoproterozoic and later times.