EXTENSIONAL TECTONISM AND TERRANE DISPERSION IN THE CORDILLERA—EXAMPLES FROM NORTHERN ALASKA AND BAJA CALIFORNIA

Terrane analysis has yielded a useful assessment of the fault-bounded stratigraphic packages that compose the North American Cordillera, resulted in identification of allochthonous fragments that have been accreted to North America, and led to construction of a general tectonic history for the region. Although recent studies have revealed new details of the timing of terrane deformation, amalgamation, accretion, and post-accretion displacement on large-scale right-slip faults, terrane dispersion resulting from extensional tectonic processes has received less attention. In northern Alaska, for example, allochthons were emplaced during north-directed arc-continent collision in the Late Jurassic and Neocomian, producing a structural stacking sequence, from base to top, of allochthons composed of continental rocks, basalt-chert, ophiolite, and arc lithologies. Following collision, northern Alaska was affected by an episode of regional extensional tectonism in the mid-Cretaceous that exhumed deep structural levels on major ductile extensional faults now exposed in the southern Brooks Range, Seward Peninsula, and Ruby terrane. Different structural levels were juxtaposed across these faults, leading to a complex map pattern of terranes in northern and interior Alaska. The Vizcaino Peninsula-Cedros Island region of Baja California likewise consists of a perplexing assemblage of terranes. Here, Triassic ophiolite and Jurassic arc rocks hosted Middle Jurassic extension and formation of ophiolite-floored interarc basins, renewed Late Jurassic and Early Cretaceous extension, and Late Cretaceous forearc extension. Although younger dispersion on right-slip faults has undoubtedly occurred in both areas, the magnitude of displacement thought to be required by these systems has probably been magnified by apparent displacement caused by the previous episodes of tectonic extension.