THE MEXICO-ALASKA MEGASHEAR (170-148 MA)

The Mojave-Sonora megashear, which bounded the southwestern margin of the North America plate from Middle to Late Jurassic, may be extended northward to Alaska based upon previously recognized regional elements such as transtensional basins, transpressional uplifts, overlapping magmatic belts, and terrane juxtapositions. The longer, continental-scale fault thus defined, which is called the Mexico-Alaska megashear (MAM), separated the North America plate from a proto-Pacific plate and linked the axis of ocean-floor spreading within the developing Gulf of Mexico with a regional restraining bend above which mafic rocks were obducted eastward onto Alaskan sialic crust, which converged against the Siberian platform. The fault, about 8000 km long, lies among more than a dozen large basins, postulated to be transtensional (e.g. Great Valley (GV); Inyo Mtns. Volc. Complex; Josephine basin (JB); Nechako basin; Eskay rift; and Tuxedni basin (Cook Inlet - S. Talkeetna Mtns.), that began to form at ca. 170 Ma (Bajocian). Somewhat smaller, presumably subsidiary basins including Nutzotin-Dezadeash, Wrangell Mtns., Gravina, and Tyaughton-Methow contain Oxfordian strata. By 164 Ma some of the larger basins were floored with basalt (GV; JB) or extensively intruded (e.g. Early Coast Plutonic complex, B.C.; Mt. Tallac basin, NE Sierra Nevada). Jurassic contractional structures within the Blue Mtns., which extend from the Klamath Mtns. to the Western Idaho shear zone (Oregon, Idaho) are interpreted to have formed along a restraining bend during transpression. The deformation is contemporaneous with the MAM as well as other contractional features comprising the Elko orogeny in central Nevada and western Utah where Jurassic plutons (166 to 158 Ma) distinguish a rather narrow, easterly trending band. The magmatism is compatible with subduction induced by transpression along the northern margin of the Blue Mtns. Regional contraction during the brief (ca. 153–150 Ma) Nevadan orogeny, which took place concurrently with magmatism in some basins and inversion in others, presaged the end of MAM activity by ca. 148 Ma.