TRANSTENSION AND TRANSPRESSION AMONG LARGE-SCALE STRIKE-SLIP FAULT SYSTEMS IN WESTERN NORTH AMERICA

The formation of large-scale strike-slip faults in western North America generally follows episodes of collision and coupling between the Pacific and North America plates and correlates with abrupt changes in plate motion. During Tertiary coupling events, changes are recorded by pulses of widespread extension and local, less pronounced transpression (e.g. Specter Range, NV). Deformation is recorded by temporally distinct (55-42 Ma, 35-20 Ma, 17-6 Ma) systems of complementary dextral (e.g. Las Vegas Valley shear zone) and sinistral (e.g. Lake Mead fault zone) strike-slip faults and co-genetic normal faults. Each fault system accommodates extension with a characteristic trend (275^o, 240^o, 280^o). A prominent, major, strike-slip fault (e.g. Tintina, San Andreas) commonly develops during each extensional pulse. Restraining steps may take advantage of irregularities in the distribution of continental crust and pre-existing weaknesses such as old faults.

A very long, sinistral, fault (1000’s of km) with large displacement (ca. 1000 km) formed following cessation of Middle Jurassic subduction between ca. 165 and 148 Ma. During relative movement of the North America plate with respect to the proto-Pacific plate, large basins (e.g. Bedford Canyon - Great Valley, Kahiltna - Nutzotin) and suites of basins (Bisbee – Huachuca – McCoy Mtns., Methow-Tyaughton, Gambier, Gravina) formed at releasing steps. Late Jurassic contractional structures in Blue Mountains of Oregon and Angayucham domain of Alaska and coeval magmatic rocks probably record transpression followed by collision and partial melting. Early oceanic crust in the Gulf of Mexico, central Atlantic Basin, and Mediterranean Sea probably formed in the floors of left-releasing steps along the sinistral fault. Late Jurassic contractional deformation and coeval magmatism that are characteristic of parts of the Yanshan and more northerly Mongolo-Okhotsk collisional zone may record co-genetic regional transpression.