LATE JURASSIC TRANSFORM FAULTING AND THE DISTRIBUTION OF TERRANES ALONG THE WESTERN MARGIN OF NORTH AMERICA

The recognition and characterization of terranes such as Wrangellia and Chulitna along the western margin of North America by D. L. Jones and colleagues were seminal to a broader vision of plate tectonic processes. The terrane concept forced geoscientists to carefully assess structural and stratigraphic relationships across geologic contacts and consider each of these contacts as a possible record of one or more plate processes including subduction, accretion or lateral motion. The distribution of North American cordilleran terranes provides a template useful in the assessment of interpretations of the history of plate tectonic processes.

The postulated extension of a Late Jurassic sinistral fault, the Mexico-Alaska megashear, from the Gulf of Mexico to Alaska along which about a thousand km of displacement may be recognized must be compatible with the established terrane template. The fault must wend its way among terranes so that the trace does not conflict with constraints provided mainly by paleontologic and stratigraphic information either before or after restoration. In western Canada, a previously recognized discontinuity, which lies within the Coast Belt and separates the Insular superterrane, including Wrangellia and Alexander, and Chulitna from the Intermontane superterrane, may accommodate the major fault.

Restoration of North America 1000 km southward yields the following regional geologic conditions: 1) re-opens the Kobuk Sea and closes the Gulf of Mexico; 2) restores continuity of Middle Jurassic arc rocks that intrude terranes on both sides of the fault by removing overlap in western Canada and a gap in northern Mexico; 3) brings oceanic Bridge River rocks to a position where they face the Pacific Ocean; 4) places strata at Antimonio on the Caborca block close to the Middle Norian biogeographic Nevada-Sonora province; 5) restores Stikinia, Quesnellia, and Klamath Mountains south of the Chulitna and Wrangellia-Alexander terranes.

The Late Jurassic restoration does not address the paleogeographic information derived from pre-Middle Jurassic fossils from Chulitna, Wrangellia and Alexander terranes.