Abstract

Arizona can be divided into three physiographic and tectonic provinces, as follows: (1) the Colorado Plateau, (2) the Basin and Range province, and (3) the Transition Zone that extends diagonally across Arizona between the other two. The Transition Zone slopes southwestward from the elevated Colorado Plateau toward the Basin and Range province, but the regional slope was in the opposite direction in the early Tertiary. The Basin and Range province in Arizona underwent two phases of extension. Middle Tertiary detachment faulting, core complex uplift, and voluminous felsic magmatism were followed by late Tertiary high-angle normal faulting and formation of deep basins, many of which contain thick evaporite deposits. The first, core complex phase of extension accompanied reversal in slope across the Transition Zone. Detachment faults dip beneath the Transition Zone along most of its length, and it is generally inferred that displacement of lower plate rocks southwestward from beneath the Transition Zone led to both the reversal in Transition Zone slope due to greater crustal thinning in the southwest and to exhumation of most of Arizona’s core complexes. Crustal thinning and perhaps regional post-magmatic cooling led to great reduction in the mobility of the plastically deforming deep crust, and this loss of mobility is inferred to have caused a transition in structural styles of extension from detachment faulting and core complex exhumation to high angle normal faulting and deep basin subsidence. This transition appears to have been delayed where the crust was unusually thick because of earlier crustal shortening, but was not delayed by locally voluminous magmatism. During the middle Miocene transition from detachment faulting to high-angle normal faulting, parts of the boundary zone between the Basin and Range province and the Transition Zone underwent moderate to severe extension by southwestward dipping faults that are antithetic to underlying, regionally northeastward dipping detachment faults. This episode of antithetic faulting is inferred to be a consequence of the reversal in slope of the Transition Zone.