The Bullard detachment fault is a gently to steeply dipping normal fault that flanks the Harcuvar and Harquahala mountains of the Basin and Range province in west-central Arizona. The stratigraphy of upper-plate Miocene conglomerates and the regional distribution of upper- and lower-plate pre-Tertiary units indicate that upper-plate rocks were displaced about 50 km to the northeast with respect to the lower plate during middle to late Tertiary time. Normal slip of this magnitude on the regionally northeast-dipping Bullard fault indicates that deep-seated Tertiary-Cretaceous(?) mylonitic gneisses and Mesozoic thrust faults of the lower plate were drawn out from beneath Precambrian rocks along the margin of the Transition Zone of central Arizona during middle to late Tertiary crustal extension. The Transition Zone was, therefore, affected by deep-seated tectonism in both Mesozoic and Tertiary time.

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