Laramide-style shortening structures in southwest New Mexico chiefly affect Cretaceous and older rocks, making the estimation of the cessation of shortening in the Paleogene problematic. The cessation of Laramide shortening is generally thought to be about 40 Ma in southwest New Mexico. However, in the Silver City Range, Grant County, New Mexico, shortening younger than 34.6 Ma was documented by Copeland et al. (2011).
The Little Burro Mountains, 10–20 km south of the Silver City Range, contain a monocline trending northwest–southeast with beds dipping ∼12° in the backlimb and up to 30° in the forelimb. The youngest folded unit in the monocline is the tuff of Wind Mountain (Twt), which yielded a 206Pb/238U zircon age of 30.9 ± 0.5 Ma. Normal faults strike orthogonal to the axial trace of the fold with low displacement (10s of meters), which we interpret to have formed synchronous with the monocline to accommodate variations in shortening along strike. Field observations, trishear fault-propagationfold modeling, and the structural style and trends of the region are consistent with the development of the monocline related to a deeply rooted blind thrust or reverse fault. These results indicate that Laramide-style shortening in southwest New Mexico was active into the late Oligocene.