Seismic crustal structure beneath the Safford Basin and Pinaleiio Mountains: Implications for Cenozoic extension and metamorphic core complex uplift in SE Arizona

Abstract

Geologic relationships and reflection seismic data in the Safford basin and Pinaleiio Mountains metamorphic core complex suggest the northwest-trending core complex is formed along a mid-Tertiary detachment fault system that extends from a breakaway near the northeast flank of the Galiuro Mountains, to a depth of -15 km (-5 s) where it flattens beneath the Gila-Peloncillo Mountains. Top-to-the-northeast mid-Tertiary and late Cenozoic extension along this system caused isostatic uplift of the core complex, and warping of the detachment-fault system by unloading due to tectonic denudation of the upper crust and other mechanisms. The Safford basin is a northwest-trending, southwest-dipping half-graben containing a maximum of 4.5 km of late Cenozoic basin fill. Formation of this basin, and renewed uplift of the core complex, began in the middle Miocene (17 Ma 7) with partial reactivation of the detachment fault system on the northeast flank of the core complex, and extension along a 50°-20° northeast-dipping listric range-bounding fault which merges with or cuts the detachment fault. An increase in basement seismic reflectivity forms a convex-upward arch-shaped zone with an apex beneath the northeast flank of the core complex at a depth of -4 km (-1.9 s) in the vicinity of Eagle Pass. The top of this arch-shaped zone extends along the base of the inferred detachment fault beneath the Safford basin, but separates from the detachment fault near the apex, dipping to the southwest and flattening at -13.5 km (-4.8 s) beneath the northeast flank of the Gila Mountains. The reflective fabric of this arch-shaped zone probably formed below -13 km as a ductile continuation of the detachment fault and a regional, subhorizontal shear zone associated with differential movement across a major brittle-ductile transition. Crustal extension was accommodated by brittle detachment faulting and block faulting above the transition, and ductile pure shear or anastomosing shear zones below. Uplift of the fabric during and after formation resulted from core complex development. Crustal reflectivity within and below the arch-shaped zone is divided into domains of variable reflection amplitude and continuity, possibly caused by variations in compositional layering, ductile strain, and partial melts. The Moho beneath the Safford basin is indicated by a series of strong reflections or decrease in reflectivity, and occurs at depths of -29-31 km (-9.5-10.5 s), apparently increasing towards the Pinalefio Mountains. Sparse mantle reflections at -11-11.3 s may be from intrusions or partial melts.