Tectono-magmatic interactions and extensional folding in the Union Pass area, northern Colorado River extensional corridor, northwest Arizona

Field and laboratory studies define the geology of the Union Pass area (NW Arizona) and elucidate both links between magmatism and extension and origins of folds in Miocene strata. The Union Pass area lies within the Colorado River extensional corridor (CREC) of the Basin and Range province and contains well-exposed Miocene volcanic centers, multiple generations of faults, and extensional folds. The advance of major extension through the CREC stalled ∼18.5--16 Ma near Union Pass, coincident with development of the volcanic centers. Major faulting was likely preempted by upper-crustal magma bodies that elevated the brittle-ductile transition, altered local stress fields, accommodated extensional strain by diking, and lacked inherited structural flaws. Diking locked some early normal faults and transferred strain to a set of antithetic faults. Overlapping, opposed fault systems produced an accommodation zone containing extensional folds. Structural analysis indicates clockwise rotation of principal strain axes in the CREC by ∼16 Ma.