THE JURASSIC (170-150 MA) MEXICO-ALASKA MEGASHEAR, A CONTINENTAL-SCALE FAULT BETWEEN THE GULF OF MEXICO AND ALASKA

The Mojave-Sonora megashear, which bounded the Jurassic southwestern margin of the North America plate from 169 to 148 Ma, may be linked northward to Alaska via the discontinuity between the Insular and Intermontane terranes. Transtensional basins, transpressional uplifts, and overlapping Jurassic magmatic belts distinguish the continental-scale fault called the Mexico-Alaska megashear (MAM) that linked ocean-floor spreading within the Gulf of Mexico with a restraining bend along which the Amgayucham terrane was obducted onto Alaskan crust. The MAM, about 8000 km long, extends west-northwesterly from the Gulf of Mexico across northern Mexico to southern California where it curves northward probably coincident with the San Andreas fault. The southern end of the Great Valley corresponds with a releasing-step that accommodated formation of a major regional pull-apart approximately between the Melones and Coast Range faults. The Coast Range and the more northerly Josephine ophiolites formed within pull-aparts.

In Oregon, MAM steps east within the Blue Mountains where contraction suggests transpression. The Methow basin and the Relay Mountain Group of the Tyaughton basin in British Columbia probably lie NE of MAM. From southern Alaska and northwestward correlative transtensional basins include Gravina, Dezadeash-Nutzotin, Wrangell Mountains, Matanuska Valley (southern Talkeetna Mountains), Tuxedni (Cook Inlet) and southern Kahiltna basins.

Principal fault segments, inferred to correspond with the trace of MAM, include Mojave-Sonora megashear, Melones-Bear Mountain, Siskiyou and Soap Creek Ridge faults, Ross Lake fault zone, as well as Harrison Lake, Bridge River suture, Lillooet Lake and Owl Creek faults. In western British Columbia, MAM corresponds with the mega-lineament along the southwest edge of the Coast Range batholithic complex. The Kitkatla and Sumdum thrust faults, probably formed initially as Late Jurassic strike-slip faults as did the Denali fault and more northerly extensions including Talkeetna, and Chilchitna faults.