Tectonics of Mid‐Tertiary Extension along a transect through west central Arizona

Large‐magnitude Miocene extension in west central Arizona occurred primarily along three imbricate, northeast dipping normal faults. The structurally highest of these faults, the gently dipping Buckskin‐Rawhide detachment fault, accommodated approximately 66 km of crustal extension, whereas the two structurally lower faults accommodated a total of about 20 km extension. Due to this large‐magnitude extension, an area at the Earth's surface that was 10 to 20 km wide is now over a 100 km wide, and crystalline rocks with mid‐Tertiary mylonitic fabrics, uncovered by detachment faulting, are exposed over roughly 2000 km² in the Harcuvar metamorphic core complex. Most of the upper plate of the Buckskin‐Rawhide detachment fault was largely undeformed by internal extension; only the thin, tapered end of the upper plate was highly extended. During extension the lower plate must have flexed to conform to the listric underside of the upper plate and to have flattened to its present subhorizontal form as it was uncovered. Grooves on the underside of the upper plate were apparently imposed on the pliable lower plate as it was denuded, forming extension‐parallel folds in the lower plate. Low flexural strength characterized the lower plate during denudation, and a highly mobile, low‐viscosity deeper crust must have effectively decoupled the upper crust from the mantle lithosphere.