There are numerous well documented examples of Laramide-style shortening in SW New Mexico. Ranges such as the Big Hatchets and the Florida Mountains exhibit classic examples of Laramide thrust faulting. Unfortunately the youngest part of the stratigraphy for this region is Cretaceous, rendering it difficult to bracket the age of Laramide termination. From sparsely known Laramide ages in the region, it is suffice to note that Laramide shortening in SW New Mexico has proven to be more complex than most modeled interpretations are capable of deducing. Recent observations by Copeland et al. (2011) show that shortening is younger than 34.6 Ma in the Silver City Range of southwest New Mexico. This has important implications for the timing and nature of the switch from regional shortening to regional extension.

The Little Burro Mountains were targeted for further studies due to structural similarities with the Silver City Range (Paige, 1916). The stratigraphy of the Little Burros consists of a Prtoterozoic granite basement unconformably below Cretaceous Beartooth quartzite and Colorado shale. Unconformably above the Cretaceous units are a series of Tertiary volcanic rocks. They consist of andesite lavas and breccias succeeded by the tuff of Indian Peak and the tuff of Wind Mountain. A Tertiary basaltic andesite and fan deposits unconformably cap the tuff of Wind Mountain.

Field mapping focused on the Tertiary volcanic rocks and how they relate to the Laramide deformed Cretaceous beds. The results clearly show a monoclinal fold with an axial trace trending NW-SE. This corresponds with the structural style of Laramide for the rest of the region, which is generally agreed to be caused by a NE propagation of shortening (Bird, 1988; Seager, 2004). The Little Burro monocline displays modestly dipping beds of ~12° in the backlimb, which steepen to ~30° in the forelimb. Field evidence and trishear faultpropagation- fold modeling supports the idea that the broad interlimb angle of the Little Burro monocline is derived from a deeply rooted blind thrust fault. Normal faults run orthogonal to the axial trace of the fold with low displacement (10s of meters). They are interpreted to have formed from variations of shortening along strike of the thrust fault as well as accommodation of flexure in the structure. The youngest folded unit associated with shortening is the ash-flow tuff of Wind Mountain. A sample of this felsic tuff was dated by U-Pb on zircons using LA-ICPMS. Using Plešovice as a standard (Sláma et al., 2008), the age Pd206/Pd208 came out to 28.83 ± 0.46 Ma.

These results suggest Laramide-style shortening was active in the late Oligocene. Examples of rift-related extension and arc volcanism are known to pre-date this age (Mack, 2004; Chapin et al., 2004; McIntosh et al., 1991), suggesting that compressional shortening, arc volcanism and extension were all active in this region during the late Eocene and Oligocene. This complex regional tectonics can be explained by delamination of eclogitized continental lithosphere (Decelles et al., 2009) followed by continued crowding of basal continental lithosphere from a shallow subducted Farallon plate.