Ancient Landscapes of Western North America by Ronald C. Blakey & Wayne D. Ranney

Author:Ronald C. Blakey & Wayne D. Ranney
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

Slab flattening of the Farallon plate shifted the arc magmatism inland; the width of the shifted belt of arc magmatism was approximately the width of the zone of collision between the oceanic plateau and the western margin of the continent (Fig. 8.12). Normal arc magmatism continued along the Cordillera to the north in Idaho and Canada, and to the south in central Mexico. As the flat slab subducted, the Farallon plate split along a developing east/west-trending oceanic ridge – the Farallon plate maintained to the south while the new Kula plate formed to the north (Fig. 8.14). All the while, smaller terranes and other sea floor material entered the trench, which now extended from South America, through Central and North America, to Alaska and even eastern Asia. Rocks of the Franciscan assemblage continued to become jumbled as they were partially subducted and accreted to the coast in California.

Fig. 8.14Paleogeography of SW North America ca. 70 Ma. The Cordilleran arc between Northern Mexico and Central Nevada has completely shut down as the subducted slab migrates eastward in the subsurface. The Franciscan forearc zone is undergoing tectonic erosion and northward transport via transform faults (ridges in forearc). The axis of subsidence in the Western Interior Seaway has shifted well to the east. In 3–4 million years the seaway will be gone

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