EVOLUTION OF MIOCENE EXTENSIONAL FAULT AND FOLD SYSTEMS AND INFLUENCE OF MAGMATISM IN THE NORTHERN COLORADO RIVER EXTENSIONAL CORRIDOR, UNION PASS AREA, NORTHWEST ARIZONA

The Union Pass area of NW Arizona lies within the highly extended Colorado River extensional corridor (CREC) of the Basin and Range province. USGS EDMAP funding facilitated detailed geologic mapping of a major Miocene volcanic center, multiple generations of faults, and extensional folds in the north ½ of the Union Pass Quadrangle. Structural analysis indicates a genetic link between fold geometries and fault kinematics and suggests clockwise rotation of principal strain axes in the CREC during the early-mid Miocene. The magmatic system near Union Pass at least partially controlled brittle deformation in the area and slowed the N-ward advance of major Miocene extension in the CREC. Peak extension is bracketed between ~19 and 17 Ma south of the Union Pass area, whereas major extension directly to the north began ~16 Ma. NW-striking dikes intrude Proterozoic basement in the area and reflect an early Miocene ~NE-SW extension direction prevalent in the CREC to the south. These dikes likely invaded fractures formed ahead of a N-ward advancing system of extensional faults. The N-ward sweep of major extension stalled ~18.5 Ma near Union Pass, coincident with development of a suite of early-mid Miocene felsic to intermediate volcanic centers. Major faulting may have been preempted by large upper crustal magma bodies that elevated the brittle-ductile transition, altered local stress fields, accommodated some extensional strain by diking, and lacked inherited structural flaws. Diking effectively locked some early normal faults and transferred strain to a set of antithetic faults. Overlap between opposing fault systems produced an accommodation zone and associated extensional folds. As magmatism slowed ~16 Ma, major extension resumed along ~N-striking fault systems around Union Pass in the hanging wall of the E-dipping Newberry Mountains detachment fault. The resumed N-ward advance of extension on N-striking faults suggests an ~E-W extension direction in contrast to the ~220-240° extension direction observed south of Union Pass. Fault-slip data from the Union Pass area suggest a mid Miocene extension direction of ~262°, indicating clockwise rotation of average incremental strain axes in the CREC by 16 Ma. Strain axes inferred from extensional fold geometries are essentially coaxial with those derived from fault-slip data.