Inversion of proterozoic extensional faults: An explanation for the pattern of Laramide and Ancestral rockies intracratonic deformation, United States

The Rocky Mountains, Colorado Plateau, and Midcontinent, regions of the North American cratonic platform, display similar styles and patterns of Phanerozoic deformation. In these regions, movement on basement-penetrating faults during the late Paleozoic Ancestral Rockies event and/or during the Mesozoic-Cenozoic Laramide event generated flat-topped uplifts bordered by outward-verging, monoclinal forced folds. We suggest that these structures, divided into two sets on the basis of orientation (west to northwest and north to northeast), formed by inversion of Proterozoic extensional-fault systems. In this model, Proterozoic rifting events formed weak faults in the cratonic platform crust, and these faults were reactivated by stress transmitted during Phanerozoic compressional orogenies. If this model is correct, the pattern of Ancestral Rockies and Laramide contractional structures reflects the trends of Proterozoic extensional faults, and regional variation in forced-fold vergence reflects the control of antecedent fault dips on fault-propagation fold geometry during inversion. Late Proterozoic rifts formed throughout Rodinia, so inversion tectonics likely occurred in cratonic platforms worldwide.