Proterozoic multistage (ca. 1.1 and 0.8 Ga) extension recorded in the Grand Canyon Supergroup and establishment of northwest- and north-trending tectonic grains in the southwestern United States
The Grand Canyon Supergroup records at least two distinct periods of intracratonic extension and sedimentation in the late Mesoproterozoic and Neoproterozoic. New 40 Ar/ 39 Ar age determinations indicate that the Mesoproterozoic Unkar Group was deposited between ca. 1.2 and 1.1 Ga. Basins in which the Unkar Group was deposited and the related northwest-striking faults were created by northeast-southwest extension, which was contemporaneous with regional northwest-southeast “Grenville” contraction. New U-Pb data indicate that the Neoproterozoic Chuar Group was deposited between 800 and 742 Ma. Sedimentary and tectonic studies show that Chuar deposition took place during east-west extension and resulting normal slip across the Butte fault. This event is interpreted to be an intracratonic response to the breakup of Rodinia and initiation of the Cordilleran rift margin. Laramide monoclines of the Grand Canyon region have north and northwest trends, reactivate faults that originated at the time of Unkar and Chuar deposition, and can be traced for great distances (hundreds of kilometers) from the Grand Canyon. We use the distribution of monoclines in the Southwest to infer the extent of Proterozoic extensional fault systems. The 1.1 Ga northwest-trending structures and ca. 800–700 Ma north-trending extensional structures created regional fault networks that were tectonically inverted during formation of the Ancestral Rocky Mountains and Laramide contraction and reactivated during Tertiary extension.