Tertiary structure and thermal history of the Harquahala and Buckskin Mountains, west central Arizona: Implications for denudation by a major detachment fault system

Abstract

The Harquahala and Buckskin mountains lie in the footwall of the Whipple‐Buckskin‐Bullard detachment system. In the Harquahala Mountains, Mesozoic fabric and structure are progressively more intensely overprinted by penetrative Tertiary deformation toward the northeastern pan of the range. Tertiary mylonitic deformation is recognized by the presence of deformed Miocene mafic dikes and characteristic textural features. Lineations in the mylonite trend 040°–060°, and megascopic kinematic indicators mostly indicate top‐to‐the‐northeast sense of shear. The crest of an antiform defined by Tertiary mylonitic fabric does not coincide with the topographic crest of the range. Reorientation of Mesozoic foliation in the vicinity of this antiform suggests that it originated as two shear zones, as opposed to a single zone that was then bent. The northeastern part of the Brown's Canyon granite acts as a large, low‐strain lozenge in the southeast limb of the foliation arch. Mylonitic foliation along the southeast side of the range, above this lozenge, is strongly oblique to the trend of the detachment fault, but the relative orientation is consistent with left‐oblique normal shear. Evidence of Tertiary plastic deformation is absent southwest of Sunset Pass. Minor low‐angle normal faults in the southwestern Harquahala Mountains and Little Harquahala Mountains dip northeast The low‐angle normal faults are cut by NW trending high‐angle, right‐oblique faults. In the central and southwestern Harquahala Mountains, 40Ar–39Ar age spectra from K‐feldspar, muscovite, and hornblende and total gas ages from biotite indicate Late Cretaceous to early Tertiary cooling to argon closure temperatures. Biotite and K‐feldspar from the area northeast of Sunset Pass record rapid early to middle Miocene cooling. Reconstruction of the original geometry of the detachment system, based on thermal differences indicated by contrasting cooling histories, and the orientation of an early Miocene dike swarm indicate that the initial dip of the detachment fault was most probably between 30° and 40°. Thus the Harquahala Mountains are a lilted block exposing of the order of a 10‐km section through the pre‐Tertiary crust. Heterogeneous Proterozoic gneiss, sparse Paleozoic and Mesozoic metasedimentary rocks, and an Oligocene plutonic complex are extensively overprinted by Tertiary mylonitic fabrics in the Buckskin Mountains. Hornblende 40Ar–39Ar cooling ages suggest that most of these rocks were below hornblende closure temperature by early Tertiary time, except in the vicinity of the Oligocène plutonic complex. Feldspar and biotite 40Ar–39Ar cooling ages suggest that the footwall of the Whipple detachment system experienced a more uniform cooling history in the Buckskin Mountains than in the Harquahala Mountains; cooling ages between about 13 and 20 Ma are recorded throughout the range with no consistent spatial pattern of ages. The Eagle Eye detachment fault, southeasternmost extension of the Whipple‐Buckskin‐Bullard fault system, becomes a transfer fault along the southeastern side of the Harquahala Mountains. This fault separates two distinct extensional terranes. On the northwest, approximately 50 km of crustal extension has been accommodated on the Whipple fault system, while on the southeast a similar total magnitude of crustal extension is accommodated by a series of smaller normal faults in the adjacent Big Horn‐Lake Pleasant extended terrane. As the deformation becomes more distributed, maximum slip on any one normal fault decreases to the point where no single fault has enough slip to denude crustal levels at which Tertiary plastic deformation has occurred.