DEFORMATION OF THE NORTHEASTERN PACIFIC BASIN AND THE WEST COAST OF NORTH AMERICA

Four great bands of unusually irregular topography named “fracture zones” have been discovered in the northeastern Pacific basin, and three have been traced into western North America. Individual zones range from at least 1400 to 3300 miles long and average 60 miles wide. The zones follow great circles for most of their lengths, and all are roughly parallel. Topography within the fracture zones is characterized by great seamounts, deep narrow troughs, asymmetrical ridges, and escarpments. Two escarpments are about 1 mile high and more than 1000 miles long. Two fracture zones separate regions which differ in depth by a quarter or half a mile.

The fractured area includes 8,000,000 square miles (5 per cent of the earth's surface), and the parallel trends of the fractures indicate a single origin. It is tentatively concluded that an annular convection current rising near the Hawaiian Islands and sinking near North America stressed the crust and produced the fracture zones by plastic deformation. The San Andreas fault marks a complementary zone of plastic deformation, and the California Coast Ranges lie within the zone. The Channel Islands and Transverse Ranges of California, and the Revilla Gigedo Islands and Volcanic province of Southern Mexico lie within continental extensions of the deep-sea fracture zones and parallel the zones. Some geomorphic provinces on the sea floor are bounded by fracture zones as are the Sierra Nevada, the Great Valley of California, the Baja California Peninsula (including the Peninsular ranges), and the Gulf Trough. Evidently the provinces were formed either with or after the fracture zones, and the stresses which formed them did not transect the zones. The zones appear to cut Mesozoic structure; they contain more active volcanoes and earthquakes than is normal for the northeastern Pacific area.