INTERACTIONS BETWEEN TERTIARY MAGMATISM AND EXTENSION IN THE COLORADO RIVER EXTENSIONAL CORRIDOR, UNION PASS AREA, NORTHWESTERN ARIZONA

The Union Pass area of the southern Black Mountains in northwest Arizona lies within the highly extended Colorado River extensional corridor (CREC) of the Basin and Range province. The Union Pass area contains well exposed Miocene volcanic rocks and dikes, multiple generations of faults, and extensional folds. This area affords an excellent opportunity to evaluate interactions between extensional faulting and an evolving pre- to syn-extensional magmatic system.

The Union Pass area appears to mark a major temporal domain boundary to extension within the CREC. To the south, large-magnitude ~E-W extension is bracketed between ~21 and 17 Ma, whereas to the north major extension is constrained between ~16 and 13 Ma. A suite of early to middle Miocene felsic volcanic centers in the vicinity of Union Pass marks this temporal domain boundary. Two generations of normal faults are found in the area. Older east-dipping (Whipple domain) faults are commonly intruded by large rhyolite dikes and cut by west-dipping (Lake Mead domain) faults. The dikes appear to be associated with ~17-16 Ma rhyolite lavas and domes. Lack of cataclasis of dikes along the east-dipping faults indicates that extension ceased on these faults just prior to, or during, dike emplacement. The transfer of extensional strain to a superposed set of west-dipping normal faults produced an accommodation zone where oppositely tilted fault blocks generate several extensional anticlines and synclines. The accommodation zone in the Union Pass area is a southern extension of the Black Mountains accommodation zone (Faulds and Varga, 1998). Unlike most other parts of the Black Mountains accommodation zone, however, the Union Pass segment served as a temporal domain boundary to large-magnitude extension and effectively halted the northward advance of Miocene extension within the CREC. The developing magmatic systems in the Union Pass area may have acted as a buttress to faulting due to perturbation of the brittle-ductile transition, presence of upper crustal bodies lacking preexisting readily reactivated weaknesses, and partial accommodation of extensional strain by diking.