MARINE GEOLOGY OF THE GEOLOGIC MAP OF NORTH AMERICA - AN OLD CONTINENT VIEWED IN A NEW CONTEXT

The new Geologic Map of North America is the first published since the plate tectonics revolution and the first to depict the seafloor geology surrounding North America. The seafloor mapping is in ‘classical' style familiar to continental geologists, but it also includes a number of special features that are significant for depicting the plate-tectonic framework of North America and for understanding the ocean basin geology. Because very little of the ocean basins had previously been interpreted in a geologic-map context, most of the seafloor was mapped from original data and from sources in the published literature. The primary information used was from seismic reflection profiles, magnetic anomaly data, bathymetric surveys, cores, dredges, submersible observations and sampling, and seafloor photography. An extensive bibliography, available from the GSA Data Repository, documents the sources used for mapping the seafloor geology.

The submarine parts of the map show the following features: bathymetry at 1000 m intervals; outcrop geology (age, extent, and composition); submarine canyons, sea valleys, and mid-ocean channels; faults, shear zones, and slump scars; diapirs; seamounts; impact structures; sediment drifts; hydrothermal vents and hydrocarbon/brine seeps; polymetallic sulfide, iron-manganese, and phosphorite deposits; and special rock types (e.g., ultramafic, felsic). Present and former plate boundaries are variously associated with normal, transform, and thrust faults plus pseudofaults, and current spreading centers are highlighted, with off-axis oceanic crustal ages identified. These features readily demonstrate the major plate-tectonic relationships that associate offshore crustal creation, evolution, and subduction with the architecture of the continental and island geology.

Because the ocean basins must be studied largely by remote access, much remains to be learned about their geology. Nonetheless, the new Geologic Map provides the basic framework of the seafloor geology and it thus properly orients the subaerial geology in the context of a plate-tectonic world. The map will be useful to a wide audience of scientists, managers, and geoscience educators.