LATE JURASSIC TRANSTENSIONAL BASINS ALONG THE MARGIN OF WESTERN NORTH AMERICA: AN INDICATION OF SINISTRAL LATERAL FAULTING FROM ALASKA TO THE GULF OF MEXICO?

Basins, at the western and southwestern margin of the craton of North America, that contain mainly Late Jurassic and Cretaceous strata distinguish a belt from northeastern Mexico to Alaska (Valle San Marcos and La Mula, Coahuila; Bisbee, Arizona and New Mexico, and Sonora; Batamote, Sonora; McCoy Mountains, California; Upper interval of Inyo Mountains volcanic complex, California; Methow, Washington; Tyaughton and Gambier, British Columbia; Gravina, Dezadeash, Nutzotin, Wrangell Mountains, Kahiltna, Alaska). Formation of the basins began as early as late Bathonian (~163 Ma) after subduction magmatism, mainly between Bajocian (174 Ma) and Bathonian (160 Ma), formed a belt along the margin of western North America. As subduction waned in Bathonian, the basins formed almost synchronously within or inboard of the slightly older magmatic belt. In southwestern North America the basin boundaries generally coincide with faults the geometry of which suggests formation as pull-apart structures at releasing bends of a sinistral fault set. The probable relationship to lateral faulting has been previously proposed in Canada and Alaska, although the sense is not clear. The inferred traces of the principal fault segments, which accommodated displacement, correspond with previously recognized faults, most of which have been extensively re-activated. These include: Mojave-Sonora megashear, Melones-Bear Mountain, Harrison Lake, and faults extending northwestward between the Bridge River and Stikine domains and the Wrangellia-Alexander-Peninsular Ranges composite terrane. Where the inferred trace emerges at the coast, it corresponds with the megalineament along the southwest edge of the Coast Range batholithic complex. Dextral faults such as the Denali and other right-lateral faults obscure the trace of the older inferred fault northwestward, although the Bathonian-Callovian basins persist near their traces. Igneous rocks (~160-145 Ma) that are commonly bimodal, have textures associated with shallow emplacement and are commonly localized within or at the margins of the basins probably record the period of most active sinistral transtension.