Along-strike upper-plate deformation in response to metamorphic core complex emplacement, SE Arizona

Abstract

Metamorphic core complexes represent concentrated zones of crustal extension; universally, the hanging walls of these systems are extensively fractured, attesting to the significant horizontal extension above the evolving detachment zone. Recent analysis of a large grid of 2-D seismic reflection lines within the Tucson Basin of southeastern Arizona has facilitated a nearly three-dimensional interpretation of subsurface features related to Cenozoic crustal extension. Within the northern Tucson Basin, the Catalina detachment fault dips 23–35° to the southwest from the western flank of the Catalina/Rincon Metamorphic Core Complex. In the southern portion of the basin, the NE-trending Santa Rita normal fault dips 15–20° to the northwest from the western flank of the Santa Rita Mountains and is cut by the Catalina detachment beneath the central Tucson Basin. The orientation of the Santa Rita fault is problematical in that its orientation is nearly perpendicular to extension directions in the region, while geologic and seismic reflection evidence indicates that the Catalina detachment and Santa Rita fault were active synchronously. One possible explanation is that the Santa Rita fault is accommodating along-strike upper-plate deformation in response to core complex emplacement. To test this, we employed a finite-element modeling approach. A two-dimensional model consisting of a homogenous elastic material undergoing uniform extension is used to study changes in stresses and displacements in proximity to a zone of weakness representing a detachment fault in the upper crust. Away from the detachment fault, primary principal stresses are oriented parallel to the regional extension direction as expected. Near the detachment, extension in the system produces a rotation of the primary principal stresses of nearly 60° with respect to the regional stress field around the end of the fault. Additionally, the model predicts mechanical failure of the upper-plate of the detachment system to the south of the Catalina core complex. This model supports our interpretation that the orientation and early displacement noted on the Santa Rita fault is the result of a perturbation in the regional stress field caused by the Catalina detachment and the associated brittle failure of the upper-plate from the extreme crustal extension associated with core complex emplacement.