Subsurface occurrences of Mesoproterozoic Unkar Group (∼1.25–1.1 Ga) and Neoproterozoic Chuar Group (∼0.9–0.7 Ga) rocks have been delineated in regions east and southeast of the Grand Canyon of northern Arizona. Gravity and aeromagnetic data, combined with gravity modeling, an analysis of seismic velocities, geologic cross sections, and geologic structures, have identified two major Proterozoic extensional regimes in the region: (1) a Mesoproterozoic northwest to southeast trend indicating northeast-southwest–directed extension throughout the study area and (2) a Neoproterozoic, mostly north-south–directed trend indicating predominant east to west extension, evident mostly in northern regions of the study area. A more ancient, northeast-southwest–aligned system of basement shear zones has also been identified. These shear zones correlate with many of the exposed major fault systems of the region such as the Bright Angel and Mesa Butte fault systems. Unkar Group graben and half-graben formation was constrained by the preexisting northeast-directed shear zones. Chuar Group basin formation was influenced by the reactivation of northwest to southeast Unkar Group basin structures, the more ancient northeast-trending shear zones, and by Neoproterozoic age north- to south-oriented extension. This superimposed network of deep-seated normal faults led to the development of a complex array of surface structures during the Laramide. By delineating Proterozoic basin structures in the region, this study helps to delineate the probable subsurface occurrences of hydrocarbon source rocks of the Walcott Member of the Neoproterozoic Chuar Group.

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