Seismic reflection evidence for detachment polarity beneath a major accommodation zone, west-central Arizona

Interpretation of reflection seismic data across a major accommodation zone in west-central Arizona shows that a uniform polarity detachment fault can extend beneath opposing polarity tilt-block domains. A dip line, which roughly parallels the extension direction, shows that the White Tank detachment fault extends beneath both the southwest-tilted Whipple-Camelback and northeast-tilted Vulture domains. Top-to-the-northeast mid-Tertiary displacement along this detachment fault was responsible for formation of the White Tank metamorphic core complex. A second profile, which is a strike line approximately perpendicular to the extension direction, intersects the dip line near the core complex in the southwest-tilted domain. Basement reflectivity on both profiles extends from the White Tank detachment or mylonitic front down to the base of the crust. Relations between the basement reflectivity, core complex, and upper-plate tilt blocks suggest that the White Tank detachment fault is the dominant detachment in the area. The accommodation zone that separates the opposing tilt-block domains consists of a crystalline-cored rollover anticline that formed above the White Tank detachment fault as two oppositely tilted half grabens developed above inwardly dipping, upper-plate listric normal faults. Southwest-dipping normal faults associated with the northeast-tilted domain appear to sole directly into either the through-going northeast-dipping White Tank detachment or a minor southwest-dipping low-angle normal fault. This minor southwest-dipping low-angle normal fault either soles into the White Tank detachment or offsets it by no more than 3 km. This suggests that faults associated with the northeast-tilted Vulture domain are essentially antithetic faults in the upper plate of the White Tank detachment. This arrangement shows that a major detachment fault associated with a more regional tilt-block domain can extend beneath a sizable domain of opposite polarity. It also shows that the tilt direction of fault blocks is not always a good indicator of the dip of the dominant underlying detachment fault.