The Colorado River extensional corridor in southwestern North America is one of Earth’s most highly extended regions of continental crust. The central part of the belt includes three imbricate, regionally northeast-dipping extensional detachment faults. The Plomosa detachment fault in the northern Plomosa Mountains in western Arizona, the middle of the three faults, dips northeastward beneath the giant Harcuvar metamorphic core complex. Approximately 1 km of lower Miocene clastic sediments, lava flows, and rock-avalanche breccias were deposited in the northern Plomosa Mountains before initiation of the Plomosa detachment fault and division of the strata into two basins with different stratal accumulations following breakup. Both the detachment-fault lower plate and upper plate were then broken and tilted by normal faults. The upper plate was fragmented into numerous fault blocks and its extension-parallel width was approximately doubled. Application of critical-taper theory to delayed basin fragmentation suggests that southwestward tilting of the land surface and underlying normal faults led to normal-fault initiation and wedge breakup.

A seismic-reflection profile northeast of the northern Plomosa Mountains reveals strong, southwest-dipping reflectors that project up dip to metasedimentary tectonites in the southern Buckskin Mountains in the Harcuvar core complex. Restoration of displacement on the Buckskin and Plomosa detachment faults aligns the reflectors and tectonites with a Mesozoic shear zone in the footwall of the Plomosa detachment fault. In this restoration the combined shear zone dips northeastward rather than southwestward and projects up dip to the folds and thrusts exposed to the south and west of the northern Plomosa Mountains. This zone is interpreted as a segment of the Mesozoic Maria fold-and-thrust belt that influenced the geometry of younger detachment faults.

The southwest-tilted, mylonitic lower plate of the Plomosa detachment fault includes, at its northern end, Orocopia Schist, which is a Cretaceous subduction complex that is better known from locations farther southwest and closer to the continental margin. Restoration of tectonic extension suggests that Orocopia Schist extends under the Harcuvar core complex and that a buoyant crustal root inherited from Cretaceous thrusting could not have been the cause of core-complex uplift unless the schist was emplaced by a mechanism other than subduction underplating. We propose that the rolling-hinge detachment-fault model combined with a highly mobile deep crust could account for Harcuvar core-complex genesis without a buoyant crustal root.

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