Distributed Neogene faulting across the western to central Arizona metamorphic core complex belt: Synextensional constriction and superposition of the Pacific–North America plate boundary on the southern Basin and Range

Abstract

Abstract We present fault data from a belt of Miocene metamorphic core complexes in western and central Arizona (USA) to determine patterns of brittle strain during and after large-magnitude extension, and to evaluate the magnitude of postextensional dextral shear across the region. In the White Tank Mountains, coeval WNW- to NW-striking dextral, normal, and oblique dextral-normal faults accommodated constrictional strain with extension subparallel to the direction of ductile stretching during core complex development. Northwest-striking oblique dextral-normal faults locally accommodated similar strain in the Harquahala Mountains, whereas in the South Mountains, constriction was primarily partitioned on NE-dipping normal faults and conjugate NW- and north-striking strike-slip faults. We interpret brittle constrictional strain to have developed during the late stages of large-magnitude extension associated with core complex development and folding of detachment fault corrugations. The oblique orientation of the Arizona core complex belt with respect to the extension direction likely resulted in a minor component of dextral transtension, accounting for much of the constrictional strain. In addition, far-field stresses associated with the transtensional Pacific–North America plate boundary may have contributed to constriction, which characterizes most Neogene detachment fault systems in the southwest Cordillera. Following cessation of detachment fault slip across the Arizona core complex belt (ca. 14–12 Ma), distributed NW-striking dextral and oblique dextral–NE-side-up (reverse) faults modified the topographic envelope of corrugations to an orientation clockwise of the core complex extension direction. Based on our analysis of this misalignment, we interpret the postdetachment fault dextral shear strain to increase northwestward from 0.03 across the South Mountains (0.5–0.6 km total slip across 18 km) to >0.03–0.07 across the Harquahala and Harcuvar Mountains (1.2–2.5 km of total slip across ∼35 km) and ∼0.2 across the Buckskin-Rawhide Mountains (7–8 km across 36 km). This along-strike variation in dextral shear is consistent with the regional pattern of distributed strain associated with the Pacific–North America plate boundary, as cumulative dextral offset in the lower Colorado River region increases toward the eastern Mojave Desert region to the northwest.