An archlike zone of seismic reflectivity, interpreted as an uplifted zone of ductilely deformed middle and lower crust, is imaged below the Pinaleño Mountains core complex in southeast Arizona. The top of the reflective zone coincides with the base of an inferred mid Tertiary detachment fault beneath the Safford basin but diverges from the detachment fault as an apparent mylonite front to form a culmination at ∼1.9 s (∼4 km) beneath the Pinaleñlo Mountains. From this culmination, the zone of reflectivity dips to the southwest below the Eagle Pass detachment fault and flattens at ∼4.8 s (∼13.5 km) beneath the relatively unextended upper crust of the Galiuro Mountains. Most of the reflective fabric probably formed during mid-Tertiary extension, although some of it may be older. These data suggest that mylonite zones form not only as the continuation of detachment faults into the brittle-ductile transition, but also along a regional zone of decoupling between the middle and upper crust. Highly extended and relatively unextended domains in the Basin and Range may be separated by zones of discrete (simple) shear in the upper crust, but both are rafted above regional bulk pure shear in the middle and lower crust.

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